We tried to establish modbus communication between Yokogawa DCS CS3000 and Honeywell PLC Safety Manager through RS485 serial link. But DCS displays a message saying error code A3C6 and it recovers after few seconds. Again the error appears and recovers. Thus this cycle repeats continuously. The description of the error code is "response message error, Received data length error". We rechecked all the parameters set for communication at both PLC and DCS end and ensured same values are set. But we could not find a way to solve the problem. Suggestions of the members are highly appreciated.
RS485 is a Half Duplex communications physical layer. In other words each device (MASTER or SLAVE) must KEY UP, delay, SEND TX, delay and KEY DOWN. So if the Slave RESPONSE does not wait until the Master has KEYED DOWN you would have this kind of experience.
Using the modscan software we could see the value of each bit on a laptop. But the problem occurs when connected to the DCS