I'm using eqmod cable and driver for my GP-DX, which has one of the EQ3/5 upgrade kits installed. Initially, before getting the eqmod cable, I had used the HC to connect, and ISTR that the HC has to be put into a special mode so it listens on the input port and translates it into proper commands for the motor controller. Did you do that?
Depending on what cable you ordered, if it is an eqmod cable (which includes AFAIK an USB-to-RS422 converter) it would have to be plugged to the motor control box instead of the HC.

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