When a synchronous position is used, the PLC (LMC in this case) sends the cyclically (usually every 1/2/4 or 8 ms) required position to the servo drive. The servo drive goes to this position according to the tuned PID regulator. If the motor is not able to follow the required position (the difference between the actual and required position is bigger than the threshold, the threshold can be adjusted), the following error (A320) appears. So, the error is detected in the servo drive and LMC only got an error signal. From PLC point of view, it is open-loop control.
