Sinumerik Alarm 300101 Bus communications failure

Sinumerik Alarm 300101 Bus communications failure

This alarm indicates that there is still no power supply to the drives although the NC is already running. This message comes only if no drive module has been signaled. (In theory, it could also be a bus error interrupting the connection to the 1st module).

– NC not ready.
– Channel not ready.
– NC Stop on alarm.
– NC Start disable in this channel.
– The NC switches to follow-up mode.
– Alarm display.
– Interface signals are set.

1) If the NCU has its own power supply, then the drives still have no power. – If possible, switch on the power supply for the drives at the same time as the NCU.
2) If the NCU and the drives have the same power supply then not even the first module could be recognized. Check whether the red LED on the first drive module is illuminated. If this is not the case, then usually the module does not have any power. – Check the connections of the ribbon cable running from your I/RF or monitoring unit to the module. – If after switching on the I/RF or monitoring unit, no LED of a module which is connected to it is illuminated, then check the I/RF or monitoring unit and, if required, replace the ribbon cable.
3) Check whether all drive bus connectors have correctly snapped into place and that the bus terminator is connected.
4) If you have not been able to detect an error by now, the module is defective. – Replace the module.

Program Continuation:
Clear alarm with the RESET key in all channels.Restart part program.


  1. Siemens Diagnostics Guide by Siemens
Sinumerik Alarm 300101 Bus communications failure