Threading - no rpm in display


Gentlemen, I kindly ask for your support

My cnc 5 hangs, when the program reaches G78. In manual mode it shows rpm. When I switch from manual to cnc mode the indicator turns dark. Obviously it's because it cannot read the rpm. When I switch back to manual mode it starts cutting the thread, then alarm 6 is displayed.
Any suggestions or tips how I can solve this problem?

Thank you guys!


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