interlock program in PLC

Hi experts,

Sorry for about my question, may be it's simple for you, but I would like to learn from experts in this site.
My question is about "interlock" program in the ladder logic.
Can somebody explain me what the meaning, function, and if possible give me a sample using RSLogix500.
Thanks in advance.

Deer

Interlock

Interlocks are conditions that must be TRUE in order for a particular output to be allowed to be energize.

Standard motor circuit:

   START       STOP                     PUMP1

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

           |

   PUMP1   |

----| |----+

`
But suppose we never want to dead-head the pump (run the pump without someplace for the fluid to go), and there are two valves, either of which can block the flow.

And we never want to run the pump if there is an E-stop (even though the E-stop will be wired to an MCR (master control relay), it's still better to be safe than sorry.)

And we don't want to run the pump if the high level switch is made.

And we don't want to run the pump, when pump 2 is running (because they both feed the same pipe, and the pressure would be too great).

All of those become the interlock for the pump.

   START       STOP     VALVE_A     ESTOP   HI_LEVEL  PUMP2     PUMP1

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

           |          |         | 

   PUMP1   |          | VALVE_B |

----| |----+          +---| |---+

Note that the presence of the interlock conditions do not cause the pump to start - they just prevent it from starting if they aren't there.

The term interlock orginialy started from mechanical inter-locks, where one object would literally lock another object in place if the first one was moving, and vice versa. The term has broadened to include any prohibitive condition.

You can understand the "inter-" part of the term when you realize that the code for pump2 would be the same as pump1, except that the pump1 contact would be the interlock instead of pump2

Some programmers (including me) take all the interlock conditions and have them power a single coil. Whether you program that coil such that -|/|- or -| |- is used on the rung is a matter of personal taste. It's also the source of much semantic confusion:

"Pump1 is interlocked with pump2" - Does this men that you can't run pump1 if pump2 is running, or do you mean that you can't run pump1 until pump2 is running?

"Right now, valve1 is Interlocked" - Does this mean that there is a condition preventing it from openning, or that it's OK to open it?

When you are working with someone on the interlock list, make sure you both understand which way you are talking.