Mitsubishi FX Series PLC Data Communication Practical Manual

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)