Mitsubishi Electric MELSEC Q Series Reference Manual page 546

(2) When a programming tool connected station is faulty
Control station
No.1
Normal station
No.4
544
Normal station
No.2
Normal Selected
station
station
No.3
Select the own station.
A communication error has occurred between the OUT-side of the station
No.3 and the IN-side of the station No.4.
An error has occurred in the station No.3.
Click the error details button.
The Error Factor field indicates that a cable is disconnected or a line is
being established.
Take corrective actions according to the troubleshooting.