JP3298290B2 - Multiplex communication device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multiplex communication apparatus for transmitting and receiving various information via a multiplex transmission line provided between a plurality of control units.

[0002]

2. Description of the Related Art A multiplex communication apparatus is known which performs communication via a multiplex transmission line between a master station control unit and a plurality of slave station control units arranged in various parts of a vehicle (see, for example, Japanese Patent Application Laid-Open No. Hei 5 (1993)). -236563). In this type of multiplex communication apparatus, a microcomputer is provided in each of the master station control unit and the plurality of slave station control units, and operation information of operation members connected to each slave station control unit is mastered via a multiplex transmission line. The master station control unit determines the drive of the corresponding terminal device based on the operation information, and transmits the drive information to the slave station control unit to which the terminal device is connected via a multiplex transmission line. are doing.

Communication between the master station control unit and the slave station control unit is performed in a predetermined order. When there is no response from the slave station control unit of the communication destination, a communication error with the slave station is recorded and recorded. The communication error is utilized as information on a failure diagnosis device connected to the outside.

[0004]

Incidentally, the microcomputer has a so-called power-on reset function. When the power supply voltage supplied to the power supply terminal Vcc falls below the reset level Vrs, the microcomputer is automatically reset, and the power supply is reset again. The operation starts when the voltage exceeds the reset level Vrs. The reset level Vrs differs from microcomputer to microcomputer due to variations in elements, and therefore, even when power is supplied from the same power supply to a plurality of control units, the reset timing may differ by several milliseconds from microcomputer to microcomputer.

[0005] For this reason, when the battery voltage is lowered at the time of cranking of the vehicle or at the time of failure of the power supply, there are microcomputers which are reset due to the variation of the reset level Vrs and microcomputers which are not reset. In such a case, even if the transmission from the control unit having the microcomputer that was not reset to the control unit having the microcomputer that was reset was performed, there was no response from the reset control unit. The control unit will record a communication error with the reset control unit. Usually, the cranking of the vehicle is performed many times and is completed in a short time. In such a case, it is desired to avoid recording the communication error even if the communication actually becomes abnormal.

However, in a conventional multiplex communication device,
It is not possible to distinguish between a communication error caused by a drop in power supply voltage due to cranking, etc., and an original communication error caused by a failure in a communication circuit, microcomputer, control circuit, etc. There is a problem that a communication error is recorded.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a multiplex communication apparatus which does not recognize a communication error that occurs when a power supply voltage drops due to cranking or the like as a communication error.

[0008]

The present invention will be described with reference to FIG. 1 showing an embodiment. The present invention automatically resets when the voltage of the common power supply BAT drops to a specific value. Arithmetic control circuits CPU1, CPU2, CPU
A plurality of control units CTR each having:
, And CTR2, CTR3,... Are applied to a multiplex communication apparatus that communicates with each other via a common multiplex transmission line LL. A voltage drop detection circuit (R1, C1, D1, CPU1) for detecting that the voltage Vb of the common power supply BAT has dropped below a predetermined value higher than the eigenvalue of each of the arithmetic and control circuits CPU1, CPU2, CPU3,. When a communication error occurs during communication between the control units, the communication error is temporarily recorded as a temporary error, and thereafter, when the communication error does not clear for a predetermined time, the temporary error is switched to a communication error, and the communication error detection circuits CPU1, CPU2, CPU3,.・
And communication abnormality detection circuits CPU1, CPU2, CPU3,
When the provisional error is recorded and the voltage drop detection circuit detects that the voltage Vb of the common power supply BAT has dropped below a predetermined value, the provisional error release circuits CPU1, CPU2, and CPU3 release the provisional error. , ...
To each of the control units CTR1, CTR2, CTR3,.
In this way, the above object is achieved. The voltage drop detection circuit of the multiplex communication apparatus according to the second aspect of the present invention is configured such that when the voltage Vb of the common power supply BAT increases, the output voltage Vc increases with a delay, and when the voltage Vb of the common power supply BAT decreases, the output voltage Vc decreases. (R1, C1,...)
D1), and when the output voltage Vc of the delay circuit is lower than a preset value, it is determined that the voltage Vb of the common power supply BAT is lower than a predetermined value. further,
The invention according to claim 3 is a microcomputer which is connected to a common power supply BAT.
Control units CTR1, CT each having
Are applied to a multiplex communication device which includes R2, CTR3,... Which communicate with each other via a common multiplex transmission line LL. When the voltage Vb of the common power supply BAT increases, the output voltage Vc increases with a delay, and when the voltage Vb of the common power supply BAT decreases, the output voltage Vc decreases almost at the same time (R1, C1, D1). , And control units CRT1, CTR2, CTR3,... By microcomputers CPU1, CPU2, CPU3,.
When a communication error occurs in the communication between the devices, the error is temporarily recorded as a provisional error, and when the output voltage Vc of the delay circuit (R1, C1, D1) falls below a preset value, the provisional error is released. To achieve. The multiplex communication apparatus according to claim 4, wherein the microcomputers CPU1, CP
U2, CPU3,..., When a predetermined time has elapsed since the recording of the provisional error, if the communication error is not resolved and the provisional error is still recorded, the provisional error is switched to the communication error. .

