The self-diagnostic function diagnoses the presence of an error within the PLC function itself.
The self-diagnostic is executed at power-on, at reset, when each instruction is executed and when the END instruction is
• At power-on, at reset.
Diagnoses whether the operation can be executed.
• When each instruction is executed
An error occurs if the operation of each instruction of the sequence program could not be executed.
• When the END instruction is executed
Watchdog error supervision is performed.
Operation mode when an error is detected
There are two types of PLC function operations for when an error is detected by self-diagnosis; the operation stops or
Even if the operation is set to continue, some errors can cause operation stop with the PLC function settings. (Refer to
• If an operation-stop error is detected by the self diagnosis, the operation is stopped as soon as the error is detected,
and all the outputs (Y) turn OFF.
• If an operation-continued error is detected, the faulty program area is skipped, and the operation continues from the
Confirmation of fault record
When an operation error occurs, SM0 (self-diagnosis error) turns ON and an error code is stored in SD0 (self-diagnosis
error). Particularly when the operation is set to be continued, use in the program and to prevent a malfunction in the
For details on the fault record detected by self-diagnosis, refer to the error code list on
Operation mode when there is an operation error
The PLC function can be set to either stop or continue operation of the sequence program when an operation error occurs.
The setting of whether to stop or continue operation is set in the PLC function parameters.
• The initial status of the operation mode and PLC function status when there is an operation error are as shown in the
An error such as an
attempting BCD conversion
on a value that exceeds 0 to
9999 (or 0 to 99999999)
occurred in the sequence
PLC function status