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Connecting RS232 cable to machine locks control up

This was the most interesting CNC problem of the year!  Had a customer with a Brother TC32B CNC machine that has had a problem from day one when they got the machine. What happens is when they connect the RS232 cable to the main NC Board the machine locks up. No alarms, screens change but cannot handwheel or perform any physical movements. The machine could communicate via RS232 with the cable connected. They tried a new computer as well as a cable but the same thing occurred. First though was the NC board must be bad, but the customer had already replaced it.
 
First thing I did was try my laptop with a USB to serial converter which worked just fine. I decided to install the USB to serial port drivers on there Windows 7 computer since there’s used a PCI card. I then plugged in the RS232 cable and the machine locked up again.
 
Next I decided to disconnect all cables plugged in to their computer machine locked up again. I then powered down the computer and tried it with the power off. I couldn't believe it! Same thing. So next I disconnected the power plug and tried it. It worked! So next I wondered if it had some kind of ground short.  I then decided to cut off the ground lug for the computer power plug.  After doing so the computer could be plugged in and powered up and it did not lock up. So I proved it was going through the PC ground.
 
Next I had to find out where and how the power was coming from. After all the machine was grounded! Next I started disconnecting cables to determine which cables or boards to try and isolate the problem. Of course this was not an easy task since many cables will cause alarms.
 
 
Eventually after 6 hours I isolated it to the feed hold button of the control panel. There were two wired that tapped off it and when to another feed hold button added to the front of the machine. The wires powered a 24VDC LED light bulb. The power that it was tapped into was 5VDC. So the circuit for one did not function and was not designed properly. Some how don't know how this was drawing down a power circuit on one of the boards or grounding out in some shape or form. Wish I knew more of how this occurred, but definitely one of the most unusual service calls.

Head scratcher

 Good stuff congrats on finding that  one!

Hitechredneck 

thanks.

thanks for sharing. I enjoyed your post!

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