Often a PLC will be used to control multiple field devices, each with its own 24V rail. For a small PLC that does not have swap-able I/O modules, generally one of these field supplies will be selected to supply power to the PLC, or a local supply will be used. I've been looking around for multiple-input PLCs that can auto-switch, but so far have just come up with external solutions involving stand-alone isolators and redundancy switchers. Is anyone aware of a PLC that can auto-select without down-time?
"a PLC will be used to control multiple field devices" ...Yes
"each with its own 24V rail" ....not a good idea..
It's unclear what you are trying to achieve, but it sounds as though it is going to cause total confusion from the maintenance point of view even if it ever gets commissioned.
"multiple-input PLCs that can auto-switch" ....what does this mean?
If you have a field device that appears 'off' - is it 'off' or has it lost it's supply?
If you want a PLC (or any other device) to be powered off dual +24vDC supplies then you can diode connect (for AC supplies use relays), but then do you want to alarm if any supply fails?
For a good design you need to know what you want and then work out what you can achieve control-wise. That's what separates Control Design Engineers from Electrical Design Engineers!
I'm not exactly sure if what you're looking for exists, but I've bought from these guys before and they have a lot of AB with product info to help you figure out what you need.
Hope this helps.