Beginner-LogixPro Simulator-Stop button

[g19code]

I tried some searches but I didn't find an answer. In the Door simulation in LogixPro I'm a little confused about using the Stop button in part 2. It is listed as NC switch and when I look at the data table I:1/2 has a 1 in its normal state. Using electrical logic for the NC PB switch it is closed (in state 0) and therefore it would output (1) to the input register, correct? That's what seems to be happening and, if correct, this makes sense to me at this part.

I thought this would then be a XIO in the program, since it was a NC PB. If nothing was done to the switch, it would continue to stay energized and allow the rung to stay true. If the PB was pressed, it would show the button as opened (state 1) and therefore the PB would output (0) to the input register.

In my program, and all the programs I've seen, it actually needs to be XIC in the rung to allow for a true statement in the simulation. Seems backwards. I must be missing something very basic. I have attached a pic of my program for just the opening and closing of the door in exercise #2 that allows it to work correctly, but, again, seems backward. Can someone point me in the correct direction?

 

[Lancie1]

In my program, and all the programs I've seen, it actually needs to be XIC in the rung to allow for a true statement in the simulation. Seems backwards. I must be missing something very basic. I have attached a pic of my program for just the opening and closing of the door in exercise #2 that allows it to work correctly, but, again, seems backward. Can someone point me in the correct direction?

It seems backwards, but the reason is that you (like most everyone else) have set in your mind the equality: Physical closed switch = PLC closed instruction.

That is incorrect, and only true for 1/2 of the time:
Physical normally-open switch = PLC open (XIC) instruction, BUT
Physical normally-closed switch = PLC open (XIC) instruction.

That is why they gave these instructions different names other than NO and NC. "Examine If Closed" means to output a 1 (True or enabled) if the physical switch is Closed. "Examine If Open" means to output a 1 if the physical switch is Open. Now which instruction (XIC or XIO) will give us a 1 when a Normally Closed physical switch is closed?

The key is to remember that you are dealing with two separate things: the external physical device, and the internal PLC memory. They are not the same thing.

The neumonic for the way I remember it: XIO gives an "Opposite" result.

 

[Mickey]

See link below for a good read on the XIC/XIO issue. It may help clear it up, then again it might make your head spin.

http://www.patchn.com/index.php?opt...d=58:xioxic-explained&catid=20:plc&Itemid=118

 

[g19code]

Thank you both for the quick responses. I think I have a handle on it now. I get the separation of physical and internal.

Why didn't they make it XI0 and XI1?

XI0 will be true if the bit is 0, false if 1
XI1 will be true if the bit is 1, false if 0

Or is that simplifying it too much?

 

[g19code]

Oh, XI0=XIO and XI1=XIC