PLC and then RTU takes control

Charbel

Member
Join Date
Jan 2012
Location
Beirut
Posts
307
Dear,
I was looking at design requirement it says the following "In normal operation the PLC must control the station. When the PLC fails the RTU must take over control seamlessly"

Practically what does it mean? I understand that all the I/O points will be connected to the PLC and then PLC will communicate to the RTU the alarms.

What they are saying is that looks like I/O are also connected to the RTU without passing by the PLC? does this require configuration that I/O's are connected to both PLC and RTU? I really got confused.

Thanks a lot for any feedback on this issue,

charbel
 
Dear,
I was looking at design requirement it says the following "In normal operation the PLC must control the station. When the PLC fails the RTU must take over control seamlessly"

Practically what does it mean? I understand that all the I/O points will be connected to the PLC and then PLC will communicate to the RTU the alarms.

What they are saying is that looks like I/O are also connected to the RTU without passing by the PLC? does this require configuration that I/O's are connected to both PLC and RTU? I really got confused.

Thanks a lot for any feedback on this issue,

charbel
Sounds that way to me. They appear to be looking for redundancy in the control design, using separate systems. That only way to accomplish that will be with either redundant I/O, or a Local PLC that can use the same separate local I/O rack as what the RTU uses. That is possible, for example by using something like FlexI/O or PointI/O from Rockwell or ET200/300 I/O from Siemens talking to a local PLC and an RTU system that is compatible at the same time.
 
Redundancy on two different devices is just double the work. They should either do two RTU's and not even have a PLC (if you have to replicate logic on the RTU anyway) or a redundant PLC setup (which just about everyone offers out of the box with no extra work on your part).
 
Some IO systems provide this functionality as standard but it is not often used. In the case of Rockwell you should look at "Device Logix" which will do what you want. With a Compact Block IO module on DeviceNet the IO module can have its own simple PLC type programme which can be set to run if the PLC connection is lost.
 

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