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20-12

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

C79000-G7076-C504-02

20.6 Master Control Relay On/Off

The Master Control Relay On (MCR<) instruction triggers an operation that

pushes the RLO to the MCR stack and opens an MCR zone. The instructions

shown in Table 20-4 are influenced by the RLO that is pushed to the RLO

stack when the MCR zone is opened. The MCR stack works like a LIFO

(Last In, First Out) buffer. Only eight entries are possible. If the stack is

already full, the Master Control Relay On instruction produces an MCR stack

fault (MCRF).

Table 20-8 Master Control Relay On Element

LAD Element Parameter Data Type Memory Area Description

MCR<

None – – Opens an MCR zone

The Master Control Relay Off (MCR>) instruction closes the MCR zone that

was opened last. The instruction does this by removing the RLO entry from

the MCR stack. The RLO was pushed there by the Master Control Relay On

instruction. The entry released at the other end of the LIFO (Last In, First

Out) MCR stack is set to 1. If the stack is already empty, the Master Control

Relay Off instruction produces an MCR stack fault (MCRF).

Table 20-9 Master Control Relay Off Element

LAD Element Parameter Data Type Memory Area Description

MCR>

None – –

Closes the MCR zone that was

opened last

The MCR is controlled by a stack which is one bit wide and eight entries

deep (see Figure 20-7). The MCR is activated until all eight entries in the

stack are equal to 1. The instruction ––(MCR<) copies the RLO to the MCR

stack. The instruction ––(MCR>) removes the last entry from the stack and

sets the released stack address to 1. If an error occurs – e.g. if more than

eight ––(MCR>) instructions follow one another, or you attempt to execute

the instruction ––(MCR>) when the stack is empty – this error activates the

MCRF error message. The monitoring of the MCR stack follows the stack

pointer (MSP: 0 = empty, 1 = one entry, 2 = two entries, ..., 8 = eight entries).

MCR On

MCR Off

Program Control Instructions

1 2 ... 241 242 243 244 245 246 247 248 249 250 251 ... 317 318

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