Programmable Logic Controllers - With Activities Manual and Access
Programmable Logic Controllers - With Activities Manual and Access
5th Edition
ISBN: 9781259732430
Author: Petruzella
Publisher: MCG
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Question
Chapter 9, Problem 1RQ
Program Plan Intro

a.

Master Control Reset (MCR) instructions:

  • The MCR instruction is used to clear all set outputs within the fenced zone.
  • In other words, it turns off all the non-retentive outputs in the fenced zone.
  • Non-retentive outputs cannot retain their memory when they are de-energized.
  • The MCR instruction within the zone are still scanned, but the scan time is reduced due to the false state of non-retentive outputs.

Expert Solution
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Explanation of Solution

Programming procedure:

  • In order to control a program section, two MCR output instructions are programmed.
  • The fenced zone which needs to be controlled begins with one MCR instruction and the other MCR instruction at the end.
  • An MCR rung with conditional inputs is placed at the beginning of the program section to be controlled.
  • An MCR rung with no conditional inputs is placed at the end of the program section to be controlled.
  • If the first MCR instruction becomes true, then all the outputs present in between the two MCR instructions will act according to the logic.
  • If the MCR instruction becomes false, then all the non-retentive outputs will be de-energized and all the retentive outputs will retain their previous state.

Explanation of Solution

b.

False-to-true transition:

  • When the MCR instruction makes a false-to-true transition, all rung outputs within the program section will be controlled by their respective input conditions.
  • Initially, when the MCR instruction in the rung is false, all the rungs within the zone are made inactive and de-energizes all non-retentive outputs.
  • Hence, all retentive devices such as latches will remain in their previous state.

Explanation of Solution

c.

True-to-false transition:

  • When the MCR instruction makes a true-to-false transition, all non-retentive outputs within the program section will be de-energized.
  • At the same time, all the retentive outputs within the fenced zone will remain in their previous state.
  • Initially, when the MCR instruction in the rung is true, all the rungs within the zone are scanned and the outputs are energized and updated based on their logic.
  • When the MCR instruction undergoes a transition from true-to-false, the scan ignores the input and de-energizes all the non-retentive outputs.
  • Hence, all the retentive devices like latches, timers will remain in their previous state.

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Students have asked these similar questions
A notice is recorded on VA page 30 if an instruction is denied. Why does this matter? A TLB controlled by software would operate more quickly than one controlled by hardware in the following scenarios:
What would occur if an instruction were to be disregarded? A software-managed TLB is speedier than a hardware-managed TLB in the following situations:
If an instruction is not accepted and it writes to VA page 30, what will happen? A TLB that is handled by software would perform better than a TLB that is controlled by hardware in the following situations:
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