JPH02171054A - Data communication system - Google Patents

Abstract

PURPOSE:To simplify the constitution by applying input/output control of a body electronics with a conventional communication equipment and using the communication equipment used together with a microcomputer for the control of an intelligent device. CONSTITUTION:Simplex stations 10, 11 are communication equipments and duplex stations 20, 21 are also communication equipments. The simplex stations 10, 11 and the duplex stations 20, 21 are connected by a communication line 5 of balanced transmission system. Body electronics 40, 41 such as switches and lamps are connected respectively to the simplex stations 10, 11. Moreover, the duplex stations 20, 21 are used together with microcomputers 30, 31 and the duplex stations 20, 21 and the microcomputers 30, 31 constitute an interface section of control sections 42, 43 of an intelligent device. Interface sections 60, 61 constitute the interface with the body electronics 40, 41.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a data communication system for mutually transmitting digital data between a plurality of communication devices in, for example, an automobile.

[Prior Art] FIG. 5 is a block diagram showing a conventional in-vehicle data communication system disclosed in, for example, Japanese Unexamined Patent Publication No. 57-147392.
11 is a simplex unit for receiving only, 12 is a simplex unit for transmitting, 13
40 to 43 are external loads such as switches and lamps, and 5 is a communication path.

FIG. 6 shows an example of the simplex unit 10 for transmission only, and FIG. 7 shows an example of the simplex unit 11 for reception only. Both examples are constructed using microcomputers 30 and 31.

In both Figures 6 and 7, IIA is a transmitting/receiving circuit, IIB is a serial-to-parallel conversion circuit that converts the digital signal format from serial to parallel, and 11C is a parallel-to-serial conversion circuit that is the opposite of the serial-to-parallel conversion circuit 11B. A conversion circuit, 11D is an interface, and IIE is a driver circuit.

By the way, in the example shown in Fig. 5, the simplex stations 10 and 11 for reception only and for transmission are each made of gold film, but it is not necessary to provide one of them, or the data link can be used for reception only and transmission only. It may also consist of only dedicated simplex stations.

Next, the operation will be explained. The simplex stations 12 and 13 are equipped with both a signal input function and a signal output function, and the simplex station 1O, which is only for transmission, is only for signal input, and conversely, the simplex station 11, which is only for reception, is for signal output. It is exclusive.

An external load 40 such as an information source switch is connected to the transmission-only simplex station 10, and an external load 41 such as a lamp to be controlled is connected to the reception-only simplex station 11.

In this way, by connecting appropriate electrical components to each station and connecting each station with the communication path 5, predetermined signals are exchanged between the electrical components. As a result, the actual wiring harness only requires wiring between the electrical equipment and the nearest station, and as a result, the wiring harness can be significantly simplified and reduced in weight.

[Problem to be solved by the invention]

Since conventional in-vehicle data communication systems are configured as described above, only what is generally called body electronics can be connected. Body electronics refers to external loads 40 to 43 such as lamps, lock windows, seats, displays, etc.
Body electronics refers to the control of not a real-time, but not very large system.

Many of the control contents are controlled by switch input.
It gives commands such as "turn on lights" and "lock/off doors".

In recent years, not only radios, cassette decks, and air conditioners, but also telephones, televisions, and CDs (compact and
As more and more automobiles are equipped with disc players and the like as optional items, wiring harnesses have become even larger and more complex.

This simplifies not only the body electronics but also the wiring harness for the intelligent devices mentioned above.
In addition to reducing weight, connecting them with one communication path,
It has become necessary to share each information content and treat it as a system.

However, since conventional communication devices only communicate with body electronics, the data transmission environment is not ready to handle multiple different information contents of intelligent devices, and as it is, there is no transmission control means. The burden on the microcomputer used is heavy, and data transmission efficiency is also poor.

This invention was made to solve the above-mentioned problems, and it not only reduces the number of wire harnesses by sharing many communication paths, but also makes it possible to share the information content between body electronics and intelligent devices. The purpose of this invention is to reduce the burden on microcomputers for measurement and transmission control, and to provide a data communication system that enables highly efficient data transmission.

