Statement List (STL) - DCE FEL ČVUT v Praze

10.14 MCR (Master Control Relay)
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Important Notes on Using MCR Functions
Program Control Instructions
Warning
To prevent personal injury or property damage, never use the MCR to replace a
hard-wired mechanical master control relay for an emergency stop function.
Description
The Master Control Relay (MCR) is a relay ladder logic master switch for energizing
and de-energizing power flow. Instructions triggered by the following bit logic and
transfer instructions are dependent on the MCR:
• =
• S
• R
• T , T , T
The T instruction, used with byte, word, and double word, writes a 0 to the memory if
the MCR is 0. The S and R instructions leave the existing value unchanged. The
instruction = writes "0" in the addressed bit.
Instructions dependent on MCR and their reactions to the signal state of the MCR
Signal State
of MCR
= S , R T , T
T
0 ("OFF") Writes 0.
Does not write. Writes 0.
(Imitates a relay that (Imitates a relay that (Imitates a component
falls to its quiet state remains in its current that produces a value
when voltage is state when voltage is of 0 when voltage is
removed.)
removed.)
removed.)
1 ("ON") Normal processing Normal processing Normal processing
MCR( - Begin MCR Area, )MCR - End MCR Area
The MCR is controlled by a stack one bit wide and eight bits deep. The MCR is
energized as long as all eight entries are equal to 1. The MCR( instruction copies the
RLO bit into the MCR stack. The )MCR instruction removes the last entry from the
stack and sets the vacated position to 1. MCR( and )MCR instructions must always
be used in pairs. A fault, that is, if there are more than eight consecutive MCR(
instructions or an attempt is made to execute an )MCR instruction when the MCR
stack is empty, triggers the MCRF error message.
Statement List (STL) for S7-300 and S7-400 Programming
A5E00706960-01 10-17