[0009]

In the multiplex communication apparatus according to the first aspect, if a communication error occurs in the communication between the control units, it is temporarily recorded as a temporary error, and thereafter, if the communication error does not disappear for a predetermined time, the temporary error is switched to the communication error. When the provisional error is recorded, if it is detected that the voltage Vb of the common power supply has dropped below a predetermined value, the provisional error is released. The arithmetic control circuit automatically resets when the power supply voltage Vb drops to the reset level. This reset level is a unique value that differs for each arithmetic control circuit, and there are arithmetic control circuits that are reset due to variations in the reset level when the power supply voltage Vb decreases, and arithmetic control circuits that are not reset. In such a case, communication abnormality occurs between the control unit having the arithmetic control circuit that has not been reset and the control unit having the reset arithmetic control circuit. However, since it is possible to recognize that this communication abnormality has occurred due to the decrease in the power supply voltage Vb, it is possible to prevent such a communication abnormality from being recognized as a communication error. In the multiplex communication apparatus according to the second aspect, when the output voltage Vc of the delay circuit is equal to or less than the set value, the voltage V
It is determined that b is equal to or less than a predetermined value. Common power supply voltage Vb
Is lower than the eigenvalue of the arithmetic control circuit, the arithmetic control circuit is reset. When the voltage Vb of the common power supply rises again and returns to a value higher than the predetermined value, the arithmetic control circuit starts operating. At this time, the output voltage Vc of the delay circuit
Rises with a delay, so that even if the arithmetic and control circuit resets and starts operating again, the output voltage V
If the value of c is equal to or less than the set value, it can be recognized that the voltage Vb of the common power supply has decreased to a predetermined value or less. Further, in the multiplex communication apparatus according to the third aspect, the voltage Vb of the common power supply BAT is input via the delay circuit, and when a communication abnormality occurs in the communication between the control units, the communication is temporarily recorded as a temporary error, and the output of the delay circuit is output. When the voltage Vc falls below a preset value, the provisional error is released. The microcomputer automatically resets when the power supply voltage Vb drops to the reset level. This reset level is a unique value that differs for each microcomputer, and there are microcomputers that are reset due to variations in the reset level when the power supply voltage Vb decreases, and microcomputers that are not reset. In such a case, communication abnormality occurs between the control unit having the microcomputer that has not been reset and the control unit having the microcomputer that has been reset. However, since it is possible to recognize that this communication abnormality has occurred due to the decrease in the power supply voltage Vb, it is possible to prevent such a communication abnormality from being recognized as a communication error. In the multiplex communication apparatus according to the fourth aspect, when a predetermined time has elapsed after the provisional error is recorded, the provisional error is switched to the communication error if the communication abnormality is not resolved and the provisional error is still recorded. This clarifies the distinction between communication errors caused by failures in communication circuits, microcomputers, control circuits, etc., and communication errors caused by a drop in power supply voltage. Error handling can be performed.