[Means to solve the problem]

In the data communication system according to the present invention, the input/output control of body electronics and the transmission/reception of information contents thereof are performed by using a plurality of first communication devices that do not use a microcomputer, and intelligent equipment in conjunction with a microcomputer. It is capable of transmitting and receiving multiple information contents and information contents with different data lengths, can perform complex data processing, and uses a multiplex access transmission protocol that determines the right to use the communication channel based on the priority of the messages to be transmitted when the communication channel is accessed simultaneously. A plurality of second communication devices each having an interface section serving as a transmission control means are provided.

[For production]

In this invention, a plurality of first. Of the second communication device,
After the communication device that wants to transmit detects the idle state of the communication channel, it transmits the message to be transmitted in frame units via the communication channel, and when the message is transmitted from multiple other communication devices, the transmission control means of the interface section The multiple access transmission protocol determines the right to use the communication path according to the priority of the message, and allows the message to be transmitted according to the determined priority without destroying the transmitted message. from the device to the communication device that should receive the data.

[Embodiments] Hereinafter, embodiments of the data communication system of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an example of the configuration of a data link in a communication system.
0.11 is a simplex station that is a communication device, 20.21 is a duplex station that is a communication device, and a balanced transmission system communication path is established between the simplex stations 10, 11 and the duplex stations 20.21. Connected by 5.

Body electronics 40, 41 such as switches and lamps are connected to the simplex stations 10 and 11, respectively.

Further, each of the duplex stations 20.21 is used in conjunction with a microcomputer 30.31, and these duplex stations 20.21 and the microcomputer 30.31 control the control unit 42 of each intelligent device.
．． 43. 62.6
3 is an interface section with the body electronics 4041.

FIG. 2 shows a rough block configuration of an embodiment of the simplex stations 10, 11. In FIG. 2, white lines indicate parallel data buses, solid lines indicate serial data buses, The dashed line indicates the control bus.

Reference numeral 101 is a line receiver, and its two input terminals include the communication path connection Vt terminal 122, BUS, and BU.
Each is connected to Sf-1. Communication path connection terminal 1
22 BUS (GND) is grounded.

The output of the line receiver 101 is input to the input terminal of a PWM composite circuit 103 via a filter 102. The output of this PWM composite circuit 103 is a 24-bit serial-parallel conversion circuit 104, an error code check circuit 111, a flag detection circuit 110, . Idle detection circuit 108. The data is sent to the collision detection circuit 109.

These error code check circuit 111.flag detection circuit 110. The output of the idle detection circuit 108 and the collision detection circuit 109 is transmitted to the transmission control circuit 11 as transmission control means.
The error code check circuit 111 is configured to send and receive data to and from the 24-bit serial-to-parallel conversion circuit 104.

The control signal of the transmission control circuit 112 is transmitted to the line receiver 10.
1. PWM code circuit 117. line driver 11B,
PWM composite circuit 103. Framing circuit 115, error code generation circuit 116, 16 bit parallel to serial conversion circuit 120, data mask circuit 106, 24 bit parallel to serial conversion circuit 114. Each of the signals is sent to an output buffer 107.

The 16-bit data from the 24-bit serial-to-parallel conversion circuit 104 is sent to the data mask circuit 106, and the 8-bit data from the data mask circuit 106 is sent to the received message data output terminal 125 through the output buffer 107. It is supposed to be done.

Received message ID (Identification)
n) 8-bit data is sent from the setting terminal 124 to the received message ID comparison circuit 105, and this received message is compared 10 times! The output of 105 is sent to transmission control circuit 112.

4-bit data is sent from the node number setting terminal 123 to the 4-to-16 line decoder/latch circuit 119, and
The 4-to-16 line decoder latch circuits 119 to 16 convert the bit data into 16-bit data.
The signal is sent to a bit-parallel-serial conversion circuit 120.

The output of the 16-bit parallel-to-serial conversion circuit 120 is sent to a PWM code circuit 117.

This PWM code circuit 117 has a framing cycle! 11
5 is also input, and the PWM encoding circuit 117 inputs the output of the framing circuit 115, performs PWM encoding on the output of the 16-bit parallel-to-serial conversion circuit 120 for each frame, and converts the output into the line. - Output to the communication path connection terminal 122 via the driver 118.

Transmission message ID setting terminal 126, transmission message
8-bit setting data is sent from each mask data setting terminal 127 to the 24-bit parallel-to-serial converter circuit 114, and transmitting message data is sent from the transmitting message data input terminal 128 to the input data change detecting circuit 113. It has become so. Two 8-bit data are output from the input data change detection circuit 113.
The signal is sent to a 4-bit parallel/serial conversion circuit 114.

