Mitsubishi MELSEC-A series, Programming Manual

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7. APPLICATION INSTRUCTIONS
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MELSEC-A
(3) Devices (D1) and (D2) must be reset before execution of the CHK instruction.
If devices (D1) and (D2) are not reset after execution of the CHK instruction, the
CHK instruction cannot be executed again. (Contents of (D1) and (D2) are
retained till they are reset by the sequence program.)
(4) Always provide pointer P254 to the head of the CHK instruction block.
(5) The CHK instruction can be written to any desired step in the sequence
program. However, it is impossible to use it at 2 or more points simultaneously.
(6) Set check condition with the LD or AND instruction before the CHK instruction.
Other contact commands cannot set check condition.
If the ANI instruction is used to set check condition, the processing about the
check condition will not be performed.
The error numbers mentioned in (8) below are assigned also to this ANI
instruction.
(7) Error check is performed in order of contact numbers. If two or more errors are
detected, error codes of high priority only are stored.
P254 X005 X009 X01A X006 X002
CHK (D1) (D2)
Check is not performed.
P254 X005 X009 X01A X006 X002
CHK (D1) (D2)
Priority High Low