Chapter 15 Modbus RTU

Chapter 15 Modbus RTU

15.1 Modbus Protocol Outline
MODBUS protocol that is the specified open protocol used for server-client is operated by data reading/writing based
on function code. The communication that adopts MODBUS protocol applies server-client function dealt with only
one client.

Characteristics RTU Mode

Signal system 8 bit binary code
The Starting bit 1
number of Data bit 8
data per Parity bit Even, Odd, None
1 character Stop bit 1 or 2
Error check CRC (Cyclical Redundancy Check)
Start of frame 3.5 Characters no response time

15.1.1 Frame Structure

The frame structure of MODBUS RTU mode is as below.
Classification Start Station No. Function code Data Error check End
Size(byte) Idle time 1 1 N 2 Idle time

(1) Characteristics of RTU mode

● It adopts hexadecimal number for communication.
● Starting character indicates unit No. and the end of frame indicates CRC error check for identifying the
frame.
● It identifies start and end of the frame by adding idle time of 1 bit.
● It has at least 3.5 character time of interval between frames and when passing 1.5 character times, it is
regarded as independent frame.

(2) Address Area

● It is composed of 1 byte.
● When using XGT Cnet I/F module, 0~31 can be applied for the unit No.
● 0 is used for client local number.
● Server includes its address to the response frame to show a client’s response.

(3) Data Area

● It transmits data through hexadecimal number(Hex.) and data structure changes depending on each
function code.
● It replies with response data when receiving normal frame.

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● It replies with error code when receiving abnormal frame.

(4) Error Check Area

● Through CRC checking method of 2 byte, it judges whether the frame is normal or not

(5) MODBUS Address Regulations

● The address of data starts from 0 and it is equal to the value obtained by subtracting 1 from MODBUS
memory. Namely, MODBUS address 2 is the same as address 1 of data.

15.1.2 Displaying Data and Address

In terms of displaying data and address of MODBUS protocol, the main characteristics are as below.

● It basically adopts hexadecimal number(Hex.).

● The meaning of each function code is as below.

Code(Hex) Used for Using area Address Max. Response data

Reading individual
01 Bit output 0XXXX 2000Bit
/continuous bit
Reading individual
02 Bit input 1XXXX 2000 Bit
/continuous bit
Reading individual
03 Word output 4XXXX 125 Word
/continuous word
Reading individual
04 Word input 3XXXX 125 Word
/continuous word
05 Writing individual bit Bit output 0XXXX 1 Bit
06 Writing individual word Word output 4XXXX 1 Word
0F Writing continuous bit Bit output 0XXXX 1968 Bit
10 Writing continuous word Word output 4XXXX 120 Word

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15.2 Wiring Diagram

The wiring method of InfoU and PLC connected through MODBUS communication may be different depending on
the manufacturer. For proper wiring, refer to each PLC’s manual.
The description on wiring of LS Industrial Systems’ XGT PLC will be provided hereupon.

15.2.1 RS-232C
When connecting MODBUS communication through RS-232C mode, the wiring method is the same as general
wiring method for RS-232C.

15.2.2 RS-422
When connecting MODBUS communication through RS-422 mode, the wiring method is the same as general
wiring method for RS-422.

15.2.3 RS-485
When connecting MODBUS communication through RS-485 mode, the wiring method is the same as general
wiring method for RS-485.

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15.3 I/O Driver Setting

InfoU can be connected through various kinds of devices that adopt MODBUS(RTU) communication with RS-232C,
RS-422/485 mode. Setting methods are different depending on the manufacturer so for more details, refer to each
device manual. For example, description on XGK PLC will provided hereupon.

15.3.1 PLC Setting

The communication for PLC(XGK) is set at XG-PD.

(1) Connection Setting

● Select [Online]  [Connection Setting].
● After setting connection options for user environment, click Connection.

(2) Reading I/O Data

● Select [Online]  [Reading I/O Data] to interpret the information of module embedded in the current base.

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(3) Basic setting

● Double-click the related Cnet I/F module to execute basic setting screen and establish communication
mode, communication speed, modem type, data bit, stop bit, unit No. of connection setting menu.

(4) Operation mode

● Select RTU server for operation mode.
● If you select MODBUS RTU server for operation mode, executing MODBUS is active.

(5) Setting MODBUS

● Starting address of bit reading area : Indicates the starting address of bit reading area with 5 digits. The
former 4 digits indicate word value and the remaining 1 digit means bit value.
Ex.) In the case of M00000 : The 0th bit of 0th word of M device is set as starting address of bit reading
area.
● Starting address of bit writing area : Indicates the starting address of bit writing area with 5 digits.
The former 4 digits indicate word value and the remaining 1 digit means bit value.

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In the case of M00100: The 0th bit of 10th word of M device area is set as starting address of bit reading
area.
● Starting address of word reading area: Indicates the starting address of word reading area with 4 digits.
Ex.) In the case of M00200: The 200th word of M device area is set as starting address of word reading
area.
● Starting address of word writing area: Indicates the starting address of word writing area with 4 digits.
Ex.) In the case of M00300: The 300th word of M device area is set as starting address of word writing
area.

(6) Writing Parameter

● Click [Online]  [Writing parameter].
● After clicking the module whose basic setting is completed at basic setting, click OK button.
● After clicking OK button, complete Parameter Writing and reset the module individually.

