Mitsubishi Electronics manual 87

9LOADING AND INSTALLATION

(3)Fail-safe measures against failure of the PLC

MELSEC-Q

Failure of a CPU module or memory can be detected by the self-diagnosis function. However, failure of I/O control area may not be detected by the CPU module.

In such cases, all I/O points turn ON or OFF depending on a condition of problem, and normal operating conditions and operating safety cannot sometimes be maintained.

Though Mitsubishi PLCs are manufactured under strict quality control, they may cause failure or abnormal operations due to unspecific reasons. To prevent the abnormal operation of the whole system, machine breakdown, and accidents, fail-safe circuitry against failure of the PLC must be constructed outside the PLC. Examples of a system and its fail-safe circuitry are described below:

<System example>

Power supply module CPU module Input 16 points Input 16 points Input 16 points Input 16 points Output 16 points Power supply module Output 16 points Output 16 points Output 16 points Output 16 points Y80 to Y8F Vacant

Output module for fail-safe purpose 1

1: The output module for fail-safe purpose should be loaded in the last slot of the system. (Y80 to Y8F in the above system.)

<Fail-safe circuit example>

ON delay timer

Internal program

Y80

T1

1s

SM412

OFF delay timer

Y80

T2

1s

External load

MC

Y81

Y80

L

Y8F

L

24V

0.5s

0.5s

0V

-

+

CPU module

24V DC

Output module

T1

T2

2

MC

2: Y80 repeats turning ON and then OFF at 0.5s intervals.

Use a no-contact output module (transistor in the example shown above).

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