rsdoran
Lifetime Supporting Member
JesperMP said:This nonsense has been stated several times, but that doesnt make it more true.
As if it is written in stone "though shalt not use an interposing relay".
The "purpose" of a PLC is whatever you chose to use it for.
I am going to throw my thoughts out there whether correct or not. I do not think the PLC was created to replace "THE RELAY" but to replace the issues with hard-wired relay logic.
For the most part the PLC is still in its infancy, there are (or were) issues with outputs going bad, depending on the type of card involved. You can look thru threads on here and see where people have had issues with different types of outputs with different types of loads etc. One of the biggest marketing "hypes" today is the power capability of outputs.
10 years ago I worked on converting a hardwired system to incorporate a PLC 5/20 with RIO, we had different voltages and devices that needed a higher output then the card was rated for; therefore we used interposing relays where needed.
Personally I have only designed a few things but have done 100's of retrofits, what you do depends on what is needed. I doubt, at least never have yet, used interposing relays for every output but since I deal primarily with older machines I always do not have the option to decide whether it will use multiple voltages or not but have layed out a din rail with the capability to support relays for each output if needed.
I am not familiar with plc's that have hot-swapping capabilities so I disagree that it would be just as easy to swap a card as a relay, for most plc's you need to power down i.e. shutdown the system to change a card; whereas you do not have to do that to replace a relay. There are many systems out there that do not need to be shutdown just because a "point" fails.
It was mentioned that someone only used interposing relays to "kill" output power, undoubtedly that is in a Category 0 ESTOP, what if you have a Category 1 ESTOP?
What it boils down to, in my humble opionion, is ALL of it is design options, you do what you feel is required or necessary. I doubt anyone can tell the original poster YES or NO to using them, only he can look at his system and decide if they may be required or necessary but I agree that noone should arbitrarily use them for every output.