(7) Checking operation

● Click [Online]  [System Diagnosis].
● After clicking the related module, press the right mouse button for frame monitoring and status by services
to check whether communication is normal or not.

15.3.2 InfoU Setting: ModbusRTU

(1) Add Channel

Select “ModbusRTU” from the I/O driver list and press “Add”.

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● Channel Name: Input a channel name.

● Description: Input some information on the channel.
● COM Port #1: Select a communication port.
● Baud Rate: Select a communication speed.
● Parity Bit: Select a parity bit.
● Data Bit: Select a data bit.
● Stop Bit: Select a stop bit.
● Time Out : It refers to a certain time period during which any response to the request for data is not made
and after passing such a time period, the system will declare timeout to move on to the next process. The
time period to be set will be a base to judge communication errors.
● Retry: Set up the number of times to retry when communication fails.
● Use RTS: Check √ in the box only when using Channel 485 and 422.
● Delay Time (Before): Information used only in Channel 485 and 422. The delay time right before requesting
Data to PLC.
● Delay Time (After): Information used only in Channel 485 and 422. The delay time right after requesting
Data to PLC
● Start Offset Address: Select either 0 or 1. It sets up whether to start from either 0 or 1.
● Save: If ‘Save’ button is pressed, Channel information will be saved and the saved information will add to
the left “Configuration Information” tree.

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(2) Add Station

● Select [New Station] from “Configuration Information” tree.

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● Station Name: Input a station name.

● Description: Input some information on the station.
● Station Number: Input the prefix number of PLC Cnet Module.
● 32bit Data Swap: 32bit can be expressed in the combination of [Upper level word + Lower level word] or
[Lower level word + Upper level word] and for this, it is available to use swap of upper level word and lower
level word.
● Save: If ‘Save’ button is pressed, Station information will be saved and the saved information will add to the
left “Configuration Information” tree.

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(3) Add Block

● Select [New Block] from “Configuration Information” tree.

● Block Number: This number is a unique code of the block. The user needs to designate a different code to
each block.
● Description: Input some information on the block.
● Start Address: Input the Block’s Start Address. There are four kinds and each address is designated as
the following ways respectively:
- Address delimiter 0(Coil Status), 1(Discrete Input Status), 3(Input Register), 4(Holding Register)
- Its address is 0~65536 and if “Start Offset Address” is set up to start from 1 in the configuration
information, it will be 1~65536
● Period: Input an interval to collect data of the relevant block (unit: msec).
● Count: A number to read for each corresponding delimiter.
● Save: If ‘Save’ button is pressed, Block information will be saved and the saved information will add to the
left “Configuration Information” tree.
● Delete: If “Delete” button is pressed, the currently selected Block will be deleted.

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(4) I/O Address

1) Analog
Delimiter (3 or 4) +0~65535
(if “Start Offset Address” is set up to start from 1 in the configuration information , it will be 1~65536)

Example 1) If the user wants to read Oth(0 of 0~65535) of No. 3 (Input Register: Read only),
Set up 300000 or 30 if 0 is selected for “Start Offset Address”
Set up 300001 or 31 if 1 is selected for “Start Offset Address”

Example 2) If the user wants to read 17th(17 of 0~65535) of No. 4 (Holding Register),
Set up 400017 or 417 if 0 is selected for “Start Offset Address”
Set up 400018 or 418 if 1 is selected for “Start Offset Address”

2) Digital
Delimiter (0 or 1, or 3 or 4) +0~65535
(if “Start Offset Address” is set up to start from 1 in the configuration information , it will be 1~65536)

Example 1) If the user wants to read Oth(0 of 0~65535) of No. 0 (Coil Status : Read/Write),
Set up 000000 or 00 if 0 is selected for “Start Offset Address”
Set up 000001 or 01 if 1 is selected for “Start Offset Address”

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Example 2) If the user wants to read 17th(17 of 0~65535) of No. 1 (Discrete Input Status: Read only),
Set up 100017 or 117 if 0 is selected for “Start Offset Address”
Set up 100018 or 118 if 1 is selected for “Start Offset Address”

Example 3) If the user wants to read 10th bit of 17th(17 of 0~65535) of No. 3 (Input Register: Read only),
Set up 300017.A or 317.A if 0 is selected for “Start Offset Address”
Set up 300018.A or 318.A if 1 is selected for “Start Offset Address”

Example 4) If the user wants to read 15th bit of 0th(0 of 0~65535) of No. 4 (Holding Register),
Set up 300000.F or 30.F if 0 is selected for “Start Offset Address”
Set up 300001.F or 31.F if 1 is selected for “Start Offset Address”

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15.4 Available Device

The available device for InfoU is as below.
Area allowed for connection
Device Reading Writing Remarks
Available range
(function code) (function code)
Output contacts 0 - 65535 Available (01) Available (05) -

Input contacts 0 - 65535 Available (02) Available (05) -

Output register 0 - 65535 Available (03) Available (06,16) -

Input register 0 - 65535 Available (04) Available (06,16) -

Notice
☞ For the details on device, refer to MODBUS Protocol manual of Schneider.
☞ Use it within device area.
☞ The maximum device value may be different depending on PLC so refer to the PLC manual to be connected.

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