Mitsubishi FX Series PLC Data Communication Practical Manual

Mitsubishi FX Series PLC Data Communication Complete Guide: Practical Technical Analysis from N: N Network to Remote Maintenance

Mitsubishi Electric FX series programmable logic controllers (PLCs) occupy an important position in the field of industrial automation due to their flexibility and powerful communication capabilities. Its data communication function covers multiple network protocols and interface standards, which can meet various needs from simple device interconnection to complex remote monitoring. This article is based on the FX series PLC data communication manual, systematically sorting out its communication types, system configurations, parameter settings, and troubleshooting methods, providing engineers with a comprehensive and practical technical reference.

The FX series PLC supports multiple physical interfaces such as RS-232C, RS-485/RS-422, USB, and Ethernet, and provides various communication protocols including CC Link, N: N network, parallel link, computer link, inverter communication, non protocol communication, MODBUS communication, programming communication, and remote maintenance. Each communication method is designed for specific application scenarios, and engineers can flexibly choose based on system size, transmission distance, number of devices, and real-time requirements.

Analysis of Core Communication Types

1. N: N network

N: N Network is a small data link system that allows up to 8 FX series PLCs to automatically exchange data via RS-485 bus. This network supports three data link modes (mode 0, 1, 2), corresponding to different device point configurations:

Mode 0: 4-point device (D register) per station, only suitable for small PLCs such as FX0N and FX1S

Mode 1: 32 point devices (M relays) and 4-point devices per station

Mode 2: 64 point devices and 8-point devices per station (applicable to FX1N, FX2N, FX3 series)

N: The total extension distance of N network can reach 500 meters (when using 485ADP), and the communication baud rate is fixed at 38400bps. The master station completes configuration by setting special registers such as D8176 (station number), D8177 (number of slave stations), D8178 (refresh range), etc. The slave station only needs to set its own station number, and other parameters are automatically obtained from the master station.

2. Parallel linking

Parallel link is dedicated to high-speed data exchange between two PLCs of the same series, providing two options: regular mode and high-speed mode:

Normal mode: Transmission of 100 point devices and 10 point devices (FX0N/FX1S for 50 point M and 10 point D)

High speed mode: only transmits 2-point devices, but the link cycle is significantly shortened

In high-speed mode, the link time of FX3 series PLC is only "5ms+master station scanning cycle+slave station scanning cycle", which is suitable for situations with high real-time requirements. Parallel links are initiated through M8070 (master declaration) and M8071 (slave declaration), with M8162 used to switch high-speed mode.

3. Computer connection

The computer link adopts a dedicated protocol (format 1 or format 4), allowing the upper computer (or personal computer) to serve as the master station and connect up to 16 FX or A-series PLC slave stations. This communication supports two physical layers: RS-232C (1:1 connection, distance of 15 meters) and RS-485 (1: N connection, distance of 500 meters).

The core instructions for computer connectivity include:

BR/BW: Read/Write Bit Devices

WR/WW: Read/write word devices or 16 bit devices by word

QR/QW: Extended word read/write (applicable to FX3 series)

BT/WT: Random Test Write (bits/word)

RR/RS: Remote RUN/STOP Control

PC: Read PLC model code

GW: Global command (simultaneously controls all slave stations)

TT: Loop Test

The communication format is set through D8120 (ch1) or D8420 (ch2), including data length (7/8 bits), parity check (none/odd/even), stop bit (1/2 bits), baud rate (300~38400bps), and whether to add header/terminator/checksum. The station number is designated by D8121 (ch1) or D8421 (ch2).

4. Communication of frequency converter

The FX series PLC can communicate with Mitsubishi FR-E700, D700, A800, F800 and other series frequency converters through RS-485 to achieve operation monitoring, frequency setting, parameter reading and writing, and start stop control. For FX2N/FX2NC, use the EXTR instruction (K10~K13); For the FX3 series, more specialized IVCK, IVDR, IVRD, IVWR, IVBWR, and IVMC instructions are used.

The IVMC instruction (multifunctional inverter control) is particularly powerful, as it can simultaneously write operation commands and set frequencies in a single communication, and read the inverter status and output frequency, greatly improving communication efficiency. This instruction supports FX3S (V1.00 or above), FX3G/FX3GC (V1.40 or above), and FX3U/FX3UC (V2.70 or above).

The parameter settings for frequency converter communication include station number (Pr.117 or Pr.331), baud rate (Pr.118 or Pr.332), data format (Pr.119/Pr.333), parity check (Pr.120/Pr.334), etc. The operation mode of the frequency converter needs to be set to "network operation mode" (Pr.79=0 or Pr.340=1), and the communication timeout and retry times should be configured correctly.

5. Non protocol communication (RS/RS2 instructions)

Non protocol communication is the most common data exchange method, allowing PLC to freely format data transmission and reception with any device with RS-232C/RS-485 interface (such as printers, barcode scanners, instruments).

The RS instruction (applicable to all FX series) supports half duplex communication, with a maximum data length of 256 bytes (early models) or 4096 bytes (FX2N/FX3 series). By switching between 16 bit/8-bit processing modes through M8161, D8124 and D8125 respectively define the header and terminator.

