Final answer:
In PLC programming, output devices within MCR's fenced zone are turned off during a true-to-false transition of the MCR instruction.
Step-by-step explanation:
When the MCR (Master Control Relay) instruction in a programmable logic controller (PLC) makes a true-to-false transition, all the output devices within the fenced zone controlled by that MCR are turned off. This is because the MCR's function is to control the operation of a block of outputs, usually for safety or organizational purposes. When the MCR goes from true to false, it sends a signal to all outputs in its zone to return to their off state.