GEI-100696B

GE Energy
WorkstationST* Modbus®
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 2008 General Electric Company, USA. All rights reserved.

Revised: 080812
Issued: 071202

g
* Trademark of General Electric Company
Modbus is a registered trademark of Schneider Automation.
 Contents

Section Page
Overview ........................................................................................................ 3
Configuration.................................................................................................. 3
Enabling the Modbus Feature .................................................................. 5
Configuring the Modbus Feature ............................................................. 7
Modbus Properties ................................................................................. 12
Hardware Interface Properties................................................................ 14
Master Message Properties..................................................................... 15
Slave Properties...................................................................................... 15
Data Structure Properties ....................................................................... 15
Scaling.................................................................................................... 16
Spare Values .......................................................................................... 19
Importing or Exporting .csv Files .......................................................... 20
Error/ Warning Messages....................................................................... 21
Runtime ........................................................................................................ 25
WorkstationST Status of Features.......................................................... 25
WorkstationST Additional Status........................................................... 26
Tracing ................................................................................................... 31
Ethernet Messages.................................................................................. 32
Serial Messages...................................................................................... 33
Monitoring Live Values ......................................................................... 34
Troubleshooting............................................................................................ 35
Slave Exception Responses.................................................................... 35
Terms............................................................................................................ 37

2 • WorkstationST Modbus GEI-100696B

 Overview
* ®
The ToolboxST application allows you to configure the Modbus feature that runs
*
in the WorkstationST component. Modbus is an application layer messaging
protocol for client/server communication between components connected on
different types of busses or networks. The WorkstationST application supports both
serial and Ethernet communication, and can operate as both a Master and a Slave. It
is possible to have serial and Ethernet Masters and Slaves all running at the same
time, within the limits of the computer used.

Configuration
When the Modbus item in the component Tree View is selected, the properties that
display in the Property View apply to all connections. When the Modbus item is
expanded, Master and Slave items display. These are organizational categories with
no configuration properties. When you insert a hardware interface into either a
Master or a Slave, several configuration properties display in the Property Editor.
One or more Slaves may be inserted under each hardware interface. Slave definitions
consist of four types of data structures:

• Holding registers
• Holding Coils
• Input registers
• Input Coils
The Slave data structures represent different data depending on whether the Slave
definition being configured is under the Masters or the Slaves item in the Modbus
Tree View. A Slave definition under the Slaves item is the WorkstationST Modbus
data structure sent to the remote Master. A Slave definition under the Masters item
reflects a local copy of the remote Slave data that is mapped to ToolboxST variables.
Each Slave under the Masters item also contains configuration properties for each
Modbus message to be sent by the Master, including the rate at which the messages
are sent.

GEI-100696B WorkstationST Modbus• 3

 WorkstationST - Modbus Tab Remote
Device
ToolboxST items Modbus Data items
Variable Name
Type* Data Type
Eng Min Raw Min
Eng Max Raw Max

Slaves WorkstationST Master

defines Slave Map
Consumed ------- I xxxx -------- > I xxxx
or ------- HC xxxx < ------- > HC xxxx
Produced ------- IR xxxx -------- > IR xxxx
Variables ------- HR xxxx < ------- > HR xxxx

Masters Remote Slave Slave

defines Slave Map
Consumed < -------- I xxxx ------- I xxxx
or < ------- > HC xxxx ------- HC xxxx
Produced < -------- IR xxxx ------- IR xxxx
Variables < ------- > HR xxxx ------- HR xxxx

*assigned at variable definition (not in modbus tab)

Modbus Data Flow
Modbus Slaves are configured in two steps:

• Creating the Modbus data structures, which comprise the memory map (address
and datatype) of every message to be sent or received by Modbus.
• Attaching each Modbus data point to a ToolboxST variable, which is then used
to transfer data to or from the corresponding Modbus data point.
Each data structure contains Modbus data points that are created by connecting a
ToolboxST variable to a Modbus address in one of the data structures. Each Modbus
data point is configured with an address and a data type to be sent or received over
Modbus. When the WorkstationST is operating as a Master, the data structure to be
configured is determined by the remote Slave component. This means that the user
must know the details of the remote Slave component to configure the
WorkstationST Master data structure.

When the WorkstationST is operating as a Slave, the data structure to be configured

is primarily determined by WorkstationST. Any address and datatype can be
assigned. However, in this case, you must also communicate with the remote Master
to determine if any special addressing or data types are required. Once the data
structures are determined, the Modbus data points are connected to the ToolboxST
variables.

Once the Slave definitions are configured, the Modbus Master Messages must be
configured for each Slave under the Masters item. These messages determine the
values to be read from, or written to the Slave, and at what frequency.

4 • WorkstationST Modbus GEI-100696B

 Enabling the Modbus Feature
To enable the Modbus feature
1 From either the System Editor Tree View or the Summary View, double-
click the WorkstationST component. The WorkstationST Component
Editor displays.

From the
General tab
Tree View,
select
Features.

From the Summary View, enable OPC DA

Server and Modbus by setting each toTrue.

Note The EGD Configuration Server must be set to True in one of the configured
WorkstationST components (it does not have to be the WorkstationST with Modbus
enabled).

GEI-100696B WorkstationST Modbus• 5

 2 From the Tree View, select Network Adapter 0.

From the Property Editor, enter

the IP Address to match the
computer on which
WorkstationST runs.

6 • WorkstationST Modbus GEI-100696B

 Configuring the Modbus Feature
To configure a Modbus Master
1 From the Modbus tab, right-click the Masters item, then select Insert New
HW Interface.

From the Property

Editor, select Serial or
Ethernet as the Interface
Type. If you are using
Ethernet, enter the IP
Address of the Slave with
whom you will be
communicating.

GEI-100696B WorkstationST Modbus• 7

 2 From the Tree View, right-click the interface you inserted, and select Insert
New Modbus Slave.

From the Property Editor ,

change the Slave ID
to match your
configuration.

3 From the Tree View, select the Slave you inserted. From the Summary
View, Holding Registers and Input Registers tabs, add variables. From the
Holding Coils and Input Coils tabs, enter Boolean variables.

Note When you connect a Modbus variable, the data type defaults to the data type of
the variable that was connected. Some data types use more than one register. (Refer
to the section, Data Types.) If you want the data type to be different than the variable
that was connected, it is recommended that you change the data type before
connecting the next variable. This is because the program reserves as many registers
as needed for that data type and changing the data type later could leave you with
either holes or overlapping registers. For example, variables A and B are LREALs in
the system, but you want to send them over Modbus as REALs. Variable A would be
put at address 00001 and variable B at address 00005 unless you changed A’s data
type to a REAL before connecting B.

8 • WorkstationST Modbus GEI-100696B

 From the Tree View, right-click
the slave you just inserted, then
select Insert New Modbus
Master Message. From the
Property Editor, select a
Function Code .

The Property Editor Message Settings are as follows:

Function Code is the function code used for this message.

Number of Registers is the number of registers or coils read or written for this
message.

Period Multiplier is the number multiplied by the Master Message Base Period to
configure how frequently this message is sent.

Start Address is the start address for this message. Depending on which Function
Code was set, this refers to either a coil or register number.

Note The lowest start number is one.

GEI-100696B WorkstationST Modbus• 9

 To configure a Modbus Slave
1 From the Modbus tab, right-click Slaves, then select Insert New HW
Interface.

2 From the Property Editor, select the Interface Type (Ethernet or Serial)
configured for the master. If you are using Ethernet, select the IP Address of
your computer.

10 • WorkstationST Modbus GEI-100696B

 From the Tree View,
right-click the new
interface you
inserted, then select
Insert New Modbus
Slave.

From the Property

Editor, enter the
Slave ID to match
your configuration.