In the means and means for solving the above-mentioned problems which explain the constitution of the present invention, the drawings of the embodiments are used to facilitate understanding of the present invention. However, the present invention is not limited to this.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the multiplex communication apparatus of the present invention is applied to a vehicle to control various on-vehicle devices will be described. FIG. 1 is a functional block diagram showing the configuration of the embodiment. A master station control unit (hereinafter, simply referred to as master station) CTR1 arranged near the driver's seat of the vehicle has a plurality of slave station control units (hereinafter, referred to as master stations) arranged at various parts of the vehicle via a multiplex transmission line LL. CTR2, CTR3,... Are simply connected. The master station CTR1 includes a microcomputer (hereinafter referred to as a CPU) 1 and an interface circuit IF.
1, a ROM storing a communication control program and the like, a battery backup RAM for recording provisional errors and communication errors during communication, and a power supply circuit PS1. CPU
1 communicates with the slave stations CTR2, CTR3,... Via the respective interface circuits IF1, IF2, IF3,... And drives the corresponding terminal equipment based on the operation information of the operation members received from the slave station. Is determined, and the drive information is transmitted to the slave station to which the terminal device is connected.

Power is supplied to the power supply terminal Vcc of the CPU 1 from the battery BAT via the terminal TB1 and the power supply circuit PS1. Normally, the terminal voltage of the on-board battery BAT is 1
Since the voltage is 2 V, the battery voltage Vb is reduced to a predetermined voltage by the power supply circuit PS1 and supplied to the CPU 1. Further, the COM terminal of the CPU 1 is connected to the ground line of the battery BAT via the terminal TB2. In addition, the master station CT
A delay circuit in which a resistor R1 and a capacitor C1 are connected in series is connected between terminals TB1 and TB2 of R1, and a diode D1 is connected to the resistor R1 in parallel in the direction shown in the drawing. The connection point of the capacitor C1 is connected to the AD terminal of the CPU1. This AD terminal is C
This is an input terminal of an AD converter built in PU1.

FIG. 2 and FIG. 3 are diagrams showing the time change of the battery voltage Vb and the terminal voltage Vc of the capacitor at the time of cranking of the vehicle. FIG.
FIG. 3 shows the case where the battery voltage Vb is lower than the reset level Vrs and lower than a predetermined voltage Vo (here, Vo> Vrs). During cranking of the vehicle, a large amount of power is supplied from the battery BAT to the starter, so that the terminal voltage Vb decreases. When the engine starts and power to the starter is stopped, the battery terminal voltage V
b recovers to almost the original voltage level. Since the capacitor C1 is charged by being supplied with power from the battery BAT via the resistor R1, the capacitor terminal voltage Vc is substantially equal to the battery voltage Vb in a steady state. When the battery terminal voltage Vb decreases during cranking, the capacitor C1 discharges via the diode D1, and the capacitor terminal voltage Vc decreases almost in the same manner as the battery voltage Vb. The battery voltage Vb drops sharply at the start of cranking, but thereafter gradually rises with some oscillation. At this time, the charging of the capacitor C1 is started again with the rise of the battery voltage Vb, and the capacitor terminal voltage Vc is changed according to the time constant determined by the capacitance of the capacitor C1 and the resistance value of the resistor R1 as shown by a broken line in the figure. To rise. When the cranking is stopped at time t2 in FIG. 2, the battery voltage Vb suddenly recovers to almost the original level, but the capacitor C1
Is charged via the resistor R1, so that its terminal voltage Vc
Rises along the broken line according to the above time constant.

The CPU resumes its operation from time t2 when the battery voltage Vb exceeds the predetermined voltage Vo and recovers to the original level. Therefore, by the CPU that has resumed operation, the capacitor terminal voltage Vc and the predetermined voltage Vc are not changed until time t3 when the capacitor terminal voltage Vc exceeds the predetermined voltage Vo.
o, the capacitor terminal voltage Vc becomes the predetermined voltage V
If it is lower than o, it can be recognized that the CPU has been reset due to a decrease in the battery voltage Vb due to cranking.

Even when the battery terminal voltage Vb does not drop to the reset level Vrs, as shown in FIG. 3, at the cranking start time t5, the capacitor terminal voltage Vc drops sharply almost simultaneously with the battery voltage Vb. At the cranking end time t6, the battery voltage Vb rapidly recovers to almost the original level, but the capacitor terminal voltage Vc
Rises along the broken line according to the above time constant. In this case, since the CPU has not been reset, time t5
Between time t7 and when the capacitor terminal voltage Vc exceeds the predetermined voltage Vo, the capacitor terminal voltage Vc is compared with the predetermined voltage Vo. If the capacitor terminal voltage Vc is lower than the predetermined voltage Vo, the battery voltage is increased with cranking. It can be recognized that Vb has decreased.

