Communication problem

[WKla 0]

Hello, I've got an project with the next hardware architecture: - FX3U CPU - FX3U-485BD The PLC is communicating via 485BD with an modbus slave device and with an PC (proprietary protocol). Because I didn't want to send the modbus communication dialog to the PC, the PLC interrupts with the help of an relay the two rs485 lines that are connected to the PC. Every x minutes I change the communication parameters of the FX3U-485BD for reading the modbus slave device. At the end of the polling the port is reprogrammed for sending data to the PC. The differences between the two configuration of the 485BD port are: - the modbus function works on 8 bit, the communication with the PC on 16 bit - there are two different RS instructions with different buffers The RS instructions are driven when a flag (BOOL) is on or off. (modbus configuration when the flag is off, and the other configuration when the flag is on). The problem I have is that sometimes the M8122 bit remains set. The communication is working well on both configuration, but only a few hours. As I is not permitted to run a reset instruction of the M8122 bit in program, the communication blocks. The only solution I've found is to manually force an STOP and then RUN on the PLC. I have made a test configuration the same as described earlier (only it is not having the relay), in the office and the communication didn't blocked after 3 days of testing. Do you know another solution for reseting this bit? Did you ever encountered this problem? The same configuration of port I have applied on FX1N plc and works well.

[kaare_t 155]

Why not add a FX3U-485-ADP-MB card for the computer/modbus so that you have one channel for each communication device?? It sounds a bit "strange" the setup you are using now....

[Inntele 60]

Wkla, Follow the advice of Kaare. Such money savings could be finished with PLC damage, because 485BD is not isolated, and in this case instead of 130 Euro for 485ADP-MB, you'll pay for FX3U base module! Count your money in the pocket each time, before you wish to saving.