3 From the Tree View, select the Slave you inserted. From the Summary
View, add variables to the Holding Registers and Input Registers tabs.
Enter Boolean variables to the Holding Coils and Input Coils tabs.

GEI-100696B WorkstationST Modbus• 11

 Modbus Properties

From the Tree View,

select the Modbus
item to display
Modbus properties.

12 • WorkstationST Modbus GEI-100696B

 Modbus Properties
Lower/Upper Limits
Property or Allowed Values Description
Big Endian for Data True/False If set to True (default), all data in the body of the Modbus message is
On Modbus converted to Big Endian before being placed on Modbus. It is assumed
that all data in the body of a received message is Big Endian, and if
required by the local computer, is converted to Little Endian before being
used locally.
Big Endian For True/False If set to True (default), the fields in the Modbus Application Protocol
MBAP (Tcp Hdr) Header MBAP are converted to Big Endian before being placed on
Modbus. It is assumed that in received messages, these fields are in Big
Endian, and if required by the local computer, are converted to Little
Endian before being used locally.
Swap Words For True/False All Dint data types are 32 bits. If set to True, the words are swapped
Dints before being sent over Modbus, and before being used when read from
Modbus. For example,
32 bits Data 0x01234567
Transmits 0x45, 0x67, 0x01 and 0x23
Swap Words For True/False All LReal data types are 64 bits. If set to True, the words are swapped
LReals before being sent over Modbus, and before being used when read from
Modbus. For example,
64 bits Data 0x0123456789ABCDEF
Transmits 0x45, 0x67, 0x01, 0x23, 0xCD, 0xEF, 0x89 and 0xAB
Swap Words For True/False All Real data types are 32 bits. If set to True, the words are swapped
Reals before being sent over Modbus, and before being used when read from
Modbus. For example,
32 bits Data 0x01234567
Transmits 0x45, 0x67, 0x01 and 0x23
Swap Words For True/False All UDint data types are 32 bits. If set to True, the words are swapped
UDints before being sent over Modbus, and before being used when read from
Modbus. For example,
32 bits Data 0x01234567
Transmits 0x45, 0x67, 0x01 and 0x23
Clamp Values True/False If set to True, the value sent on Modbus is clamped between the RawMax
and RawMin values. The EngMax/EngMin and RawMax/RawMin are used
to determine how to scale a value when writing to/reading from Modbus.
However, if the actual Engineering value at runtime is greater than
EngMax, or less than EngMin, then a value could be sent on Modbus that
is greater than RawMax or less than RawMin. Some jobs may not find this
desirable, however, if the values are being sent to a computer that only
supports 12 bits of data, these limits must be enforced.
Command Limit 0 / 25 Limits the number of EGD Command Message Protocol (CMP)
commands that any given register map writes to a controller in a second.
This limit is designed to prevent the controller from being overloaded with
requests. However, if both the Holding Registers and the Holding Coils
each contain 10 signals that are on a controller EGD page, and if the
Master sends messages to all 20 signals every second, it's possible for 20
CMP messages to be sent to the controller.
Ethernet Port 502 / 503 The Ethernet port that Modbus uses to communicate, if there are any
Ethernet Modbus connections. (A Slave listens on this port, a Master tries
to connect to this port.) The Modbus Protocol Specification default is 502,
but if 502 is already used, it can be changed to 503.
Timeout NAK 4/6 When data cannot be obtained from the OPC Server before the
SlaveSendTimeout time, the Slave returns a Slave Exception Response
to the Master. Most jobs want a NAK Code 6 (Slave component Busy) in
this situation, but this value gives you the option to return a NAK Code 4
(Slave Component Failure).
Master Message 200 / 2147483647 This number defines how frequently a Modbus Master Message can be
Base Period sent. This number is muliplied by the Period Multiplier on the Modbus
Master Message to indicate, in ms, how often a given message is sent
over Modbus.

GEI-100696B WorkstationST Modbus• 13

 Lower/Upper Limits
Property or Allowed Values Description
Master Read 500 / 6000000 The Master Read Timeout is the time duration, in ms, to wait for a
Timeout response from a Slave before proceeding to the next message.
OPC Data Rate 100 / 15000 The OPC Data Rate is the rate, in ms, at which the Modbus feature gets
updated values from the OPC Server. It should be twice as fast as you
expect the Master to be asking for values over Modbus.
ReConnect Time 500 / 6000000 The ReConnect Time is the time duration, in ms, before the Master tries to
reconnect to the Slave after a connection error. (Also, if Serial Slave has a
problem communicating over a serial port, it closes the serial port and
waits for this time duration before attempting to re-open the port.)
ReStartCycle Time 500 / 6000000 The ReStartCycle Time is the time duration, in ms, before restarting the
Read/Write or Write/Read cycle after an error. If this time duration is too
short, and the error does not clear itself, too many processing cycles are
used.
Slave Read 500 / 6000000 The Slave Read Timeout is the length of time, in ms, to wait for a request
Timeout from a master before marking this transport as being ready to be
shutdown. (This only applies to Ethernet Slaves.)
Note The program is still waiting for any new connection from a Master,
regardless of the time set here.
Slave Send 100 / 6000000 The Slave Send Timeout is the length of time, in ms, to wait for data from
Timeout the OPC Server before sending back a Timeout NAK response to the
Master.

Hardware Interface Properties

When the hardware interface is selected in the Tree View, the following properties
display in the Property Editor.

Note You can only change Ethernet Settings when the Interface Type is Ethernet,
and Serial Settings when it is Serial.

Lower/Upper Limits or
Property Allowed Values Description
Interface Type Serial / Ethernet Modbus Hardware Interface type
IP Address Valid IP Address format IP Address for Ethernet connection. For a Master, this is the IP
Address of the Slave that you are connecting to. For the Slave,
this is the IP Address at which you'll be listening for a
connection from a Master. If you're a Slave, verify that the
WorkstationST has a Network Adapter defined to support this IP
Address.
Baud Rate 300 / 600 / 1200 / 2400 / Communication baud rate for serial network
4800 / 9600 / 14400 / 19200
Data Bits 8 Communication data bits for serial network
Parity None / Even / Odd Communication Parity for serial network
Port COM1 / COM16 Communication Port for serial network
Response 0 / 999 Additional delay time, in ms, before the Slave responds to a
Delay Master request. Change this from zero only if the Master
cannot handle a quick response.
Stop Bits 1/2 Communication Stop Bits for serial network

14 • WorkstationST Modbus GEI-100696B

 Master Message Properties
Lower/Upper
Limits or Allowed
Property Values Description
Function Code FuncCode1 / 1 - Read Holding Coils
FuncCode2 / 2 - Read Input Coils
FuncCode3 /
FuncCode4 / 3 - Read Holding Registers
FuncCode5 / 4 - Read Input Registers
FuncCode6 /
5 - Write Single Holding Coil
FuncCode8 /
FuncCode15 / 6 - Write Single Holding Register
FuncCode16 15 - Write Multiple Holding Coils
16 - Write Multiple Holding Registers
8 - Diagnostic, sub Function Code 0, Return Query Data
This sends a piece of canned data to the Slave, which the Slave then returns.
This code can be used to test the health of a Slave without needing to know
the register map.
Number of Registers 1/65535 The number of Registers (or Coils) to be read or written for this message.
Note: This is not the number of variables. A 4-byte data type uses two
registers, and an 8-byte data type uses four registers.
Period Multiplier 1/2147483647 This number is multiplied by the Master Message Base Period to determine
how frequently, in ms, this message should be sent. (The Master Message
Base Period is an item in the Property Editor when Modbus is selected in the
Tree View)
Start Address 1/65535 The starting address for this message. This refers to either a coil number or
register number, depending on which function code is configured. A variable
must be connected to this address, or you cannot validate the Modbus
configuration and select the Build command.