When there is no response from the slave station of the communication destination, the CPU 1 temporarily sets a provisional communication error with the slave station (hereinafter, referred to as a temporary error).
(Referred to as a provisional error) in the RAM of the battery-back amplifier. Thereafter, if there is no response from the slave station even after a predetermined time has elapsed, the provisional error is changed to a communication error, and predetermined error processing is performed. When the provisional error is recorded and the capacitor terminal voltage Vc is detected to be lower than the predetermined voltage Vo, the CPU 1 determines that a provisional error with the slave station has occurred due to a decrease in the battery voltage Vb due to cranking. Then, the provisional error recorded in the RAM is released.

The slave station control unit CTR2 is installed near the driver's seat of the vehicle, and includes a CPU 2 and an interface circuit IF.
2 and similar devices to the master station CTR1. Although not shown, the slave station CTR2 is the master station CTR.
1 is provided with the same power supply circuit as the battery BA.
Power is supplied from T to the CPU 2. This slave station CTR2
Are connected to a small lamp switch for turning on and off a lamp for meter illumination, a head lamp switch for turning on and off a head lamp, a turn signal switch for turning on and off a turn signal lamp, and the like. The slave station control unit CTR3 is installed near the driver's seat of the vehicle, and includes the same devices as the master station CTR1, such as the CPU 3 and the interface circuit IF3. Although not shown, the slave station CTR3 has a power supply circuit similar to that of the master station CTR1.
Power is supplied to U3. A switch for opening and closing the window of the passenger seat, a switch for opening and closing the left and right windows of the rear seat, a door lock switch, and the like are connected to the slave station CTR3. Note that, in addition to the above-mentioned slave stations, various slave station control units are connected to the vehicle via the multiplex transmission line LL with the master station CTR1, but the description of these slave stations will be omitted.

FIGS. 4 and 5 show the CPU of the master station CTR1.
3 is a flowchart showing a control program executed in Step 1. The operation of the embodiment will be described with reference to these flowcharts. When power is supplied from the battery to the master station control unit CTR1 and a predetermined voltage is supplied to the terminal Vcc of the CPU1, the CPU1 executes a predetermined reset sequence and then starts executing the communication control program shown in FIG. . In step S1, after transmitting the driving information and the like to the slave station CTR2, the process proceeds to step S2 and proceeds to the slave station CTR2.
Wait for a response from. If there is a response from the slave station CTR2, the process proceeds to step S8; otherwise, the process proceeds to step S3. In step S8, the operation of the corresponding terminal device is determined based on the operation information of the switches transmitted from the slave station CTR2, and the process proceeds to step S9 to transmit the drive information and the like to the slave station CTR3.

If there is no response from the slave station CTR2 in step S2, the battery backup RA is returned in step S3.
A provisional error is recorded in M, and the process proceeds to step S4, where the CPU
1. Start the timer built in 1. In a succeeding step S5, it is determined whether or not the provisional error has been released by a timer interrupt routine described later, and if the provisional error has been released, the process returns to step S1 to transmit to the slave station CTR1 again.
If the provisional error has not been cleared, the process proceeds to step S6. In step S6, it is determined whether or not the set time T1 of the timer has elapsed.
Otherwise, return to step S1. If the set time T1 has elapsed without being canceled by the timer interrupt routine after the recording of the provisional error, the provisional error is changed to a communication error in step S7, a predetermined communication error process is performed, and the process proceeds to step S9. . Since the communication processing for the slave stations CTR3,... After step S9 is the same as the processing for the slave station CTR2 described above,
Description is omitted.

The CPU 1 has a predetermined time, for example, 10 msec.
A timer interrupt occurs for each c, and the CPU 1 executes a timer interrupt routine shown in FIG. Step S
At 21, it is determined whether or not the capacitor terminal voltage Vc is lower than a predetermined voltage Vo.
Proceed to 22, otherwise return. Step S
At 22, it is determined whether or not a provisional error is recorded in the battery backup RAM, and if so, the process proceeds to step S23, otherwise returns. If the provisional error is recorded and it is detected that the capacitor terminal voltage Vc is lower than the predetermined voltage Vo, it is determined that a provisional error with the slave station has occurred due to a decrease in the battery voltage Vb due to the cranking. RA in S23
The provisional error recorded in M is canceled and the routine returns.

