PLC multi linking to DCS
PLC multi linking to DCS
(OP)
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
Regards
SmilyMiley





RE: PLC multi linking to DCS
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.
RE: PLC multi linking to DCS
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
RE: PLC multi linking to DCS
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.