Slave Properties
Property Lower/Upper Limits or Allowed Values
Slave ID The Slave ID on the Modbus network

Data Structure Properties

Property Lower/Upper Limits or Allowed Values
Raw Min The value used to scale the data before it is sent or read
Raw Max from Modbus.

Eng Min Ymod − RawMin X Eng − Eng Min

=
Eng Max RawMax − RawMin Eng Max − Eng Min

where Ymod is the value to be sent over Modbus and XEng

is the value of the connected variable.

GEI-100696B WorkstationST Modbus• 15

 Data Types
When variables are sent over Modbus, the following data types are allowed. Note the
number of registers that each data type uses on a Modbus page.
Data Type Number of Registers Needed
BOOL 1 (Can pack up to 16 Booleans
into one register).
INT 1
UINT 1
DINT 2
UDINT 2
REAL 2
LREAL 4

Scaling
The relationship between Y (the value sent over Modbus) and X (the engineering
value) is linear. This equation:

can be converted into the slope intercept form:

y = mx + b

Where m = slope

b = offset

For the engineering value in the system:

For the value that is sent over Modbus:

RawMax − RawMin Eng Max RawMin − Eng Mix RawMax

YMod = X Eng +
Eng Max − Eng Min Eng Max − Eng Min

16 • WorkstationST Modbus GEI-100696B

 If all min/max values are zeros, a gain of 1 and an offset of zero are realized. If the
Clamp Values property is set to True, the value sent on Modbus is clamped between
the RawMax and RawMin. The EngMax/EngMin and RawMax/RawMin are used to
determine how to scale a value when writing to or reading from Modbus. However,
if the actual Engineering value at runtime is greater than EngMax, or less than
EngMin, a value could be sent that is greater than RawMax or less than RawMin.
Therefore, RawMax and RawMin are not true minimums and maximums unless the
Clamp Values Boolean is set. They are only used to determine the scale factors.
However, before values are sent over Modbus, they are always clamped, regardless
of the state of the Clamp Values Boolean, based on the range of the Modbus data
type. If values are sent to a computer that only supports 12 bits of data, verify that
your RawMax and RawMin values respect the 12 bits of data limitation, and set the
Clamp Values Boolean to True.

When a Master writes to a WorkstationST Slave, the Slave converts the Modbus
value to an Engineering value. If the calculated Engineering value is outside the
range of the Engineering variable’s data type, that value will not be set, and a Slave
Exception Response (Code 3, Illegal Data Value) is returned to the Master. Any
other valid values in the write request are written. In this case, the OPC Server Write
Errors counters increase.

When a Master reads from a WorkstationST Slave, the Slave converts the
Engineering value to a Modbus value. If the calculated Modbus value is outside the
range of the Modbus data type, the value is clamped by the Modbus data type range.
This clamping based on data type occurs even if the Clamp Values property is not
set. The clamped value is returned to the Master. In this case, no Slave Exception
response is returned.

Before a WorkstationST Master writes to a Slave, the Master converts the

Engineering value to the Modbus value. If the calculated Modbus value is outside the
range of the Modbus data type, the value is clamped by the Modbus data type range
and the clamped value is written to the Slave. This clamping based on data type
occurs even if the Clamp Values property is not set.

After a WorkstationST Master reads from a Slave, the Master converts the Modbus
value to an Engineering value. If the calculated Engineering value is outside the
range of the Engineering variable’s data type, that value will not be set, and the OPC
Server Write Errors counters increase. Any other valid values in the write request are
still written.

At runtime, you can determine the number of times a value was clamped by
monitoring the corresponding counters in the Additional status detail window (for
example, Slave 1 on 3.29.17.149 HR has 0 Current, 0 Total Modbus Values being
Clamped occurrences).

GEI-100696B WorkstationST Modbus• 17

 Scaling Examples
EngMax = 8.0 inches

EngMin = 0 inches

Engineering variable is an IEEE float (in ToolboxST, referred to as REAL).

First example:

RawMax = 65000

RawMin = 0

Engineering variable current value is 8.5.

Modbus data type is an unsigned 16-bit integer (in ToolboxST, referred to as UINT).

65000 − 0 8 * 0 − 0 * 65000
ModbusValue = * 8 .5 +
8−0 8−0
65000
ModbusValue = * 8.5 = 69062.5
8

Modbus Value = 65535 if Clamp Values = False. (just clamped based on Modbus
data type range, for unsigned 16-bit integer is 0 to 65535.)

Modbus Value = 65000 if Clamp Value = True. (the tighter of the two clamps
(RawMax/RawMin versus the data type range) is applied.)

Second example:

RawMax = 32000

RawMin = -32000

Engineering variable current value is 8.07

Modbus data type is a signed 16-bit integer (in ToolboxST, referred to as INT).

(32000 − (−32000)) 8 * (−32000) − 0 * 32000

ModbusValue = * 8.07 +
8−0 8−0
64 000
ModbusValue = * 8.07 − 32000 = 32560
8

Modbus Value = 32560 if Clamp Values = False. (No clamp was reached.)

Modbus Value = 32000 if Clamp Values = True. (RawMax clamp was reached.)

18 • WorkstationST Modbus GEI-100696B

 Spare Values
A Master cannot read data that contains an empty register. If registers 1 through 100
are being read by a single read, and there are no values connected to one or more
registers, an Illegal Data Address error occurs.
To repair empty registers
1 From the WorkstationST component, OPC DA Server tab, select Client
Driven Variables.

From the Summary View, add an

INT variable in the Name column.

Note The recommended variable name is Spare (for example, SpareInt1).

Note If more registers need to be filled, the data type can be INT (uses one register),
REAL (uses two registers) or LREAL (uses up to four registers).

2 Set the External Access to Read/Write.

3 Set the Initial Health to TRUE. If the Initial Health is not set to TRUE, the
Master gets Exception Response: Slave Component Failure 04 when a
Read/Write is attempted that includes this variable.
4 If required, set the Initial Value.
5 Select the Modbus tab to connect this new variable to the empty register.

6 Select the Build and Download commands .

7 Restart the OPC Server.

Note If Booleans are packed into a register, all 16 bits within a register do not have
to be mapped.

GEI-100696B WorkstationST Modbus• 19

 Importing or Exporting .csv Files
To export or import a .csv file from the Modbus item

From the Tree View,

right-click the Modbus
Slave and select Export
to .csv file or Import
from .csv file.

The Export to .csv file option includes:

• Register Type (Example: HoldingReg, Coil, InputReg, Input)

• Register Address (Example: 00001)
• Signal Name (Example: TVNA)
• Writeable or Read Only (Example: W)
• Modbus Data Type (Example: UDINT)
• Signal Data Type (Example: UDINT)
• Scale Modbus Minimum
• Scale Modbus Maximum
• Scale Engineering Minimum
• Scale Engineering Maximum
• Signal Units
• Signal Description

The Import from .cvs file option includes:

• a .csv file that you exported
• a .csv file exported from a Mark VI or Mark VIe Modbus Master configuration

20 • WorkstationST Modbus GEI-100696B

 Error/ Warning Messages
Any errors/warnings in the Modbus configuration display in the WorkstationST
Component InfoView when the Build command is selected. Double-click an
error/warning to display the location that the error/warning occurs.

