Isolated Modules?

When designing a Control Logix system why would I need isolated AC or DC input cards? What situations are these useful in?

Also in the DC input cards seems like I can get them with isolated or diagnostic but not both. I know the diagnostic helps with open wire detection and more by why don't they have an isolated module with the diagnostics?

I can't answer the second question, but as for the first, consider the following (by way of example):

Let's assume that we have a 600V motor operated valve. Internal to the actuator, there is a 120V transformer for the discrete controls -- local/remote, open/close etc. ("Field powered")

Let's also assume that we have a couple of pressure switches, and some limit switches on other valves; these are generally dry-contact devices that require power from a power supply, or lighting or UPS circuit. ("System Powered")

A non-isolated card shares -Ve/Com, and if you wire all of these devices to a non-isolated card, a problem exists because the commons are at different potentials. However, if you wire ONLY the "field powered" or ONLY the "system powered," it would be fine (as long as the "system powered" devices are supplied by the same circuit/power supply/etc).

Input channels on an isolated card do not share commons, so one can have different voltage levels on each channel.

Isolated input and outputs have a optocpoupler in their electrical path.

An optocoupler is a LED and a phototransistor together. The LED shines on the phototransistor which then passes current trough it.

That means that it is only light that completes the connection. In the case of inputs it doesn't matter what voltage or what potential that voltage has as long as it can power the LED of the optocoupler. Any surges, spikes or whatever will not reach beyond the LED.

Even if you put 220VAC on a 24VDC input you will probably destroy that input (the LED) but nothing will happen to the rest of the inputs or outputs. If the inputs or outputs are not isolated then anything could happen since all non-isolated inputs and outputs are electrically interconnected.

PS. Looking at the ControlLogix I/O modules they all seems to be isolated (and they should be). But there are modules that are individually isolated and that is perhaps what's confusing. To save input terminals it is common to hook up the common/ground/0V to more than one isolated input. Individually isolated have two terminals per input and no interconnection to any other input. So each and every input could have a different potential to any other input.

This was the one I looked at: http://literature.rockwellautomation.com/idc/groups/literature/documents/td/1756-td002_-en-e.pdf

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Isolated input modules are used when an inputs on the same module have a completely different power source. For example two machines with their own 24vdc power supply. An isolated input would have separate input and negative input. Same for AC inputs with a different common. Most of the time a PLC is on one system with it's own power so isolated inputs are not required. Mostly you see them used in interlocking IO from different systems. But these days that is usually done over communications.

All AB inputs use an opto isolation to protect the micro circuitry from the higher power IO, but this is not what an isolated input module is.

JaxGTO said:

All AB inputs use an opto isolation to protect the micro circuitry from the higher power IO, but this is not what an isolated input module is.

Click to expand...

Please, don't misuse the word isolated. If there is an optocoupler on the input, it is an isolated input, end of story.

What you are talking about is what AB calls their "Individually Isolated" modules. "Individually" being the key here.

So the diagnostic modules are isolated too, but every group of 4 inputs have their common/ground tied together.
So you can use this and hook up to completely different voltage supplies as well, but you have to keep the same potential / voltage supply in a group of four inputs.

PS. Sorry if I seemed all worked up about this but it's a pet peeve of mine. I've worked as an electronics designer on embedded control systems and I'm always surprised how many electrical engineers out there don't know what an isolated input is and how to hook them up - usually resulting in rows of relays that are completely unneeded.