Another chance for a discourse
theDave2 said:
And to make even more confusion, A/B inverts the terms sometimes and makes them relate to what the field device is doing, not what the I/O card is doing. Grrrrrrr.
Actually they do not, they just used terms that were not intuitive...ie XIC and XIO. What does XIO mean? Examine if ON, OFF, OPEN?
What we can deduce is that XIC means Examine if CLOSED and XIO should be the opposite so it should mean Examine if OPEN. These are just "inverted" representations of the same thing.
It all depends on what you need and where you need it.
If a Normally Open PB is used at input 1 with an XIC then the XIC will be true when the PB is pressed and allow rung continuity. An example is a Start Pushbutton.
A normally closed device, like a STOP button, will provide a signal when the pushbutton is NOT pressed and the use of an XIC will provide continuity in the rung UNTIL the PB is pressed.
Why would you need XIO? In the STOP PB situation you needed the ALWAYS ON condition to provide continuity to the rungs involved. Lets say you have a prox sensor with a normally closed output...ie with nothing in front of the prox it is ALWAYS ON. This prox could be used for counting product etc etc, now you need to KNOW when the INPUT signal is OFF...ie the prox senses product and the output turns off...NOW you need the rung to have continuity when the prox input is OFF (0) so would use an XIO.
NC prox
|----|/|------------|counter|--|
In this case when the prox senses a product the Normally closed contact will open and NO signal will be present at the PLC input..ie a 0 will be present in the input table. This in turn will make the XIO instruction be ON (True) which allows continuity in the rung...ie the counter will count.
With the STOP PB you needed to disallow rung continuity when it was OFF. With the prox you needed to allow continuity when it was OFF.
In either case of buttons, sensors, switches and using NPN/PNP devices the "logic" can be inverted and make the code work...the issue is that doing so can make it confusing for others to understand.
Its natural for us to think...the input comes ON then the instruction comes ON and action occurs..WHY? Because we do this everyday, flip a light switch and the light comes ON..ie flip switch, power (signal) goes to bulb and it turns ON. Its not intuitive to think in another fashion so inverted logic can make things confusing.
In general what matters is understanding what you need and how to use it...there have many situations where the wrong device was put on a machine...operator/mechanic error, correct device not available etc where I have just inverted the logic in the PLC.
I will not go into the relay analogy but you can read it here:
http://www.patchn.com/xio_xic_explained.htm