how to turn off

I can turn on a coil in the plc simulator. How do I later turn it off using a different switch?

You can connect the OFF switch in series to turn the coil off. something like this...

  

 

       ON Switch             OFF Switch          Coil

        I/0                  I/1                 O/0

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

 |                 |

 |      O/0        |

 |     Coil        |

 |------| |--------|

 

Rohan's logic is correct for two n/o switches.

In the real world the stop pushbutton will be n/c, therefore both contacts in the PLC logic will be n/o.

PeterW said:

Rohan's logic is correct for two n/o switches.

In the real world the stop pushbutton will be n/c, therefore both contacts in the PLC logic will be n/o.

Click to expand...

Not necessarily. You have to know what your hardware is, then program accordingly.

Or, in the infamous words of Ron Beaufort (if I am learning everything correctly from him - man I need to take his classes one day), what you really want to do is to "write" a zero to the bit at the instruction address associated with the "coil" (not really a coil, just an address).


Don't limit yourself to the ways that were suggested. Even though they are correct, there are many other ways to accomplish the same thing. Please keep in mind that I am mostly familiar with the SLC500 processor and these ideas might not work on all other lines of processors.

You can program an UNLATCH at that address.
You can MOV a zero into that word.
You can COP a zero into that word.
You can use a MCR if they are non-retentive outputs.

All I am trying to say is that there are many ways to write a zero into the address associated with that instruction. I am sure that Ron (and others) will
1) correct what I said that is wrong and
2) tell in much more detail what is going on

There are many people here that know much more than I do. I'm just giving you some food for thought.


EDIT: OK after re-reading the OP, the previous suggestions are more in line with what you want to do.

OZEE said:

Not necessarily. You have to know what your hardware is, then program accordingly.

Click to expand...

I beg to differ here unless you are perhaps talking HMI signals. Stop = n/c is I thought a universal standard, at least it is in the UK and I have never seen a stop push button wired as normally open in 30 years.

OZEE said:

Not necessarily. You have to know what your hardware is, then program accordingly.

Click to expand...

I agree with OZEE, the logic should match the hardware. The hardware in this case is the PLC simulator ... Besides, a stop button is almost always NC but an input to turn off a coil need not be a NC contact.

rohan tandon said:

I agree with OZEE, the logic should match the hardware. The hardware in this case is the PLC simulator ... Besides, a stop button is almost always NC but an input to turn off a coil need not be a NC contact.

Click to expand...

I recently work on a multi-million pound project, where on-site we had to re-program all the stop push-buttons, simply because it had been 'simulated' and the simulator designer made all stop buttons n/o.

I agree, if the hardware gives a certain signal then you have to program to suit, that is obvious.

In this case, even if someone is simulating, I believe a new learner should be taught along the lines of what the actual hardware is likely to be.

Industrial standard control hardware will in almost all cases be n/c for a stop push-button.

I remember years ago, being given a test on an interview, where I had to review some code to show I knew my stuff, one of the 'deliberate mistakes' they made to catch people out, was to code the stop push-button as a n/o button.

I don't disagree that the "standard" is for a stop button to be n/c. BUT never assume that it is! That was my point.

This "standard" goes back to doing start/stops with hardwired controls, or hardwired relay logic. But with PLCs n/c is NOT required for a stop button.