The 8 Results of 2 Digital Input Commands

XIC and XIO: For you newer PLC programmers, if you get it clear in your mind about how the two common digital input commands work, (XIC and XIO for Allen-Bradley RSLogix users), then you will be well ahead of the learning curve. There are at least 8 possibilities, if you consider whether the physical device circuit power is on or off. I made up a chart to show them.

And people complain that Siemens is complicated!!!

randylud,
I see your point about the "power off" states not being important. Most of the time they are not, but I think it is wise to at least be aware of those other four states. Remember too, I said ".. at least 8 possibilities". What about the conditions when you use subroutines or state relays? Then you could double the number of possible output states to 16!

I do remember several times troubleshooting a circuit in a larger plant, and looking at a XIO bit, and saying, "Hey, that PLC input bit should be off", and then spending a good deal of time checking the logic and the PLC to make sure it was not faulted and not offline, then finally checking the control power for that circuit and finding a blown fuse for that input. With a blown fuse or a disconnected wire, an XIO bit will be on, even if it should be off when the control power is on. (See the last two lines in my table). Sometimes it does matter about those other 4 states.

RMA,

How are the equivalent two inputs for Siemens any different, or any less complicated? In my experience with Siemens, they work exactly the same.

The logic may not be any different, but the mnemonics are much more easily understandable. XIC and XIO must be the nadir of all mnemonics!

RMA,

I checked my Siemens ladder, and it calls them "Normally Open" and "Normally Closed". The NO is okay, but the NC is a misnomer, a paradox. If you didn't know how it worked, the name would mislead you. I think that why Allen-Bradley chose different names was to emphazise that the PLC Input bit is NOT the same thing as the physical device. What does a "Normally Closed" input say to Siemens users?

In hindsight I am sure Rockwell would pick different ones if it didn't mean a largely unecessary product change, but they go back right to the earliest PLC's in the 70's and it really isn't at all complicated. I just think of them as:

XIC = eXamine If Closed = True at Logic = 1

XIO = eXamine IF Open = True at Logic = 0

The single biggest hurdle that most beginners need to leap over is that a PLC processor solves rungs according to flow of logical TRUTH, whereas relays solve according to the flow of electricity.

This is easy as long as you always use positive logic, but PLC's allow negative logic as well and this just does not have a physical equivalent in the electrical world. This is very like what happens when most students first encounter imaginary numbers in the world of maths (sqrt -1).

Having started the subject grounded in a physical model (ie real numbers/electrical relays) there is a moment of floundering when the model becomes imaginary. Most folk make the leap, some don't.

Same with me Phillip, once I saw the light, it was never a problem again! It is the same way for me with load/load not. And we get paid for this too!

There is another even simpler way to look at it. Think of EVERY Boolean flag in the PLC as a relay coil.

If the Boolean is set to 1 the coil is energised.

If the Boolean is set to 0 the coil is de-energised.

Now imagine every one of these relays has an unlimited number of NO and NC contacts attached to it, and you can use as many of them as you need wherever you like in the program.

In the AB world an NO contact is called XIC, an NC contact is called XIO.

PhilipW said:

There is another even simpler way to look at it. Think of EVERY Boolean flag in the PLC as a relay coil.

If the Boolean is set to 1 the coil is energised.

If the Boolean is set to 0 the coil is de-energised.

Now imagine every one of these relays has an unlimited number of NO and NC contacts attached to it, and you can use as many of them as you need wherever you like in the program.

Click to expand...

That's exactly the way I explain it around here. To get the older guys of the dime I explain the PLC (aka mystrey box), I simply represent it as a "relay replacer" w/math capabilities. For the most part it seems the most effective approch.

Makes sense to me and that is why I love this stuff! It is also why it drives some people crazy! HeHeHe, job security?

What does a "Normally Closed" input say to Siemens users?

Click to expand...

I dont see how its a misnomer, the contacts in a plc work just as they would with a relay. Regardless of how many states "YOU CREATE" using the instruction(s), it still boils down to is the input ON or OFF.

Terry Woods did an explanation of the AB instructions XIO and XIC and states it was a dumb idea.
http://www.patchn.com/xio_xic_explained.htm