Question?

What effect does a MCR zone have on the outputs in that zone?

Robmo

When the MCR is activated, the rung it is in goes false, the outputs inside the MCR zone are all deactivated if they are no retentive. Meaning if they are not latched on by some means. And what question number is that on your homework, quiz, or test? I bet the answer is in your text. It is in the one I use to teach PLCs.

I must be stressed out or something, many of the replies do not make sense to me. I think this will be my last post for a while.

When the MCR is activated, the rung it is in goes false, the outputs inside the MCR zone are all deactivated if they are no retentive.

Click to expand...

My understanding of MCR is that any rung in the MCR "zone" will not be active until the MCR is ON.

If this is AB then the MCR must contain 2 MCR instructions, one to turn the "zone" on and one to turn it off. When the MCR is active all the outputs can be active, when the MCR is not active all the non-retentive outputs will be inactive...this means that if the MCR is OFF then any rung in the MCR zone may be "true" but the output will be OFF.

My apologies if I have made any statements that may have offended anyone.

Just goes to show you what stress will do to you, Ron. My explanation is about how the AB MCR works and there may be some that operate in the opposite manner. The MCR instruction is meant to mimic the old MCR relay, so that when you drop the MCR relay, you get the action you are after. When the MRC relay is on, everything operates like it does normally. SEE NOW?

i dont know Randy but i would say Ron hit it spot on...

From rs5000 on line help...

The MCR instruction, used in pairs, creates a program zone that can disable all rungs within the MCR instructions.
When the MCR zone is enabled, the rungs in the MCR zone are scanned for normal true or false conditions. When disabled, the controller still scans rungs within an MCR zone, but scan time is reduced because all the outputs in the zone are disabled.
When the MCR zone is disabled, the rung-condition-in is false for all the instructions inside of the MCR zone.
When you program an MCR zone, note that:
§ You must end the zone with an unconditional MCR instruction.
§ You cannot nest one MCR zone within another.
§ Do not jump into an MCR zone. If the zone is false, jumping into the zone activates the zone from the point to which you jumped to the end of the zone.
§ If an MCR zone continues to the end of the routine, you do not have to program an MCR instruction to end the zone.
Important: The MCR instruction is not a substitute for a hard-wired master control relay that provides emergency-stop capability. You should still install a hard-wired master control relay to provide emergency I/O power shutdown.
Do not overlap or nest MCR zones. Each MCR zone must be separate and complete. If they overlap or nest, unpredictable machine operation could occur with possible damage to equipment or injury to personnel.Place critical operations outside the MCR zone. If you start instructions such as timers in a MCR zone, instruction execution stops when the zone is disabled and the timer is cleared.

D

I know I am stressed now.

randylud said:

Just goes to show you what stress will do to you, Ron. My explanation is about how the AB MCR works and there may be some that operate in the opposite manner. The MCR instruction is meant to mimic the old MCR relay, so that when you drop the MCR relay, you get the action you are after. When the MRC relay is on, everything operates like it does normally. SEE NOW?

Click to expand...

To me an MCR is a master control relay, the controls can not operate until the MCR is turned ON. I have rarely used the MCR instruction but it follows that same principle. If you drop an MCR, in my experience, nothing wired behind it will work.

With AB you must have 2 MCR instructions to create a "zone", when the MCR is "true" the outputs in that zone can be active, when the MCR is "false" then ALL outputs in that zone are false, even if the rung shows "true". This allows you to turn OFF a group of outputs at the same time for the same duration.....didnt I just say that?

Randy this is the statement that confused me...like that is hard to do.

When the MCR is activated, the rung it is in goes false,

Click to expand...

That doesnt seem to follow the principle of an MCR.

Yes, Ron, you just said that....
and that is how I've always used an MCR, and I ain't blown anything up..(lately).

rsdoran said:

Randy this is the statement that confused me...like that is hard to do.

That doesnt seem to follow the principle of an MCR.

Click to expand...

A better way to state this is, anything between the MCR instructions won't happen.

Semantics

Ron and Randy are correctly describing the actions of the MCR zone but using slightly different terminology.
Randy says the "active" state of the zone is when the MCR rung is false and outputs are turned off.
Ron says the "active" state of the zone is when the MCR rung is true and outputs are enabled.

For my part, I prefer to think
enabled = active
disabled = inactive

So, it may be a matter of semantics that I say the MCR is active when the rung goes false

Click to expand...

That may be true in a way but its misleading, to me anyway, because if you are viewing the program the MCR must be ON for the outputs in that zone to work.

Personally the use of MCR as an instruction is a misleading concept to begin with...a hardwired MCR turns ALL CONTROL POWER OFF....zone control (ZCR) or anything like that would have made more sense.

It's like this: MCR is a Master Control Relay. Anything between the MCR and MSL (or whatever instructing ends the section), is totally dependent on the condition (on or off) of the MCR coil.

For example:

    A       B

----||------||---------(MCR)

 

     C   

-----||-----------------(output 1)

 

     D

-----||-----------------(output 2)

 

------------------------(MLR)

Contacts A & B are required to be ON for output 1 and output 2 to be ON.

So for output 1 to be ON, alll three contacts (A, B, & C) have to be on.

For output 2 to be ON, all thee contacts (A, B, & D) have to be on.

Rather simple concept...

To add more fuel to this fire, when the MCR in the LogixPro (RSLogix Simulator program) is ON, Energized, =1, then the program puts a little yellow "Active" right above the MCR. From this perspective, Ron is correct and Randy is incorrect in the semantics. I agree with Ron and the LogixPro program. If a relay is energized, then it is "active", no matter what it does elsewhere.

I think I understand what Randy is trying to say:

What Randy is saying is that when the MCR rung is true, all the rungs in the zone are scanned normally, just as though the MCR did not even exist - you could even delete it (for the time that it is true) from the program and nothing would change. However, when it is false all the rungs in the zone have to be scanned in a special way, they are scanned as false - regardless of the logical state of the bits in the rungs.

But to think of it in that way is confusing for most people. Ron's way is much more intuitive.