PLC multi linking to DCS

[Smilymiley]

We are currently supplying a client with 2 PLC's, one for system management and one for system control. The control PLC incorporated all the shutdown signals. Our client is requesting that we serial link the 2 PLC's together with an onward serial link to their overall control DCS system. Serial linking will be done through a Modbus RTU RS485. I am not in favour of this as I see it will create a 'common mode failure point' which will impact on the safety of the system. I would prefer it if each PLC were linked directly to my clients DCS but I don't have the experience to determine the Pro's and con's of each system. Is there anyone who can help me with this?

Regards

SmilyMiley

 

[Jmancan]

If your just using the RS485 link to collect and monitor data from the DCS you shouldn't have a problem. However, if it is used to do critical control (such as safety or interlocks) you probably shouldn't use it at all and just stick with discrete wiring.

On another note, RS485 is pretty reliable if installed correctly but it's always a good idea to put in some communication monitoring logic to monitor communications between controllers and DCS (like programming a heartbeat signal to generate an alarm and put things in a safe state). This should take out the worries but I don't see a problem with the clients request depending on the type of control that is being done.

 

[Smilymiley]

Thanks Jmancan. Lead you up the garden path there a bit. All critical control is to be hardwired.
Just one more point, What would be the position with the time delays that this setup will introduce?
Isn't there a chance that events may be recorded 'out of sequence' if the PLC's are wired in series?

Smily Miley

 

[bmsg]

I do not know your system configuration details, but there are other options for you, I think.
One is to link up the PLCs and DCS over an Ethernet TCP/IP bus (the link can be made redundant, if there ought to not be a single point of failure in the communication link). In practice, this link is realised through an Ethernet switch. Data communication may be achieved using OPC client-server technology. This is a cheap and industrially acceptable solution.
RS-485 communication - in my experience - is rather clumsy, as far as speed and data integrity is concerned.