Issues related to MODBUS communication between systems
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Yokogawa’s DCS communicates with several subsystems, using the RS485 interface and MODBUS communication protocol. There were issues with communication with Siemens’ PLCs: the DCS sent commands to the PLCs to switch certain values. Perhaps because certain conditions were met, the DCS kept sending these commands to the PLCs, resulting in communication failures that would then be resolved only to occur again repeatedly. This continuous fulfillment of the conditions for sending commands was the cause of the problems. At first, it was thought that the issue lay in poor wiring connections; after stopping the sending of commands, the communication problems disappeared. What is the reason for this, and are there any settings that can prevent such situations from occurring again?
I think it’s a programming issue on your end; it has nothing to do with the hardware RS485 interface. It is because there may be incorrect strings in the commands you sent, which cause CRC errors and thus affect communication. But one thing is certain: the command was sent to the PLC, and the PLC executed the corresponding program.
The communication is good at times and bad at others, and I can’t figure out why
Yokogawa’s DCS communicates with several subsystems; so apart from the Siemens PLC, are all the other communications working properly?
Upstairs, you know what? There are 3 subsystems made by Siemens, and all of them are having problems. Two of them used to work fine in terms of communication, but suddenly stopped working; the connections were checked and found to be normal, there was sufficient voltage, and no changes were made to the programs. So why did it stop working? It was ignored for a while… Now, because of this fault, it’s necessary to figure out what’s going on, as certain commands are being sent. Whenever a fault occurs, the DCS signal IOP triggers a high-level alarm, which leads to continuous faults and the sending of those commands. But whenever those commands are stopped, the fault disappears. It’s not clear what’s causing this issue. Such problems didn’t occur before when these commands were sent. It’s frustrating… I have no idea how to handle communication issues at all
I have this problem too. The DCS communicates with the valve bus controller, and sometimes communication failures occur for no apparent reason, and this happens precisely when switch commands are sent. It appears intermittently; it’s hardly a clue to the cause of the problem!