The RS2 instruction (only applicable to FX3S/FX3G/FX3GC/FX3U/FX3UC) has more powerful functions:

Supports headers and terminators of up to 4 bytes

Automatic calculation and checksum

Can specify communication channel (ch0/ch1/ch2)

Support full duplex communication

The maximum data length is 4096 bytes

The communication format of the RS2 instruction is set by D8370 (ch0), D8400 (ch1), or D8420 (ch2), and the header and terminator are stored in registers such as D8380~D8381 and D8382~D8383 (ch0), respectively. By setting M8372/M8402/M8422 (send request) and M8373/M8403/M8423 (receive completion flag), reliable data transmission and reception control can be achieved.

6. Programming communication and remote maintenance

Programming communication allows programming software (GX Works2, GX Developer) to be connected through standard RS-422 ports, USB ports, or Ethernet adapters for online monitoring, program upload/download, and debugging. Remote maintenance utilizes modems and telephone lines (or cellular networks) to achieve remote access to PLCs from different locations. In the FX3 series, the initialization settings of the modem can be simplified through the "PP modem mode (CH1/CH2)", without the need for users to input AT commands themselves.

System configuration and hardware selection

Proper selection of communication equipment is the foundation for stable system operation. The following table summarizes the available communication extension modules for each FX series:

Communication interface supports models and typical distances

RS-232C FX3U-232-BD, FX3G-232-BD, FX2N-232-BD, FX3U-232ADP(-MB), FX2NC-232ADP 15 rice

RS-485 FX3U-485-BD, FX3G-485-BD(-RJ), FX2N-485-BD, FX3U-485ADP(-MB), FX2NC-485ADP 50 Meter (BD)/500m (ADP)

RS-422 FX3U-422-BD, FX3G-422-BD, FX2N-422-BD 50 meters

USB FX3U-USB-BD (or standard built-in USB port) 5 meters

Ethernet FX3U-ENET-ADP 100 meters (segment length)

When using multiple communication channels (FX3G/FX3U/FX3UC can support ch1 and ch2), attention should be paid to the limitations of functional combinations. For example, when ch1 sets parallel links, ch2 cannot set N: N networks; But when setting up the computer link in ch1, ch2 can set the N: N network.

Communication parameter setting method

The communication parameters of FX series PLC can be set in two ways:

1. Parameter method (recommended)

In GX Works2, open the [Parameters] → [PLC Parameters] → [PLC System (2)] tab, check the "Run Communication Settings", and then configure the protocol type, data length, parity check, stop bit, baud rate, and header/terminator. This method is applicable to computer links, frequency converter communication, and non protocol communication. For N: N networks and parallel links, do not check 'Run Communication Settings', but configure them through special registers in the program.

2. Procedural Law

Write the set value to a special data register (D8120, D8400, D8420, etc.) using the MOV instruction. For example, setting the RS instruction communication format to "8-bit data, even parity, 1 stop bit, 9600bps, no header/terminator" can execute MOV H0086 D8120. The programming method needs to take effect after the PLC is powered on or when the mode is switched, and its priority is lower than the parameter method.

Troubleshooting Guide

Communication failures typically originate from the following aspects:

1. LED indicator status

SD/RD flashing: normal data transmission and reception

SD flashing, RD constantly off: only sending without receiving a response, check the slave address and wiring

RD flashing, SD constantly off: only receiving but not sending, check the send request flag

All out: No physical connection established, check power supply, cables, and terminal resistance

2. N: N Network Common Error Codes

Direction for troubleshooting the meaning of error codes

01H monitoring timeout, slave station not responding, check wiring and power supply

02H station number error. Duplicate station number or non consecutive number

03H counter error communication data abnormality, check for line noise

04H message format error, invalid data returned from the station

11H main station timeout. The main station did not poll on time. Check the scanning cycle of the main station

3. Parallel link error code

6312/3812: Character error

6313/3813: Checksum error

6314/3814: Format error

4. Computer link error (NAK response)

02H: checksum error

03H: Protocol error (command or format mismatch)

06H: Character area error (illegal device address)

0AH: PLC number error (should be fixed as FFH)

18H: Remote control error (RUN/STOP mode mismatch)

5. General troubleshooting steps

Confirm PLC version: Read the last three digits of D8001 and refer to the applicable version table in the manual.

Check hardware installation: Ensure that the expansion module is securely connected and the terminal resistance is set correctly (110 Ω or 330 Ω).

Validation parameters: Confirm that the communication format matches the device by monitoring D8120/D8400/D8420.

Check program conflicts: Avoid using RS, RS2, EXTR, or inverter instructions simultaneously on the same channel.

Clear error flag: Some errors require restarting the PLC or switching RUN/STOP to clear.

Practical suggestions

For new systems, the FX3 series PLC is preferred as it supports richer communication instructions (such as RS2, IVMC) and higher baud rates (38400bps).

N: The N network is suitable for small distributed control, but it should be noted that the link period increases with the number of stations (in Mode 2, the link time for 8 stations is about 131ms).

If computer links require frequent reading and writing of large amounts of data, it is recommended to use QW/QR commands (FX3 series) to improve efficiency.

In frequency converter communication, reasonable response waiting time (D8150/D8155) and communication retry times should be set to avoid timeouts caused by processing delays in the frequency converter.

When performing remote maintenance, it is necessary to correctly configure the modem initialization parameters (AT command) on the PLC side and register the corresponding AT command on the personal computer side.