Error Message Possible Cause Solution

Connection Bool8 is The variable that was entered If not, determine where the variable should have been
undefined (where the into the Connected variable field defined. If the variable comes from another component,
connected variable name is is not defined anywhere in the make sure that:
the name you entered) system. -Variable was defined in that component
-Variable was mapped to EGD page in that component
-The component was built on a computer that is working
online with the EGD ConfigServer so that their EGD page
maps are available.
-This component is set up to consume pages from that
component
-This computer is working online with EGD ConfigServer
to get the latest EGD Map
Point Address 00001 must For a specific Slave and register Determine which variable you want at that address. Is the
have a unique address space (such as Holding other variable no longer needed, or should it be at a
(where the address is the Registers), more than one different address?
value you entered) variable has been mapped to
the exact same address.
Raw Min must be set to zero A non-zero scale factor (Raw Put zero values in the Raw Min, Raw Max, Eng Min, and
for point HoldingReg8-00 Min, Raw Max, Eng Min or Eng Eng Max fields for this Boolean variable (on either the
when Data Type is a BOOL Max) was entered for a Boolean Holding Register or Input Register tab).
(where the point is your point.
Name, and the parameter
could also be Raw Max, Eng
Min or Eng Max)
Point Address 00001 is of A variable that takes up more Do you want the first variable to be that data type when it
type UDINT and requires 2 than one register was is sent over Modbus? When you connect a variable to a
addresses. The next address connected to an address, but Modbus page, the data type defaults to that of the
is located at 00002 (where not enough space was left in the variable that was connected, but this may not be desired.
the addresses and data types register map for it. Instead, If not, you need to move the second variable. Remember
are yours) another variable was mapped to that two registers are needed for DINT, UDINT, and
a following address that would REAL variables, but four registers are needed for LREAL
cause an overlap of data. variables.
A Slave under a Modbus The Slave ID is set to 0, and the Change the Slave ID to a non-zero value.
Serial HW Interface has to Interface Type is a serial link.
have a non-zero Slave ID,
since 0 is reserved for
broadcast messages in the
Modbus Protocol
Specification.

GEI-100696B WorkstationST Modbus• 21

Error Message Possible Cause Solution
HW I/F COM1, Slave 1, point The same variable is connected If this was a spare variable being used to fill a hole in a
Coil2 and HW I/F COM2, to two places in a Slave map register map, define as many spare variables as needed
Slave 5, point Coil8 are both under the Masters item. (It can so they can be unique. Otherwise, split your Modbus
connected to T1.L27BN. be, but does not have to be on message into two messages, one that reads the values
Both pts are being written to the same Slave, or even under before the hole, and a second that reads the values after
by values read over Modbus, the same Hardware Interface.) the hole. (Refer to the section Spare Values.)
so they will overwrite each That condition alone is okay if in
other. Either change one one location the variable is
connection or one of the being read to be sent over If this is not a spare variable, determine which variable
Modbus messages that writes Modbus, and in the other it’s you want in each instance.
to this variable (where the being read from Modbus and
Hardware Interfaces, Slave written to. However, your
numbers, point names and configuration actually has one
connected variables are or more Modbus messages
yours) configured to read from Modbus
and write to both places where
the same variable is connected.
Function Codes that read
values from Modbus and write
them to a variable are
FuncCode 1, 2, 3, or 4.
More than one Workstation More than one Hardware If you have more than one serial interface in the same
Modbus HW Interface is Interface is configured to use WorkstationST, the target computer must have enough
configured to use the COM1 the same Com Port. hardware to support it. Select the appropriate Com Port
Com Port (where Com Port is in the configuration or change one of the interfaces to
the duplicate in your Ethernet.
configuration)
Under Modbus, Masters, There are two or more hardware Determine if one of the hardware interfaces should really
there is more than one HW interfaces under the Masters be a different address, and if so, change it. Otherwise,
interface configured to use category that are using the all of the slaves need to be under one entry for this IP
the 172.20.100.151 IP same IP Address. Address. Use cut-and-paste to move the slave definitions
Address. under one hardware interface and delete the second
hardware interface.
Under Modbus, Slaves, there There are two or more Determine if one of the Hardware Interfaces should really
is more than one HW Hardware Interfaces under the be a different address, and if so, change it. Otherwise,
Interface configured to use Slaves category that are using all of the slaves need to be under one entry for this IP
the 172.20.100.151 IP the same IP Address. Address. Use cut-and-paste to move the slave
Address. definitions under one Hardware Interface and delete the
second Hardware Interface.
Under Modbus, Masters, No messages were defined for Determine if you want to read or write values to this slave
172.20.100.151: Slave 17 a given Masters slave map. map, and if so, insert a ModbusMasterMessage.
does not have any Messages
defined for it.
Duplicate Slave IDs are not The same Slave ID was used Change duplicate Slave IDs. To configure a redundant
allowed in any Modbus HW for more than one Slave, set of Slaves, the backup Slaves should be under their
Interface Configuration. underneath any given Hardware own Hardware Interface.
Interface.
An IP Address 3.29.17.201 is Under the Slaves item, an From the General tab, verify that all the Network
configured for a Modbus HW Ethernet Hardware Interface is Adapters in your WorkstationST computer have been
Interface that has Slaves on defined. However, the IP configured with the proper IP address. Double-click the
this Workstation, but the address for it does not match error message in the Log View at the bottom of the
Workstation configuration one of the IP addresses on the screen to go to the Hardware Interface with the bad IP
itself does not have a WorkstationST Network address. Select the IP Address field, then select the
Network Adapter defined to Adapters. This could have correct IP address.
support this IP Address happened if the WorkstationST
(where the IP Address is Network Adapters configuration
yours) was changed after the IP
address was selected for your
Slaves.

22 • WorkstationST Modbus GEI-100696B

 Error Message Possible Cause Solution
An IP Address ASDFG is Under the Masters item, there is Change the string to the correct IP format
configured for a Modbus HW an Ethernet Hardware Interface
Interface that has Masters, whose IP address doesn’t follow
but the format is not a valid IP the format nnn.nnn.nnn.nnn,
Address format (where where n is a numerical digit.
instead of ASDFG, it is the
string you entered)
ModbusMasterMessage0 is Double-click the error message in the Log View at the
configured to send Function bottom of the screen to go to the bad Modbus Master
Code 5, which by the Modbus Message. In the Summary View, select the Coil tab.
Protocol Specification only Determine if you want to write a single point, and if so,
sends 1 coil of data. change the number of registers to 1. If you want to write
However, the Number Of more than one point, change the Function Code to 15.
Registers configured is 12.
Consider changing this to a
Function code 15 (where the
Message Name and Number
Of Registers are your values)
ModbusMasterMessage0 is Double-click the error message in the Log View at the
configured to send Function bottom of the screen to go to the bad Modbus Master
Code 6, which by the Modbus Message. In the Summary View, select the Holding
Protocol Specification only Register tab. Determine if you want to write a single
sends 1 register of data. variable, and if so, change the number of registers to 1. If
However, the Number Of you want to write more than one variable, change the
Registers configured is 12. Function Code to 16.
Consider changing this to a
Function Code 16 (where the
Message Name and Number
Of Registers are your values)
ModbusMasterMessage0 is Under the Masters category, a Did you want to read from Modbus? If not, change your
configured to send a Slave is defined that contains a function code. If so, determine if you should be writing to
message to read a value from Read-Only variable. This a different variable, or if this variable should be changed
Modbus and write it to Bool1, Modbus Master message is to Read/Write.
but that variable is defined as either a Function Code 1, 2, 3,
Read-Only (where the or 4 which causes the Modbus
Message Name and Master to read a value from a
connected variable name are Slave, and try to store the value
yours) in this Read-Only Connected
Variable.

