5. INTERNAL RELAYS

Internal relays are built-in functions with PLC. They have other terms widely

used: auxiliary relays, markers, bit storage and coils. Their addresses are ranging

from M0.0 to M255.7

Internal Bit Storage (Markers)

Internal relay

Resetting Latch

Motor start/stop using only ONE push button (Task3)

Latching using Set and Reset Coil, S-R and R-S Flip Flop

(Task1)

Set – Reset with Reset priority (Task4)

Set – Reset with Set priority

Master Control Relay

MCR functionality is activated by the MCRA rung.

The rungs between the MCR< and the MCR> (outputs Q4.0, Q4.1) are executed

as follows:

- I0.0 = "1" ( MCR is ON ): Q4.0 is set to "1" if I0.3 is logic "1", or will

remain unchanged if I0.3 is "0" and the logic state of I0.4 is assigned to

Q4.1.

- I0.0 = "0" ( MCR is OFF): Q4.0 remains unchanged regardless of the logic

state of I0.3 and Q4.1 is "0" regardless of the logic state of I0.4.

- In the next rung, the instruction ---(MCRD) deactivates the MCR. This

means that you cannot program any more MCR zones using the

instruction pair ---(MCR<) and ---(MCR>).

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