Introduction to MCR Zones
MCR Zones are difficult for some people to understand. MCR stands for “Master Control Reset”. In this post, I’ll emphasize the common misconceptions about MCR Zones, and show examples different MCR States. It’s important to realize that MCR Zones do not necessarily shut off all outputs in a disabled zone. It’s equally important to know that the processor does not “skip over” a disabled MCR Zone. When we enable the MCR Zone, the logic operates normally. When we disable the zone, the rungs are evaluated as false.
For this example, I will just use a fan with an oscillator. It might not be the most practical example, but it’s an example that anyone can visualize easily. The purpose of this example is just to show the MCR Zone operation when using standard OTE’s vs. the Latch/Unlatch instructions.
MCR Enabled
Here we have an enabled MCR Zone. In effect, the logic will operate normally. Notice we have a conditional MCR instruction at the beginning of the zone, and an unconditional MCR instruction to end the zone.
Notice that on rung #1, the “OscillateFan” bit is on a standard OTE. Rungs 2 and 3 are to latch and unlatch the “CoolingFan” bit.
MCR Disabled
Let’s disable the MCR Zone as-is, and see what happens!
We have disabled the MCR Zone. As a result, the processor evaluates all rungs as false. Even though the OscillationSwitch is still on, the OscillateFan bit is shut off. As I have said, not all of the outputs shut off. The rungs are simply evaluated as false. Outputs that are retentive, such as the CoolingFan remain on. When the Latch instruction is evaluated as false, the CoolingFan bit remains in it’s last state (which is on!).
In another case, let’s look at what happens when we try to shut off the cooling fan.
Even though, the StopCoolingFan bit is high, the Cooling fan does not shut off. In this case, with the MCR Zone disabled, we have no control of the CoolingFan. Because both the latch and unlatch are inside of the MCR Zone, and latch/unlatch is retentive, the Cooling fan remains in it’s last state.
Re-Enable the MCR
Finally, let’s re-enable the MCR Zone. Notice that all logic operates normally.
Notice the OscillateFan bit is re-energized, and the CoolingFan has shut off.
MCR Zones are generally used to disable certain sections of equipment when not in use. Just remember the zone must start with a conditional MCR and end with an unconditional MCR. Never use the JMP instruction to Jump into an MCR Zone. Unpredictable operation could result! Also remember that MCR is not a substitute for a Master Control Relay. Under certain conditions, you must drop actual control power to disable equipment. Always remember that PLC outputs can fail open or shut at any time.
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— Ricky Bryce