ModbusMasterMessage1 is There is a hole in the signal Select the Modbus Master Message. Based on what the
configured to send a map. An example of a hole is a function code is, select the appropriate tab in the right
message with Starting message that tries to write 4 hand pane. (1, 5, 15 select Coil; 3, 6, 16 select Holding
Address 2 that takes up 2 registers starting at register 10, Register; 2 select Input; 4 select Input Register.) Look at
coils or registers. However, where there is a DINT at the hole (address) indicated in the error message.
there is no Variable register 10, no variable at Either move variables around to fill the hole, add a spare
connected to address 2 register 12, and an INT at variable to fill in the hole (refer to the section, Spares
(where the Message Name, register 13. Values), or use two Modbus messages. The first Modbus
Start Address, Number Of Note that the hole can be at the message includes variables before the hole and the
Registers, and first address beginning or end of the second includes variables after the hole.
that does not line up are message span.
yours) If the hole is at the beginning, change the Start Address
in the Message. If the hole is at the end, change the
Number of Registers in the Modbus Master Message and
verify that it does not extend beyond the end of the last
connected variable.
Note If Booleans are packed into a register, all 16 bits
within a register do not have to be mapped.

GEI-100696B WorkstationST Modbus• 23

Error Message Possible Cause Solution
ModbusMasterMessage0 is The number of registers is not Select the Modbus Master Message. Add the current
configured to send a large enough to incorporate the number of registers and the exceeded number, then put
message with Starting whole length of the last variable the result into the Number of Registers. In the example
Address 1 that takes up 12 in the message. message, you would add 12 + 1 = 13 and change the
coils or registers. However, Number of Registers from 12 to 13.
the last variable LReal1 is
long enough that it causes the
message to exceed the
Number of Registers by 1
register (where the Message
Name, Start Address,
Number Of Registers,
Connected Variable Name,
and exceeded number are
yours)
ModbusMasterMessage1 is The number of registers needed Double-click the error message in the Component
configured to send Function for the connected variable is InfoView to go to the poor Modbus Master Message. In
Code 6, which by the Modbus greater than one. the Summary View, select the Holding Register tab.
Protocol Specification only Either change the data type for the point on the Modbus
sends 1 register of data (two page so that it only takes up a single register, or change
bytes). However, the variable the Function Code to 16 to pass the point over Modbus
LReal1 connected to the with its current data type. If you change the Function
Starting Address 10 takes up Code to 16, you must also change the Number of
4 registers. Consider Registers to reflect the number of actual registers used
changing it to a Function for that data type, as indicated in the error message.
Code 16 (where the Message
Name, Connected Variable
Name, Starting Address and
number of registers needed
are yours)
ModbusMasterMessage1 is The Modbus protocol for the Add another ModbusMasterMessage and read some of
configured to send a response of a Function Code 1 the values in the first message, and the remaining values
message with 2050 coils, but (Read Holding Coils) calls for 1 with the second message.
the Maximum Number of byte to specify the number of
Coils that can be sent in any bytes of data to follow. The
single Message is 2040. maximum value for a single byte
Please split this into multiple is 255. Each byte of data can
messages. contain values for 8 coils.
Therefore, the maximum
number of coils that can be read
is 255 x 8 =2040.
ModbusMasterMessage0 is The Modbus protocol for the Add another ModbusMasterMessage and read some of
configured to send a response of a Function Code 3 the values in the first message, and the remaining values
message with 200 registers, (Read Holding Registers) calls with the second message.
but the Maximum Number of for 1 byte to specify the number
Registers that can be sent in of bytes of data to follow. The
any single Message is 127. maximum value for a single byte
Please split this into multiple is 255. There are two bytes of
messages. data for each register.
Therefore, the maximum
number of registers that can be
read is 255 / 2 =127.5, (127
whole registers).

24 • WorkstationST Modbus GEI-100696B

 Runtime
There are three ways to monitor the status of a configured Modbus.

• WorkstationST Status of Features window

• WorkstationST additional status window
• Detail Log

WorkstationST Status of Features

You can access the WorkstationST Status of Features screen in one of two ways:
From the Start menu, select Programs, GE ControlST, WorkstationST,
and WorkstationST Status Monitor.
or
From ToolboxST, open a WorkstationST component and click the Go
On/Offline button . From the Component InfoView, select the Status
tab of the WorkstationST running the configured Modbus feature.

The Modbus row displays the current

status of the Modbus feature.

The logic that drives the Error/Warning/OK status for the Modbus feature is as
follows.

Errors

• Any current connection errors over Modbus

• No connection with the OPC Server
• Any current Checksum Errors
• Any current Read Message Errors
Endian mismatch
Unsupported function code or scrambled data in the receive buffer because
of serial port parameter mismatches

GEI-100696B WorkstationST Modbus• 25

 Warnings

• Any current application errors

• Any current slave exception responses
• Any current invalid Slave ID errors (for example, a Slave receives a message
from a Master for a Slave ID that is not configured.)
• A Master with current:
Read timeouts
OPC Server write errors
Skipped periodic messages
Unbound variables
Unhealthy variables
Exceeded command limit
Unsupported Function Code
Trying to write to a read-only variable
Data conversion error

Note If detail items listed under a Master occur in a Slave, the Slave sends back a
Slave Exception Response, which triggers a WARNING status.

WorkstationST Additional Status

To view additional status information
From the Component InfoView Features column, right-click the Modbus
item and select View Additional Status Detail. The WorkstationST
additional status window displays.

26 • WorkstationST Modbus GEI-100696B

 The Error Level column that displays on the WorkstationST additional status
window has three possible values:

• OK – Value is zero
• Warning – Condition occurred at some point
• Error – Condition is still occurring

When you download Modbus, the list is refreshed. However, since Warning
means that a condition occurred at some point, any Error Level marked
Warning remains that way until you click the Zero Counters button.

Once a steady state condition has been reached, it is recommended that you set the
counters to zero to more easily identify any subsequent conditions that occur.

The following information displays in the WorkstationST additional status window,

and can be monitored at runtime.

Masters and Slaves Status Counters

Message Description
Modbus initially started at The time that the program was last started. If WorkstationST Service is configured to start
10/9/2007 2:06:24 PM. automatically when the computer restarts, and the Modbus feature is enabled, that service
starts the program. Otherwise, the WorkstationST Status Monitor can be used to stop and
start the Modbus feature.
Modbus last restarted at When you download a new WorkstationST configuration from ToolboxST, the Modbus
10/9/2007 3:02:13 PM. (Due to a program restarts, and the time of this restart is recorded. If nothing that affects the
download.) Modbus configuration changes, then Modbus does NOT restart, and an INFO message
indicating this is added to the trace log file.
When Modbus restarts, it goes offline to allow enough time for the COM ports to be
released before the new configuration tries to use them. If another component that
Modbus consumes is downloaded, the lists used to communicate with the OPC Server
Modbus are stopped and restarted. This can cause unhealthy data, which generates a
Slave Exception Response if the Master asks for that data during this timeframe. Stopping
and starting the lists prevents unbound variables (caused by moved variables or changed
data types), which continually return Slave Exception Responses to the Master for this
data.