The output of the 24-bit parallel/serial conversion circuit is sent to the above-mentioned framing circuit 115 and error code generation circuit 116.
It is designed to be sent to The output of error code generation circuit 16 is also sent to framing circuit 115.

-9 side, 130 is a reset input terminal of the transmission control circuit 112, 129 is a power input terminal, and 131 is a clock input terminal. The clock input from this clock input terminal 131 is sent out at the synchronization timing 121. This synchronous timing circuit 121 is for outputting a synchronous timing signal to each part.

FIG. 3 shows a rough block configuration of an embodiment of each duplex station 20, 21 in FIG. 1, and since the configuration is almost the same as that in FIG. The part corresponding to the circuit in Fig. 2 has r20 o.
The number J number is given to avoid redundant explanation of the structure.

As is clear from comparing this Figure 3 with Figure 2,
Codes 201 to 203, 208 to 212, 21 in the figure
5 to 223 and 229 to 231 are the same as the parts corresponding to number r100J in FIG.

Further, 204 is an 8-bit serial/parallel conversion circuit that inputs the output of the PWM composite circuit 203 and converts it into parallel.The output of this 8-bit serial/parallel conversion circuit 204 is sent to the temporary storage register 205. It has become.

- Data is exchanged between the time storage register 205 and the error code check circuit 211.

-The output of the time storage register 205 is input to the input/output buffer 23.
3.

The human output buffer 233 is connected to the input/output terminal 232 for sending/receiving message data.
The bit data is sent to an 8-bit parallel/serial conversion circuit 234.

The output of this 8-bit parallel/serial conversion circuit 234 is transmitted to the error code generation circuit 216 and the framing circuit 2.
It is set to be sent on the 15th. Reference numeral 235 is a transmission control terminal, and the signal is sent to the transmission control circuit 212.

Next, the operation will be explained. The data link of this data communication system is equipped with a multiple access transmission protocol that safely handles message data collisions. A communication device wishing to transmit can access the communication channel after detecting the idle state of the communication channel.

Furthermore, if a message is communicated by another communication device, the device stops its own transmission operation and switches to reception operation so as not to destroy the message.

Further, when a plurality of communication devices simultaneously access the communication channel through their respective transmission operations, the right to use the communication channel is determined based on the priority of the message, and the communication is made harmless.

In other words, a communication device that is sending a message with a low priority immediately stops sending and switches to receiving, and a communication device that is sending a message with the highest priority can continue sending. can.

Messages to be transmitted are communicated in frame units. The frames are as shown in Fig. 4 fa) - a monitoring frame 71 used for monitoring data links, executing control, requesting mode settings, responding, reporting abnormal conditions, etc.; , an information frame +72 in which the information content 703 can be specified as any data length between 1 and 16 bytes, which is a frame that communicates only between duplex stations 20 and 21;
As shown in C), the data 703 in the frame communicated with the simplex 10.11 has a data length of fixed 2 bytes (one byte of which is mask data). be.

The configuration of the monitoring frame 71 is as shown in FIG. 4(a), and the priority of this monitoring frame 71 must be higher than other frames.
ntification) 702 (hereinafter referred to as ID) is from the highest 0OHex to 0FHex, and the data 703 at that time is command information with special meaning.

The structure of the information frame 172 is as shown in FIG. 4 (G), and the data length of the message is specified in the first byte of the data 703.

The structure of the information frame 73 is as shown in FIG. 4(C), and the message data 703 has a fixed data length of 2 bytes (of which 1 byte is mask data). The information frame 172 and the information frame [73 are identified by the content of the ID 702 in the same way as the monitoring frame 7I, and the content of the ID 702 is arbitrarily determined between transmission and reception.

SOM (Start of Message) 7
01 is used as a flag for frame synchronization between transmission and reception. This SOM”l is used by the communication device on the receiving side.
Detection of O1 identifies the beginning of the message being transmitted.

ID702 has message indicator and message priority information. In this protocol, 0 bit is superior to 1 bit, so ID702 indicates the highest priority when it is 00Hex, and the lowest priority when FFHo=. show.

Data 703 indicates the information content of the message.

The data length varies depending on the frame type and message.

