A CNC milling machine with SIEMENS 802S CNC controller and Stepper motors, move  axes under JOG mode, the coordinate on the controller screen are moving, but the machine no response, no alarms on the CNC controller.

Check the axes stepper motors are not running, but the enable signals of the axes in the PLC are in effect, no alarm on CNC, it is confirmed that the CNC controller outputs feeding pulses already.

As the machine is using SIEMENS 802S with steppers which is open-loop system without position detecting devices, if there is no stepper driver “READY” signal in the PLC, so even if the stepper drivers are not working, the CNC controller works normal and outputting pulses. Basing on above the problem should be the connection between the CNC and the stepper drivers.

Then we checked the input power(85V) of the stepper drivers is tripped off, which caused the problem, turn on the main power the machine back to fine.