GEI-100696B WorkstationST Modbus• 27

Message Description
Modbus last zeroed counters at Once a steady state condition is reached, it’s recommended that you set the counters to
10/9/2007 3:02:13 PM. zero.
Each status line has an Error Level, Current Count, and Total Count for a specific
condition. If the condition is still occurring (non-zero current count), the Error Level
displays Error. As long as the total count is non-zero, the Error Level displays Warning,
indicating that at some point this condition existed. Setting the counters to zero returns the
Error Levels to OK.
This makes it easier to identify subsequent problems. A new problem causes the Error
Level to change to either Error (if the condition still is occurring) or Warning (if it occurred),
but has stopped, allowing you to detect transient problems.
The Zero Counters button also zeroes the counters for the number of messages sent and
received.
A Slave Manager is listening on This lets you know that the program is actively listening for any Masters that might
3.29.17.149, Port 502 with No connect over Ethernet. The message also shows which IP Address and Port the Master
Errors. There have been 3 should use when trying to connect, and how many successful connections have occurred
successful TCP connections since the counters were last set to zero. You can also see how many Ethernet Slave
since last Zeroed Counters. connections are currently running.
Currently there are 2 Active If a Master has disconnected and reconnected several times, it is possible that the total
Slave Connections. number of connections is more than the current number of Slaves. (This number could
also be zero if the counters were just set to zero.)
If a Slave connection does not receive any messages from the Master for a configured
amount of time (refer to the Slave Read Timeout property), it is shut down. The Slave
connection corresponds to the Hardware Interface configured in the ToolboxST
application. More than one Slave ID may be configured to respond over any connection.
More than one Master can talk to the same Slave map at the same time, each Master
using their own Slave connection. This might be done to support a primary/backup
redundant setup. For example, 1.1.1.1 HW Interface has Slaves 1, 2, and 3 defined under
it. One Master connects, which displays as one connection, although the Master can get
data from Slaves 1, 2, and 3. A second Master connects, which displays two connections.
This second Master can also get data from Slaves 1, 2, and 3.
Slave(s) on 3.29.17.149 are Since more than one Master can connect to a given Ethernet Slave Hardware Interface,
talking to a Master on TCP this tells you which Master connection all Status Counters following this line correspond
Remote End Point to. If another Master connects using the same listening port (but still having its own
3.29.17.191:1451 connection) after the Status Counters display for the first connection, this status line is
repeated for the second Master connection, followed by the Status Counters for the
second connection.
Slave(s) on 3.29.17.149 have This is the number of messages that have been received and sent since the Counters
1036 Received, 1036 Sent were last set to zero. For Slaves, if the received amount is significantly higher than the
Messages Total. sent amount, the Slave is having problems responding to messages received. Refer to the
additional error counters and the trace log file for information. If the Slave responds with a
Slave Exception Response, it still counts as a message sent.
Slave(s) on 3.29.17.149 have 0 If Masters send too many messages too quickly, this counter allows a field engineer to
Messages Received in the last report the rate to the person responsible for the Master so that the configuration can be
Second, 0.4 Average messages modified.
per second over the last five
seconds, 2 messages per
second Highwater Mark.
Slave(s) on 3.29.17.149 have 0 Possibly due to not being physically connected, having mismatched Hardware interface
Current, 0 Total Connection configuration parameters, or having the connection closed by another program. When a
Errors. Connection error is received, Modbus closes the connection, waits the ReConnect Time
(configuration parameter), and then attempts to reconnect. Refer to the trace log file for
more information.
Slave(s) on 3.29.17.149 have 0 If an Application Exception occurs, the connection waits the ReStartCycle Time
Current, 0 Total Application (configuration parameter) and then restarts the cycle. (The ReStartCycle Time prevents
Errors. (An Unexpected the restart from being caught in an infinite loop.) For a Master, restarting the cycle means
Exception that doesn't appear to identifying and sending out the next message. For a Slave, restarting the cycle means
be related to a connection listening for the next message from a Master.
problem.)

28 • WorkstationST Modbus GEI-100696B

 Message Description
Slave(s) on COM1 have 0 When a Checksum error occurs, the Slave ignores the rest of the message, and waits to
Current, 3 Total Checksum read the next message. No Slave Exception Response is returned. This only applies to
Errors. Serial Hardware Interfaces.
Slave(s) on 3.29.17.149 have 0 For Slaves: The Slave has waited a configured amount of time (Slave Read Timeout
Current, 0 Total Read Timeouts, property) without receiving a request from a Master before marking this transport as ready
where it waited 5 minutes for a to be shut down. This only applies to Ethernet Slaves; Serial Slaves do not have a
request. timeout.
The program waits for any NEW connection from a Master, regardless of the time set
here.
For Masters: The Master has waited a configured amount of time (Master Read Timeout
property) without receiving a response from a Slave before reading the next message.
Slave(s) on 3.29.17.149 have 0 No Slave Exception Response is returned, but the status on the WorkstationST Status
Current, 0 Total Invalid Slave ID Monitor changes to Warning if this occurs. The Master configuration or the Slave ID must
Errors, where a Master sent a be modified. If the Slave ID is changed, the Build and Download commands must then be
message for a Slave ID that isn’t selected. Use the trace log file to identify the invalid Slave ID.
configured.
Slave(s) on 3.29.17.149 have 0 Refer to the section, Exception Responses for more information about Slave Exception
Current, 0 Total Slave Exception Responses. The count does not differentiate between different error codes.
Responses that were sent.
Slave 1 on 3.29.17.149 has 0 This is the length of time the Slave waited (configured in the Slave Send Timeout). The
Current, 0 Total Send Timeouts, Slave normally sends back a Slave Component Busy 06 Slave exception response to the
where it didn't get data from the Master. However, if the NAK Code configuration parameter is set to 4, it returns a Slave
OPC Server in 0.5 seconds so it Component Failure 04 Slave exception response to the Master.
sent a Slave Exception
Response to the Master.
Slave 1 on 3.29.17.149 HR has 0 This Master/Slave tried to write values to the rest of the system through the OPC Server
Current, 0 Total OPC Server and failed. A Slave then sent a Slave Component Failure 04 Slave Exception Response to
Write Errors. the Master. If the program detects a specific cause of this error (variable is read-only,
unbound, unhealthy, and such), it does not increment this generic counter. Use the trace
log file to identify errors.
Slave 1 on 3.29.17.149 HR has 0 This is a severe error. If the Modbus program cannot connect to the OPC server, it keeps
Current, 0 Total OPC Server trying to reconnect. It will not send messages over Modbus until connected. A Slave
Connection Errors. returns a Slave Component Failure 04 Slave Exception Response to the Master if it
cannot connect to the OPC Server. Start the OPC Server, if it is not running. If the OPC
Server is running, use the OPC Server’s trace log file to identify errors.

Slave 1 on 3.29.17.149 HR has 0 The Slave returns an Illegal Data Address 02 Slave Exception Response to the Master. If
Current, 0 Total Illegal Data this error occurs, the person responsible for the Master needs to determine why a
Address Errors. This error occurs message is expected at this location. Are there data type mismatches that caused signals
when the Master asks for an to get shifted in the register map? Should a spare variable be added at this address?
address for which no variable (Refer to the section, Spare Values.) Use the trace log file to identify which variable has
was configured. the error.
Slave 1 on 3.29.17.149 HR has 0 The Slave returns an Illegal Data Address 02 Slave Exception Response to the Master.
Current, 0 Total UnBound Normally, this occurs briefly when another component is downloaded, causing Modbus to
Variable Errors. stop and restart the lists to the OPC Server. However, if another component deleted a
variable being used by Modbus and was then downloaded, this could continually occur. In
that case, talk with the component owner to see if another variable should be used. If not,
the person responsible for the Master must delete this variable. In either case, modify the
configuration, then select the Build and Download commands. Refer to the trace log file to
identify which variable has the error.
Slave 1 on 3.29.17.149 HR has 0 The Slave returns a Slave Component Failure 04 Slave Exception Response to the
Current, 0 Total UnHealthy Master. Normally this is only a momentary occurrence when another component gets
Variable Errors. downloaded, and Modbus stops and restarts the lists to the OPC Server. However, it also
occurs if a component goes offline. Refer to the trace log file to identify which variable has
the error.

