Thanks for the reply:
The PLC is a Nano-10 (by Triangle Research) and is only open collector on the 2 output available channels. These are fairly low cost and I/O limited.
The PWM module is not by the same manufacturer and is not specifically made for sure this PLC, so I'm trying to "make it work". The PWM board is a cytron MDD10A, dual channel PWM driver. It does have a forward/reverse signal input for each channel but all I'm interested in is speed control. There are forward/reverse buttons locally on the board for testing.
I'm guessing signal conversion from the PLC PWM output is possible but not sure how yet.