F CS (Frame Check 5equens
e) 704 is a sequence for frame error detection, and 705 is an 8-bit error check code (cyclic redundancy code CRC), which is a generator polynomial (% E OD (End Of Data)). This is a flag indicating the end of information content.

RS P (Response) 706 is a communication bag that connects to the data link and provides a response as to whether the message communication was carried out normally. (The number of nodes connected to the data link is up to 16.) In the acknowledgment, each bit becomes the node number of the corresponding communication device.

E OM (End Of Message) 707
is a flag indicating the end of the message.

Next, the operation of the simplex signals 10 and 11 will be explained with reference to FIG.
After signal level conversion between the communication channel and the circuit is performed, the signal is input to the filter 102, where noise is removed. Further, the PWM decoding and decoding circuit 103 performs PWM decoding of the signal, and the transmission control circuit 112 monitors the signal.

At this time, the transmission control circuit 112
8. Collision detection circuit 109. The flag detection circuit 110 identifies whether the communication channel 5 is in an idle state, whether a data collision has occurred during its own message sending operation, or whether a message has been sent.

If it is in an idle state, transmission between the simplexes 10 and 11 becomes possible. If a data collision occurs, the transmission operation is stopped and the reception operation is switched to. Also, if a message is sent, it starts receiving operation.

First, the transmission operation will be explained. The input data change detection circuit 113 checks the state of an external load such as a switch connected to the transmission message/data input terminal 128 for the simplex 10° 11, and enters a standby state for transmission operation when there is a change. At that time, if the communication path 5 is already in communication, the transmitting operation is kept in the standby state until the communication path 5 becomes the idle state and the receiving operation is performed, but when the communication path 5 is in the idle state, the transmitting operation is directly started.

When the transmission operation starts, the transmission control circuit 112 commands 2
The 4-bit parallel/serial conversion circuit 114 has a transmission message ID setting terminal 126, a transmission message mask data setting terminal 127, and a transmission message/data input terminal 1.
28 setting data is serially converted. The serial data is processed by the framing circuit 115 into 50M701. F
C3704 and EOD705 are added and framed.

Here, the FC3704 is automatically generated by serial data entering the error code generation circuit 116 at the same time as the framing circuit 115.

Then, the signal is PWM encoded by the PWM encoder circuit 117 and sent to the line driver 118, and the line driver 11B converts the signal level to match the communication path 5 and outputs it from the communication path connection terminal 122.

When the communication device on the receiving side confirms EOD705, it simultaneously sends an R3P706 response, so that the simplex station 10 on the transmitting side
, 11 waits for a predetermined time required for the RSP 706 response, then outputs the ROM 707 and completes the series of transmission operations.

Note that the simplex stations 10 and 11 do not process the contents of the sent R3P706 response, and if they confirm that data collision has been detected when outputting the ID 702, they immediately stop the transmission operation and start the reception operation.

Next, the reception operation will be explained. When a message is sent, that is, the flag detection circuit 110 detects 50M70.
1, the message PWM-decoded by the PWM composite circuit 103 is converted into parallel by the 24-bit serial-parallel conversion circuit 104, and the message I
D702 is sent to the received message ID comparison circuit 105.

The received message 10 comparison circuit 105 compares the ID 702 with the data at the received message ID setting terminal 124. As a result of this comparison, ID702 is the received message ■D
If the data matches the data at the setting terminal 124, the data mask circuit 106 masks unnecessary data with the mask data 703, which is similarly converted into parallel data, and outputs the received message data via the output buffer 107 to the received message data output terminal 12.
5 to drive an external load such as a lamp.

R5P706 response is EOD regardless of the contents of ID702
705 will be sent if it is approved by i11.

Then, after checking the ROM 707 after receiving the R5P706 response, the series of receiving operations ends.

Also, when you check 50M701, the error code
The check circuit 11'4 checks the following ID 702 and data 703 using the FC 3704, and if there is an abnormality, the transmission control circuit 112 prevents the data output from the received message/data output terminal 125 from being changed.

The same applies when the IDs 702 do not match.

Next, the operation of the duplex stations 20 and 21 will be explained in FIG. After converting the signal level between the communication path and the circuit, the signal is output to the filter 202, where noise is removed, and then sent to the PWM composite circuit 203. This PWM
A composite circuit 203 performs PWM composite on the output of the filter 202, and the transmission control circuit 212 monitors the output.

