We had the same problem at our shop on an M2 control. There are 2 ports on the control. One is the CMT port and thinking you have connected the cable to this port. There should be one next to it that you connect the EIA port to. If not you can open the cabinet and look on the inside to see if it floating in there, which is the problem we had. The cable we use is the same as most machines. PINS 2 and 3 crossed with PINS 4&5 jumpered and 6,8&20 jumpered all on the machine side.
Hope this helps.