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20-8

Ladder Logic (LAD) for S7-300 and S7-400

C79000-G7076-C504-02

20.4 Master Control Relay Instructions

The Master Control Relay (MCR, see also Section 20.5) is an American relay

ladder logic master switch for energizing and de-energizing power flow

(current path). A de-energized current path corresponds to an instruction

sequence that writes a zero value instead of the calculated value, or, to an

instruction sequence that leaves the existing memory value unchanged.

Operations triggered by the instructions shown in Table 20-4 are dependent

on the MCR.

The Output Coil and Midline Output instructions write a 0 to the memory if

the MCR is 0. The Set Coil and Reset Coil instructions leave the existing

value unchanged (see Table 20-5).

Table 20-4 Instructions Influenced by an MCR Zone

Element or Name in Box Instruction Name Section in This

Manual

Midline Output 8.5

Output Coil 8.4

Set Coil 8.8

Reset Coil 8.9

Set_Reset Flipflop 8.22

Reset_Set Flipflop 8.23

MOVE

Assign a Value 14.1

Table 20-5 Operations Dependent on MCR and How They React to Its Signal State

Signal State of

MCR

Output Coil or

Midline Output

Sector Set or Reset

SR RS

Assign a Value

MOVE

Writes 0

(Imitates a relay that falls to its

quiet state when voltage is

removed)

Does not write

(Imitates a latching relay that

remains in its current state

when voltage is removed)

Writes 0

(Imitates a component that, on

loss of voltage, produces a

value of 0)

1 Normal execution Normal execution Normal execution

Definition of

Master Control

Relay

Program Control Instructions

1 2 ... 237 238 239 240 241 242 243 244 245 246 247 ... 317 318

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