At this time, the transmission control circuit 212
8. The collision detection circuit 209 and the flag detection circuit 210 identify whether the communication channel 5 is in an idle state, whether a data collision has occurred during its own message transmission operation, or whether a message has been transmitted.

If in the idle state, the duplex stations 20 and 21 are enabled to transmit. If a data collision occurs, the transmitting operation is stopped and the receiving operation is switched to. Also, if a message is sent, it starts receiving operation.

First, the transmission operation will be explained. microcomputer 3
When 0.31 wants to send a message, use the following table 1,
As shown in Table 2, the operating status of the duplex stations 20 and 21 is identified by checking the status from the transmission control terminal 235 of the duplex station. If the status at that time is idling, the transmitting operation continues; otherwise, the transmitting operation is awaited.

(Table 1) <Table 2> First, the microcomputer 30.31 is the duplex station 20.
．． 21 sends a transmission request to duplex station 20. When 21 confirms the transmission request, if communication path 5 is in an idle state, duplex station 20
, 21 is output as message transmission in progress. and microcomputer 30.3
1 confirms its status, and thereafter outputs the ID 702 and the contents of the data 703 in order to the transmission/reception message/data input/output terminal 232 at regular intervals.

The data output from the microcomputers 30 and 31 is sent via the human output buffer 233 to an 8-bit parallel
The serial conversion circuit 234 performs serial conversion. The serial data is transmitted to 50M7 by the framing circuit 215.
01 is added for framing. Then, it is sent to the PWM code circuit 217, where it is converted into a PWM code,
The signal is sent to the line driver 218, converted to a signal level suitable for the communication path 5, and outputted from the communication path connection terminal 222.

While a transmission request is received from the microcomputer 30, 31, the serial data enters the framing circuit 1215 and the error code generation circuit 216 at the same time to automatically generate an error code.

When the duplex stations 20 and 21 receive no more transmission requests from the microcomputers 30 and 31, they transmit the data 703 to the FC 37.
04 and EOD705 are added and output to the communication path 5.

When the communication device on the receiving side confirms EOD705, it simultaneously sends an R3P706 response, so that the duplex station 20 on the sending side
．． 21 changes the status to R3P7 while receiving R5P response.
The contents of the 06 response are output to the transmitted/received message/data input/output terminal 232.

Microcomputer 30.31 has its status T
Jf! When the duplex stations 20 and 21 on the transmitting side read the data, they wait a predetermined time for the R3P706 response, output 20M707, complete a series of transmission operations, and set the status to idle.

Further, if a data collision is confirmed when the ID 702 is output, the transmitting operation is immediately stopped, the status is set to message receiving, and the receiving operation is started.

Next, the reception operation will be explained. When a message is sent, that is, the transmission control circuit 212
When 50M701 is confirmed in step 19, a reception request is issued to the microcomputer 30.31, the status is set to message reception operation, and the 8-bit serial/parallel conversion circuit 204 converts the received data into parallel data.

This parallel data is always stored in the temporary storage register 20.
5 and the input/output buffer 233 for sending and receiving messages.
The data is output to the data input/output terminal 232. After confirming the reception request, the microcomputers 30 and 31 begin the reception operation and read the data.

First, the user checks the ID 702 of the message and identifies whether it is a necessary message. If unnecessary, ignore the message data below. If necessary, the data length is checked by looking at the first byte of the next data 703, a buffer necessary for reception is prepared, and the data is read.

When the transmission control circuit 212 confirms the EOD 705, it automatically transmits an R3P 706 response. And 20M707
Once confirmed, the series of reception operations is completed and the status is set to idle. Also, when 50M701 is confirmed, the error code check circuit 211 reads 10702 and data 703 written in the temporary storage register 205.
is checked by the FC 3704, and if there is an abnormality, no reception request is issued to the microcomputer 30 or 31.

Note that the receiving operation is also performed for its own transmitted messages, but during the sending operation, no receiving request is issued unless a data collision is detected.

In the above embodiment, information frame I and information frame ■ out of the three types of frames used for transmission are explained as having a variable data length of up to 16 bytes and a fixed data length of 2 bytes, respectively. In addition, although the above embodiment was explained in the automobile field, O
The present invention may also be used in the A (office automation run) field, and the same effects as in the above embodiments can be achieved.

