I am provinding my client with 2 PLCs, one is a management system the other a control system containing all the shutdown signals for our part of the project. My client is requesting that we connect both our PLCs in series through a Modbus RTU RS485 serial link and then onward to their 'overall control' DCS system. I feel that this is not the safest way to connect the systems up as it will introduce a common mode failure point and would prefer to seperately connect each PLC to the DCS.
What I am looking for are the pros and cons of each method to enable me to confidently respond to my client's request.

 

I'm not sure safety would be my first concern, I'd wonder about throughput, but you don't provide much detail about the volume of data to be handled.

Modbus over RS-485 is known as multi-drop, meaning that all the units can be connected on a bus, or wired in parallel (not series) with one another. (while it might look like a series string the wiring is in parallel, A to A to A to A, B to B to B to B) Some call this daisy chaining: A to B to C to D and so on.

The alternative is 2 separate RS-485 lines: wiring A to B and, separately, A to C. This mode requires a separate COM port on the master (A in the example) for each line.

Throughput must take into account the number of devices on a multidrop bus and the volume for each, because any given link is still Modbus, master slave, where the master waits for a slave's response.

A COM port for each PLC probably would likely provide a greater throughput, because each line runs independent of the other. Each PLC can be 'read' at the same time, which cannot happen on a multidrop mode.

As far as reliability, I am a firm believer in RS-485 isolators to protect the COM ports on each device. Isolators are the best insurance I've ever invested in.

 

Miles,

What about doing it as a multi-drop still using the same Modbus 485 link, there's still a single point of failure, but not at your end ! Maybe you should also think about providing the shutdown signals seperately, hard-wired ?

Neil Munro