Plc

Allen Bradley used to be 'Examine if On' and 'Examine if Off'. It can be confusing the first time you see a physical 'closed contact' programmed in a PLC as 'normally open'.
 
It is best not to think of PLC instructions as NO or NC contacts.

Confusion to the beginner can be caused when a -||- PLC instruction is coming from a Normally-Closed contact in the field device, and vice-versa.

In PLC ladder language, we only want to know if the addressed memory location is on (use -| |-), or off (use -|/|-).

Once you get used to the concept that -| |- and -|/|- have absolutely nothing to do with Normally-Open and Normally-Closed field or internal memory locations it all becomes clear.

IMHO it was very wise that Allen-Bradley adopted XIC (eXamine If Closed) and XIO (eXamine If Open) as the names for their Boolean logic conditional instructions.
 
I have worked with engineers that did not understand that emergency stop and limit switch circuits are programmed XIC but wired normally closed. Differentiating between internal logic and external wiring is important.
 
if you think as 24VDC as logic 1, which it is, then NO is closed when activated (high, 24VDC) which means when logic 1, and NC is closed when deactivated (low, no voltage) which means logic 0.
what's there to confuse?
 
balash said:
if you think as 24VDC as logic 1, which it is, then NO is closed when activated (high, 24VDC) which means when logic 1, and NC is closed when deactivated (low, no voltage) which means logic 0.
what's there to confuse?
Click to expand...
becuase the way you use the outside no and nc contacts for contol purposes without a plc differ than what you see on a plc when using the exact same contact, stop and estop circuits are a good example.
 
yea that could be a problem if you think that hardware NO must match logic NO (which, IMO, is wrong in the beginning cause that's two separate systems, at least to me.)
 
I understand it ok but I have been doing this foe a long time, during that time I have seen several new people to PLC programming get confused when using a NC contact in a field device but use a xic instruction in the program such as a stop button. Yes it is easy enough to get your head around but when communicating to other people it does save confusion if talking in XIC and XIO terms. but I also agree it can be just as simple to refer to it as high and low or on and off.
 

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