GEI-100696B WorkstationST Modbus• 29

Message Description
Slave 1 on 3.29.17.149 HR has 0 The Slave returns a Slave Component Failure 04 Slave Exception Response to the
Current, 0 Total Read-Only Master. Refer to the trace log file to identify which variable has the error, then determine if
Variable Errors. (This error is the variable can be changed to Read/Write, or if another variable should be used. Modify
when it's asked to write to a your configuration, then select the Build and Download commands. This error will not
variable that was configured as resolve itself.
read-only.)
Slave 1 on 3.29.17.149 HR has 0 The Slave returns an Illegal Data Value 03 Slave Exception Response to the Master.
Current, 0 Total Data Conversion When a Master writes to a Slave, the Slave converts the Master value to an engineering
Errors. value. If the calculated engineering value is outside the range of the engineering value’s
data type, that value is not sent, and a Slave Exception Response is returned to the
Master. (Any other values in the write request will be written.) The OPC Server Write
Errors counter also increases. In this case, make changes to your data types, as required.
When a Master reads from a Slave, the Slave converts the engineering value to a Modbus
value. If the calculated Modbus value is outside the range of the Modbus data type, the
value is clamped by the Modbus data type range. This clamping based on data types
occurs even if the Clamp Value property is not set. The clamped value is returned to the
Master. No Slave Exception Response is returned. In this case, a Slave Exception
Response indicates that an unanticipated conversion problem exists. This could also
occur if scrambled data was received over Modbus. Use the trace log file to identify which
variable has the error.
Slave 1 on 3.29.17.149 HR has 0 The Command Limit (configuration property) limits the number of EGD CMP commands
Current, 0 Total Times that the that any given register map writes to a controller to prevent the controller from being
Commands Per Second Limit overloaded with requests. (For example, if the Holding Registers and the Holding Coils
was reached. each contain 10 signals that are on a controller EGD page, and the Master sends
messages writing to all 20 signals every second, it's possible for 20 CMP messages to still
get sent to the controller.) The register does not buffer these change requests and send
them at a slower rate, but instead discards any messages that exceed the limit. Even if
the command limit is reached, other non-command values are sent. The Slave returns a
Slave Component Failure 06 Slave Exception Response to the Master.
Slave 1 on 3.29.17.149 HR has 0 If the Clamp Values (configuration property) is set to True, the value sent over Modbus is
Current, 0 Total Modbus Values clamped between the RawMax and RawMin. The EngMax/EngMin and RawMax/RawMin
being clamped occurrences are used to determine how to scale a value when writing to/reading from Modbus.
However, if the actual Engineering value at runtime is greater than EngMax, or less than
EngMin, a value could be sent over Modbus that is greater than RawMax or less than
RawMin. Some jobs may not find this desirable, but if the values are going to a computer
that only supports 12 bits of data, these limits must be enforced. Also, if the Modbus value
is outside the range of the Modbus data type when an engineering value is converted to a
Modbus value, the Modbus value is clamped and the counter increases.
Slave 1 on 3.29.17.149 HR has 0 The Modbus program tries to reduce the number of unnecessary EGD commands and
Current, 0 Total Times that a minimize the chance of reaching the command limit. This is done by comparing the
Value has Not Changed so it was current value of a variable coming from another component, to the value received over
Not Sent to a Controller Modbus. If the value is the same, it is not sent, but this counter is increased by one. If the
value is different, and the command limit was not reached, a command is sent to that
component.

Masters Messages
Master talking to COM1 has 0 These could occur if either the Master tries to send, or receives a function code that it
Current, 0 Total UnSupported cannot handle.
Function Code Errors.
Master talking to COM1 has 0 This normally occurs when:
Current, 0 Total Skipped Periodic Modbus connection is down
Modbus Messages. Message
would be skipped when OPC Server connection is down
previously scheduled messages Too many messages were scheduled to be sent at the same time. In this case, modify
haven’t completed before new your configuration.
messages are scheduled to be
sent.

30 • WorkstationST Modbus GEI-100696B

 Tracing
Modbus writes messages to a trace log file to help identify problems.
To display the Modbus trace log file
From the WorkstationST Status of Features window, right-click the
Modbus feature and select Advanced, then View Detail Log to display the
contents of the trace log file.

There are four message levels in the trace log file: Error, Warning, Info, and
Verbose.

GEI-100696B WorkstationST Modbus• 31

 To change the message level
1 From Windows Explorer, select C:, then Program Files, GE Energy, and
WorkstationST Features.
2 From the WorkstationST Features folder, use Notepad to open the
GeCssModbus.exe.config file.
3 Change the ApplicationDefaultSwitch value. (The program automatically
detects that the file has changed and starts using the new level.) The values are:

1 - Errors

2 - Warnings and Errors

3 - Info, Warnings, and Errors

4 - Verbose, Info, Warnings, and Errors

Note The default value is 3. If you need to view the contents of each message
received, change the value to 4 to include Verbose. However, because this value
quickly fills the trace log file, return the value to 3 as soon as possible.

An example of an Error is when a Modbus Slave Exception Response is generated or

received. An example of a Warning would be a Master not receiving a response from
a Slave before the Master Read Timeout. An example of an Info message is
successfully opening a serial port.

Ethernet Messages
Change the message tracing level in the trace log file to Verbose to display raw data
about message content. Byte or word swapping has not yet occurred, if required.
However, when a message displays, for example, beginning to read 6 bytes to get the
message body, byte swapping has already taken place. This determines that 6 bytes
were expected for the message body.

2007-10-18 15:03:38.081 VERBOSE ModbusSlaveTcpTransport10/18/2007

5:34:32 AM.BeginRead: Slave at IP Address 172.20.100.151 beginning to read 6
bytes to get the message header.

2007-10-18 15:03:38.097 VERBOSE ModbusSlaveTcpTransport10/18/2007

5:34:32 AM.ReadCompleted: Slave at IP Address 172.20.100.151 received Message
Header 0, 0, 0, 0, 0, 6

The Message Header contents are as follows:

• The first two bytes are Transaction Identifier: 0, 0,

(The Slave returns what the Master sent it. The Master can use this to match the
response with the correct request.)

• The second two bytes are the Protocol Identifier: 0, 0,

(These should always be value 0 for Modbus protocol.)

• The last two bytes are the message length: 0, 6

(There are 6 bytes for the message body that follow this header.)

2007-10-18 15:03:38.097 VERBOSE ModbusSlaveTcpTransport10/18/2007 5:34:32

AM.ReadCompleted: Slave at IP Address 172.20.100.151 beginning to read 6 bytes
to get the message body.

32 • WorkstationST Modbus GEI-100696B

 2007-10-18 15:03:38.097 VERBOSE ModbusSlaveTcpTransport10/18/2007 5:34:32
AM.ReadCompleted: Slave at IP Address 172.20.100.151 Received Message Body
1, 3, 0, 9, 0, 5

The Message Body contents are as follows:

• The first byte is the Slave ID: 1,

• The second byte is the Function Code: 3,
(Everything after this point in the message could be different for different Function
Codes. Refer to the Modbus Protocol Quick Reference table for details.)

• The next two bytes are the starting Address: 0, 9

(The address sent over Modbus is zero-based, even though the registers are one-
based. This message starts reading at Holding Register 10.)

• The last two bytes are the number of registers to read: 0, 5

Serial Messages
Serial messages start by reading a 7-byte header. The program then displays, for
example, beginning to read 8 bytes to get the message body. This means that the
entire message is 8 bytes. When reading serial trace messages, focus on the one that
reads, Received Message Body followed by data, not Received Message Header
followed by data.

2007-10-18 14:57:31.977 VERBOSE ModbusSlaveSerialTransport10/16/2007

11:53:19 AM. ReadCompleted: Slave on Serial Port COM1 beginning to read 8
bytes to get the message body.

2007-10-18 14:57:32.007 VERBOSE ModbusSlaveSerialTransport10/16/2007

11:53:19 AM.ReadCompleted: Slave on Serial Port COM1 Received Message Body
255, 4, 0, 0, 0, 13, 36, 17

The Message Body contents are as follows:

• The first byte is the Slave ID: 255

• The second byte is the Function Code 4
The following is specific to Function Code 4

• The next two bytes are the starting Address: 0, 0,

Since the address sent over Modbus is zero-based, and the registers are one-based,
this message starts reading at Input Register 1.