Now, during communication between the master station CTR1 and the slave station CTR2, a drop in the battery voltage Vb due to cranking or the like occurs, and the slave station CTR2 is changed as shown in FIG. If the master station CTR1 is reset and the master station CTR1 is not reset as shown in FIG. 3, the slave station CTR2 cannot respond to the transmission from the master station CTR1.
Communication with TR2 is disabled. Once the master station CTR1 records a provisional error for the slave station CTR2, and thereafter executes a timer interrupt routine and detects that the capacitor voltage Vc is lower than the predetermined voltage Vo, this communication failure is caused by cranking of the battery. The CPU 2 of the slave station CTR2 recognizes that the reset has occurred due to the decrease in the voltage Vb, and the provisional error is released.

On the other hand, during communication between the master station CTR1 and the slave station CTR2, a decrease in the battery voltage Vb due to cranking or the like occurs, and the reset level Vrs of the CPU1 and CPU2 is reduced.
Assuming that the master station CTR1 is reset as shown in FIG. 2 and the slave station CTR2 is not reset as shown in FIG. 3 due to the variation, the CPU 1 of the master station CTR1 that has resumed operation after cranking ends from time t2. Until the time t3, the timer interrupt routine recognizes that the capacitor voltage Vc is lower than the predetermined voltage Vo, and resets itself (CPU1) with a drop in the battery voltage Vb due to cranking or the like, and resets with the slave station CTR2. Recognize that communication has been interrupted.

In the above-described embodiment, an example has been described in which the multiplex communication apparatus of the present invention is applied to a vehicle to control various on-vehicle devices. However, the present invention can be applied to devices other than the vehicle. Further, in the above-described embodiment, an example in which communication is performed with each slave station centering on the master station has been described. However, the present invention is also applicable to a case where each control unit executes the above-described control program and communicates with another control unit. Can be applied. Further, in the above-described embodiment, the description has been made focusing on the reduction of the battery voltage due to the cranking.
The multiplex communication apparatus of the present invention is not limited to the time of cranking and operates similarly to the time of cranking even when the power supply voltage drops due to a failure of a power supply circuit or the like.

In the configuration of the above embodiment, the microcomputers CPU1, CPU2, CPU3,... Are provided with an arithmetic control circuit, a voltage drop detecting circuit, a communication abnormality detecting circuit, and a provisional error canceling circuit.
1 and the diode D1 each constitute a delay circuit.

[0025]

According to the first aspect of the present invention, if a communication error occurs in communication between the control units, it is temporarily recorded as a temporary error, and thereafter, if the communication error does not disappear for a predetermined time, the temporary error is switched to a communication error. The provisional error is canceled when it is detected that the voltage of the common power supply drops below a predetermined value while the provisional error is recorded. The arithmetic control circuit automatically resets when the power supply voltage drops to the reset level. This reset level is a unique value that is different for each arithmetic control circuit, and there are an arithmetic control circuit that is reset due to a variation in the reset level of the arithmetic control circuit and a non-reset arithmetic control circuit when the power supply voltage decreases. A communication abnormality occurs between the control unit having the arithmetic control circuit and the control unit having the reset arithmetic control circuit. However, since it is possible to recognize that this communication error has occurred due to a drop in the power supply voltage, it is possible to prevent such a communication error from being recognized as a communication error, and to make a communication circuit, a microcomputer, a control circuit, etc. Thus, a distinction can be made between the original communication error caused by the failure and the communication error caused by the drop in the power supply voltage, and appropriate error processing can be performed only for the original communication error. Furthermore, when controlling various on-vehicle devices by applying to a vehicle, the cranking operation of the vehicle can be recognized, so communication between the control units is started before the cranking, and communication accompanying the cranking operation is performed halfway. Even if an abnormality occurs, it can be prevented from being recognized as a communication error, and various new functions can be added to the vehicle. For example, in order to prevent malfunctions due to communication errors during cranking, the anti-theft function of the vehicle that had been activated after cranking was also activated before cranking, and if theft was detected, the engine itself was started. It is also possible to prohibit. According to the second aspect of the invention, when the output voltage of the delay circuit is equal to or lower than the set value, it is determined that the voltage of the common power supply is equal to or lower than the predetermined value. When the voltage of the common power supply drops below the eigenvalue of the arithmetic control circuit, the arithmetic control circuit is reset. When the voltage of the common power supply rises again and returns to a value higher than the predetermined value, the arithmetic and control circuit starts operating. At this time, since the output voltage of the delay circuit rises with a delay, the voltage of the common power supply becomes equal to or less than a predetermined value even if the output voltage value of the delay circuit is equal to or less than the set value even in the arithmetic control circuit that has been reset and started again. Can be recognized. Further, according to the third aspect of the present invention, the voltage of the common power supply is input via the delay circuit, and when a communication abnormality occurs in the communication between the control units, it is temporarily recorded as a provisional error, and the output voltage of the delay circuit is reduced. When the value becomes equal to or less than a preset value, the provisional error is released. The microcomputer automatically resets when the power supply voltage drops to the reset level. This reset level is a unique value that differs for each microcomputer, and there are microcomputers that are reset due to variations in the reset level when the power supply voltage is reduced, and microcomputers that are not reset. In such a case, communication abnormality occurs between the control unit having the microcomputer that has not been reset and the control unit having the microcomputer that has been reset. However, since it is possible to recognize that this communication abnormality has occurred due to a decrease in the power supply voltage, it is possible to prevent such a communication abnormality from being recognized as a communication error. According to the invention of claim 4, when a predetermined time has elapsed after the provisional error is recorded, the provisional error is switched to the communication error if the communication error is not resolved and the provisional error is still recorded. The distinction between the original communication error caused by the failure of the communication circuit, microcomputer, control circuit, etc. and the communication error caused by the drop of the power supply voltage is clarified, and appropriate error processing is performed only for the original communication error. It can be carried out.

