Hello guys! Sorry by taking so long, I tried to make a decent amount of progress before showing my face here again.
At first I thought that all I would have to do is sending X number of pulses, and the driver PCB would translate them into X number of steps. But well, I was very wrong. All the stepping was made inside the microcontroller, so having or not having the board made no difference.
I tried ClydeCrashKop advice, just the existing transistors instead of the whole board. Unfortunately during this step I short circuited the PSU and the board got wrecked, my bad. Thank you by your code, I'll try it when the hardware is finished.
Following KISS advice I've identified the corresponding motor pins to make my own driver, based on a ULN2803 and a parallel port.
They work, now I'm adapting them to the existing printer belts and structure holes. Trying to remove the gears destroyed the previous stepper motors, so I opted by the horrendous solution you are about to see.
A problem I'm facing now is that the motors don't have any decent torque. At this moment I'm only turning on only one coil at a time. I guess this method is very crude. If you can recommend me a better method or some decent reading about the topic I would appreciate it a lot.
By the way I checked Mr RB driving boards and they look damn good (I don't want to be a sycophant, I just checked the finished CNC's using them). Unfortunately they are beyond my budget.
Atferrari made me think about a fact I didn't contemplated: How precise the stepper motors tend to be? can I expect that the stepping amount to be uniform, or a feedback sensing system would be required?
Thank you by all your inputs guys