How to achieve Modbus redundant communication between Siemens PLC and Yokogawa DCS
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The Siemens PLC used is S7 400, the communication card is CP341, and the DCS is Yokogawa DCS. My colleague said that if the DCS uses a hot-standby Modbus communication card (i.e., a card with two RS485 interfaces, providing redundancy), then even if I configure redundant CP341 units in the PLC using the same slave address, when receiving signals from the DCS, it will be impossible to determine whether those signals come from the DCS’s master interface or the slave interface, which leads to errors. If true redundant communication is to be achieved, then the DCS must also use two separate communication cards, rather than communication cards with redundant functions. I’m not quite sure about this; I’d like to ask everyone if this is the only solution available? There are still other ways to implement redundant communication. Thank you very much
Such redundant modules can be placed on remote racks within the control system that are not part of the controllers themselves; they receive signals from their own controllers, as determined by the redundancy strategies of those controllers. Communication between the two redundant modules is governed solely by communication protocols, and any switch in controllers does not affect the data communication and control operations of these redundant modules. If it is to be placed in the rack where the controller resides, generally speaking, due to controller redundancy, there may be a primary controller rack and a secondary controller rack. In such cases, redundant communication modules need to be installed on both the primary and secondary controllers, which means many settings are required – making the process complicated and less secure.
This post was last edited by jackgao79 on 2009-9-25 00:04. Generally, Yokogawa provides one pair of communication ports, one for each controller, to enable redundant communication; Yokogawa uses a primary and a backup setup. It can be seen in the configuration software; Siemens’ version is unclear. When I was using Siemens, it was in the steel industry, and redundancy wasn’t really a priority there. One more thing to mention: if the controller is redundant, then it doesn’t make much sense to provide two ports on a single card as a form of redundancy; perhaps it was purchased incorrectly, or one dealt with a scammer. Imagine a card that is damaged, with both of its communication ports broken. If you’re the client, ask them to make the corrections first; if you’re the contractor, just muddle along. After all, making money is what matters most these days.
Because I didn’t create the DCS, so I’m not very familiar with it either. But I know that their DCS uses MUDBUS TCP/IP for communication with other systems; however, since Siemens’ PLCs can only use MUDBUS RTU, they have installed a dedicated signal conversion station. The redundant communication module on the DCS side is located in this station. Is this what the colleague above referred to as the “non-controller remote rack”? If so, does that mean that redundancy can actually be achieved?
I haven’t heard of any redundancy in Modbus communication within the Yokogawa DCS system. Modbus communication is suitable for transmitting less important data; for parameters that are crucial and need to be sent to the DCS, hardwired connections or the more reliable PROFIBUS communication is generally used. Your PLC400 is also redundant, right? The communication card used is the CP341, so your Modbus communication card should be placed on the ET200M I/O station, not on the CPU rack. In that case, it’s not possible to achieve redundancy or hot standby for Modbus communication
Now my users say they will implement redundancy; I just need to provide two card modules. I don’t understand it, but I’ll just leave it alone. The users said they’d handle it themselves, so I’ll take a look when the time comes, hehe
I’ve encountered similar problems too; I’ll learn *it first