[Brief description of the drawings]

FIG. 1 is a functional block diagram showing a configuration of an embodiment.

FIG. 2 is a diagram showing a time change of a battery voltage Vb and a capacitor terminal voltage Vc at the time of cranking.

FIG. 3 is a diagram showing a time change between a battery voltage Vb and a capacitor terminal voltage Vc during cranking.

FIG. 4 is a flowchart showing a communication control program.

FIG. 5 is a flowchart illustrating a timer interrupt routine.

[Explanation of symbols]

CTR1 Master station control unit CTR2, CTR3 Slave station control unit CPU1, CPU2, CPU3 Microcomputer IF1, IF2, IF3 Interface circuit PS1 Power circuit C1 Capacitor R1 Resistor D1 Diode BAT Battery LL Multiplex transmission line

Claims (4)

(57) [Claims] A plurality of control units each having an arithmetic and control circuit that automatically resets when the voltage of a common power supply drops to a specific value, and communicates with each other via a common multiplex transmission line. In the multiplex communication device, a voltage drop detection circuit that detects that the voltage of the common power supply has dropped below a predetermined value that is higher than a unique value of each of the arithmetic and control circuits, and when a communication abnormality occurs in communication between the control units. Is temporarily recorded as a provisional error, and thereafter, when the communication abnormality is not resolved for a predetermined time, a communication abnormality detection circuit that switches the provisional error to a communication error, and when the provisional error is recorded by the communication abnormality detection circuit, When the voltage drop detection circuit detects that the voltage of the common power supply has dropped below the predetermined value, the provisional error for canceling the provisional error is released. A multiplex communication device, wherein each of the control units is provided with an error canceling circuit. 2. The multiplex communication apparatus according to claim 1, wherein the voltage drop detection circuit is configured to output the voltage with a delay when the voltage of the common power supply rises and to decrease the voltage of the common power supply when the voltage of the common power supply rises. Has a delay circuit whose output voltage decreases almost simultaneously, and when the output voltage of the delay circuit is equal to or less than a preset value, it is determined that the voltage of the common power supply is equal to or less than the predetermined value. Multiplex communication device. A plurality of control units each having a microcomputer connected to a common power supply;
In a multiplex communication device in which they communicate with each other via a common multiplex transmission line, the output voltage rises with a delay when the voltage of the common power supply increases, and the output voltage increases when the voltage of the common power supply decreases. The microcomputer, when a communication error occurs in the communication between the control units, temporarily records as a provisional error, the output voltage of the delay circuit is less than a preset value A multiplex communication device, wherein the provisional error is canceled when the multiplex communication device becomes available. 4. The multiplex communication apparatus according to claim 3, wherein the microcomputer is configured such that, when a predetermined time has elapsed since the recording of the provisional error, the communication abnormality is not resolved and the provisional error is recorded. A multiplex communication device, wherein the provisional error is switched to a communication error if it is not.