• The next two bytes are the number of registers to read: 0, 13

• The last two bytes are the CRC checksum: 36, 17

GEI-100696B WorkstationST Modbus• 33

 Modbus Protocol Quick Reference
Function Codes 1, 2, 3 and 4 Function Codes 5 and 6 Function Codes 15 and 16
Request: Request and Response Request :
Byte1; Slave Id follow same format: Byte1; Slave Id
Byte 2; Function Code Byte1; Slave Id Byte 2; function code
Bytes 3 and 4; Starting Byte 2; Function Code Bytes 3 and 4; starting address
Address Bytes 3 and 4; Address Bytes 5 and 6; number of registers
Bytes 5 and 6; Number of Bytes 5 and 6; Value to to be written
Registers be written Byte 7; number of bytes of data
Response: Bytes 8+; Data
Byte1; Slave ID Response:
Byte 2; Function Code Byte1; Slave ID
Byte 3; Number of bytes of Byte 2; Function Code
Data
Byte 3 and 4; Starting Address
Bytes 4 +; Data
Bytes 5 and 6; number of registers
to be written

Slave Exception Response Format

Byte 1 Slave ID
Byte 2 Originating Function Code with the most significant bit set to 1
Byte 3 Exception Code

Monitoring Live Values

To monitor live values

From the Modbus tab, select a Slave and click

the Go On/Offline button to monitor live values
on a Modbus page.

Note The value displayed is the value before it is scaled and/or clamped to send over
Modbus.

34 • WorkstationST Modbus GEI-100696B

 Troubleshooting
Symptom Problem Solution
The Master tries to talk to a Slave Could be an issue with Big Endian for Change the state (False to True or True to
over Ethernet and gets no MBAP (tcp Hdr) being in the wrong False) of the Big Endian for MBAP (tcp Hdr)
answer even though the slave is state. To check, change Tracing to property, then select the Build and Download
running and healthy include Verbose, (Refer to the section, commands.
Tracing) and check messages to see
how long the program thinks the
message body length should be. If the
value is extremely high, it is probably
an Endian issue.
Data values for 2 and 4 word May need to swap words for 4 or 8- Try changing the state (False to True or True to
variables are not as expected byte data types. False) of the Swap Words for xxx property
(where xxx is the data type of the variable with
the problem), then select the Build and
Download commands.

Slave Exception Responses

Slave Exception When it occurs Solution
Codes
01 Illegal Function Master asks Slave to perform a 1. If the Master is asking for a non-supported function code, the
function it does not support Master configuration must be modified.
2. Verify that all communication properties match between the
Master and Slave, since a scrambled message could be
interpreted as an unsupported function code.
02 Illegal Data Master asks to Read/Write to a Use the trace log file to identify which variable has the error and
Address register where there is not a what the error is.
variable connected, or the If there was no variable at that address, the user who configured
variable is unbound the Master must determine why a response is expected at this
location. Are there data type mismatches that caused signals to get
shifted in the register map? Should a spare variable be added at
this address? (Refer to the section, Spare Values.)
If the variable was unbound, was it just a momentary occurrence
when another component was being downloaded, so Modbus
stopped/started the lists to the OPC Server? In this case, no action
is needed. However, if Modbus was using a variable from another
component, and the other component deleted the variable and then
was downloaded, this error continually occurs. In that case, ask the
component owner if another variable should be used. If not, the
person responsible for the Master must delete this variable. (In
either case, modify your configuration, then select the Build and
Download commands.)

GEI-100696B WorkstationST Modbus• 35

Slave Exception When it occurs Solution
Codes
03 Illegal Data Value An exception occurs either When a Master writes to a Slave, the Slave converts the Master
when trying to convert an value to an engineering value. If the calculated engineering value is
Engineering value to a Modbus outside the range of the engineering value’s data type, that value is
value, or vice versa. This not sent, and a Slave Exception Response is returned to the
conversion incorporates Master. (Any other values in the write request will be written.) You
applying scale factors, clamps, will also see the OPC Server Write Errors counter increase. In this
and converting the data type, if case, make changes to your data types, as required. When a
required. Master reads from a Slave, the Slave converts the engineering
value to a Modbus value. If the calculated Modbus value is outside
the range of the Modbus data type, the value is clamped by the
Modbus data type range. This clamping based on data types
occurs even if the Clamp Value property is not set. The clamped
value is returned to the Master. No Slave Exception Response is
returned. In this case, a Slave Exception Response indicates that
an unanticipated conversion problem exists. This could also occur
if scrambled data was received over Modbus. Use the trace log file
to identify which variable has the error.
04 Slave Any one of the following Look at the trace log file to identify the cause of the Slave
Component Failure conditions could cause this Exception. Based on the cause, take the appropriate action:
exception:
1. Master tries to read or write 1. This occurs briefly when another component is downloaded, and
a signal that is unhealthy Modbus stopped and restarted the lists to the OPC Server.
However, it would also occur if a component went offline.
2. Master tries to write to a 2. Determine if the variable can be changed to Read/Write, or if
signal that is read-only perhaps another variable should be used. Modify the configuration,
then select the Build and Download commands. This error will not
resolve itself.
3. Modbus is not connected to 3. This is a severe error. If Modbus can not connect to the OPC
the OPC Server server, it keeps trying to reconnect. Verify that the OPC Server is
running. If it is, use the OPC Server’s trace log file to identify
errors.
4. There is an error writing to 4. Modbus tries to catch most causes of OPC Server Write errors
the OPC Server (variable is Read-Only, unbound, or unhealthy, and such), in which
case it will not increase the OPC Server Write Error counter. Use
both the Modbus and OPC Server trace log files to identify errors.
5. There was an Application Use the Modbus trace log to identify errors.
Exception
06 Slave 1. Master asks for data from 1. Verify that the OPC Server is running.
Component Busy the Slave, but the Slave does
not get the data from the OPC
Server before the Slave Send
Timeout times out. (Configure
with NAK Code to send 4 or 6
in this case.)
2. The command limit has 2. Determine if this is due to the command limit being reached by
been reached. viewing the WorkstationST additional status window. If it is,
determine
If the command limit can be raised without jeopardizing the
controllers, and
If the rate at which the Master sends commands can be reduced

36 • WorkstationST Modbus GEI-100696B

 Terms
Coil - A type of data structure (a 1-bit Modbus value)

Data Structure - a method of storing data in a computer so that it can be used

efficiently

Feature - An element of the WorkstationST runtime system, which can be

optionally enabled through ToolboxST. Examples include OPC DA Server,
Recorder, OPC AE Server and Alarm System.

OPC OLE for Process Control (OLE is Microsoft's Object Linking and Embedding)
is a standard for data exchange in an industrial environment. The OPC foundation
provides specifications for various OPC standards such as OPC DA (Data Access)
and OPC AE (Alarm and Event).

OPC DA Server - a core part of the WorkstationST product that conforms to the
OPC Data Access Specification

Register - A type of data structure (a 16-bit Modbus value)

Scaling - The act of converting one value to another value, or the values (for
example, slope and offset) used to perform this conversion. In this program, an
engineering value is converted before it is sent over Modbus, and a value read from
Modbus is converted to an engineering value before being used in the system.

Slave Exception Responses - Modbus protocol used by a slave to communicate

to a Master when there is a problem.

Tracing - Messages written by a program to a trace log file to help identify

problems. In the WorkstationST Status Monitor, use the View Detail Log to display
the contents of the trace log file.

GEI-100696B WorkstationST Modbus• 37

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