(Effects of the Invention) As described above, according to the present invention, input/output control of body electronics is performed by a communication device that does not use a microcontroller, and is used in conjunction with a microcomputer to control intelligent equipment. Since the communication device is configured to carry out the communication, the role of the microcomputer is reduced and the body electronics do not require the use of a microcomputer, making it possible to simplify the configuration and reduce costs.

In addition, the wiring harness can be simplified and lightened, and data link configurations can be made between simplex only, which does not require a microcomputer, or mixed with duplex stations used in conjunction with a microcomputer. It can be designed in any frame format, and messages to be transmitted can be specified in any frame format.

Furthermore, when a plurality of communication devices simultaneously access the communication channel, the second communication interface section includes a multiplex access transmission protocol that determines the right to use the communication channel based on the priority of the message of the information content of each device as a transmission control means. Since the system is installed in the equipment, data transmission can be made more efficient, and the system has flexibility in design and can be applied to a wide range of products.

[Brief explanation of the drawing]

Fig. 1 is a data link configuration diagram of a data communication system according to an embodiment of the present invention, Fig. 2 is a block diagram between simplex communication devices that are communication devices in the above embodiment, and Fig. 3 is a communication device in the above embodiment. Block diagram of a certain duplex station, Figure 4 (
a) to FIG. 4(b) are frame format diagrams of messages transmitted by the communication system according to the above embodiment;
Fig. 5 is a data link configuration diagram of a conventional in-vehicle data communication system, Fig. 6 is a block diagram of a simple communication device for transmission, which is a communication device in a conventional in-vehicle data communication system, and Fig. 7 is a block diagram of a conventional in-vehicle data communication system. FIG. 2 is a block diagram of a receive-only simplex communication device which is a communication device in an internal data communication system. 5...Communication path, 10.11...Simple communication, 20.21
... Duplex station, 30.31 ... Microcomputer, 40.41 ... Body electronics, 42゜43 ... Control unit of intelligent equipment, 60-6
3...Interface section. In addition, in the figures, the same reference numerals indicate the same or equivalent parts. Agent: Masuo Oiwa, Kei (Method) Changed "Figure 4 (b) J" on page 2, line 9 of the detailed statement of contents of the amendment to )” is corrected. 1. Indication of the case Patent application No. 1983-327127 2. Name of the invention Data communication system 3. Person making the amendment Relationship to the case Patent applicant address 2-2-3 Marunouchi, Chiyoda-ku, Tokyo Name ( 601) Mitsubishi Electric Corporation Representative Moriya Shiki 4, Agent Address 2-2-3 Marunouchi 2-chome, Chiyoda-ku, Tokyo 5゜Date of amendment order (shipment date) 1. Indication of the case 29 Name of the invention 3. Amendment 4, Patent Application No. 63-327127 Data Communication System Subject to Substitution Amendment Representative 6, Contents of the Amendment (1) Page 2 of the specification, lines 11-12, ``12 is a simplex station for transmission; 13 is a receive-only simplex station," is replaced with "12.1.
3 is a simplex station that can be used for both transmission and reception.'' (2) r62.63 on page 8, lines 6-8 is...
...This is an interface section. 60, 61 are the interface parts with the body electronics 40.41, and 62.63 are the interface parts with the intelligent equipment control part 42.43. ” he corrected. (3) rPWM code circuit 1 on page 11, lines 5 to 8
17 outputs the output of the rPWM code circuit 11.
7 is corrected by inputting the output of the framing circuit 115 and the output of the 16-bit parallel-to-serial conversion circuit 120. (4) Correct r219J in the first line of page 26 to r210J. that's all

Claims (1)

[Claims] Monitoring frames used for data link monitoring, mode setting requests, responses, and reporting of abnormal conditions, information frames I used for messages with variable data length information content, and information used for messages with fixed data length information content. Among the messages transmitted frame by frame using three types of frames, including frame II, a plurality of first frames transmit and receive the information frame II without using a microcomputer, receive the monitoring frame, and control the input/output of external loads. It is used in conjunction with a communication device, a balanced transmission method communication path, and a microcomputer to transmit and receive the three types of frames mentioned above, and when the above communication paths are simultaneously accessed, the communication path is determined based on the priority of the message to be transmitted. a plurality of second communication devices each having an interface section whose transmission control means is a multiple access transmission protocol that determines usage rights of the data communication system.