Modbus communication
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Recently, I tested a Yokogawa ALR121 RS485 Modbus communication module; I connected a laptop to the ALR121 communication card of the DCS using an RS232 to RS485 adapter, and ran a serial port monitoring software (named winsct). The DCS has been configured correctly. The DCS configuration address is set to A40010, with the station number being 1 and the size being 2. By monitoring the serial port, it is already possible to see the request frames sent by the ALR121. 01 03 0009 00 02 14 09. I send a response frame, formatted as: 01 03 04 00 06 00 03 CRC checksum. This information should be visible from the DCS. (A real expert did indeed demonstrate this to me on his notebook successfully.) ) Then I switched to my own laptop and tried the same steps, but I still couldn’t establish a connection. The request frames sent by DCS can be seen, but the response frames sent back are not transmitted. Error code A391. Basically, it means that reading is attempted at the same time as sending, resulting in a conflict. How to solve it? To put it simply, I just moved the interface card from his laptop to mine; the software settings are exactly the same as before. It just doesn’t work.
I think it’s a matter of timing – you need to receive the request signal before you can send a response signal; otherwise, errors will occur. When I was working on communications for SIEMENS in the past, I only used the receiving function, and I encountered this same problem with the sending module. First, you need to make sure that the request signal has been received, then you can send the response signal; and only after confirming that the response signal has been sent can you start receiving request signals again.
There can only be one Modbus master, which is the computer. Make sure that the computer’s serial port is not being used by any other software. Also, verify that the baud rate, parity, and stop bits are set correctly; if any of these settings are incorrect, communication will not be possible
It’s possible that the AREAD module within the MODBUS ACM module isn’t configured properly