Need help in understanding when to use XIC and XIO instruction

[shanky48]

Hi,
I am new to plc and i was reading the contensts in plcs.net. I have a doubt. i am not able to understand when we should go for normally open contact and normally closed contact while writing ladder logic porgram.
hope to get reply for this and thanks in advance

shanky

 

[PLCgirl]

I hope this helps...

Usage of the Examine On bit in a PLC program:
- this bit is used when you want a reaction when the hard wired switches are on, or complete a circiut. Ex: a Normally Open Limit Switch is connected to input one on the PLC module. If we want an output or an internal bit activated when the limit switch pushes close, then in the program an examine on bit is used for the limit switch I:0/1 this address or whatnact is connected to the icon that looks like two backward brackets with no line through it. This examine on bit is looking for when the switch is on, in order to complete the path for the rung. So, the limit switch is depressed activates the examine on bit that activates the load that is connected on the same rung.

Usage of the Examine off bit in a PLC program:
- Examine off bits work exactly the same way but opposite, so it all depends in what you want to happen, and what kind of hard wired switch that is being utilized. But imagine the examine off is the bit that activates when the switch is off, because it is always examining for an off.

I hope this helps .. I'll check back later .. Good luck

 

[Steve Etter]

Shanky,

While working with ladder logic inside the PLC program itself, it is probably better to NOT think of your XIO and XIC as N.O. and N.C. contacts. Rather, think of them as "Active High" and "Active Low".

For instance: A flag in your program (M3.1) is currently not satisfied. You may want an action to be allowed in this situation but not allowed if it were satisfied.

In this case, you would use an "Active Low" (XIC) in your logic:

  M3.1    M4.0

---|/|----( )-

On the other hand, you may want to allow another action only if the flag is satisfied. In that case you would use an "Active High" (XIO):

   M3.1   M4.1

---| |----( )-

This allows you to simply look at the condition of a bit and then decide under which condition is it that you want to allow your logic to be true or "Active".

If you can make the mental division, it may be best to reserve the terms "Normally Open" and "Normally Closed" for real-world I/O. And in that case, I think you will find those designations refering to the "de-energized" or "on-the-shelf" state...the state it would be in if all external power were removed.

Hope this helps.
Steve

 

[jdbrandt]

I'd have rather had them called XIT XIF

eXamine If True (see if the bit in the data table is = 1)

eXamine If False (see if the bit in the data table is = 0)

True, active, on, =1 all these terms mean the same thing.

False, idle, off, =0 "

N.O. is relative, as Steve says, to shelf condition. It
really gets ugly when you have a normally open device that is
held closed in normal duty. (I'm thinking of slack cable
switches on cranes as a good example).

PLCGrl is describing the function with relation to hardwired
inputs. These instructions are also used with memory addresses
that have nothing to do with hardware. THAT's when the n.o.
and n.c. don't make sense.

 

[barnschop]

Ladder logic was designed to assist electricians to be able to program in a method similiar to that used in electrical circuit diagrams. The only problem is that a circuit designed using relays and switches and shown on a circuit diagram will be different to a PLC ladder logic.

Siemens in the S7 manual shows a basic contactor circuit, start button, stop button and retaining contacts. The diagram is a direct translation of a circuit diagram. The stop contact on the ladder diagram is shown closed, here is where the error lies.

In the real world stop cicuits are normally closed such that a cable break will make the device stop hence be fail safe.

The contact in the ladder logic should therefore have beed shown open.

Before I create a load of replies you would in practise use a seperate emergency stop system.

I look at an open contact as a non inverted input whilst the closed contact I take as being an inverted input.

So power onto an open contact would be shown as being made when looking at it online

The opposite is true of an closed contact. Power onto the input would be shown as open when looking at it online.

You should take some time to ponder the statements above. It can take time before the penny drops and I have seen some new programmers stumble at this hurdle.

Martin