Looks like I may indeed have destroyed one of my stepstick drivers due to my own stupidity, of course you can’t measure current across 2 pins, you can only measure it in-line, now the diagram did show this, but I was obviously not thinking at the time, and the description was not idiot proof!
The dead driver is probably due to me shorting it out across 2 of the motor pins whilst attempting to measure the current.
After yet more reading about the stepper drivers, I came across the following post stating which current sense resistors are used on the various stepper drivers.
I then checked the parts list associated with the bare board version of the ones I have and they list 0.05R – same as Pololu, not the same as A4988 stepstick.
so my calculation should be: Motor Current = Vref / ( 8 * Rs ), where Rs is 0.05 (or Motor Current = Vref * 2.5).
I was now able to accurately determine how much current was being made available, I started at 1A and worked down.
I tried all sorts of settings and was becoming increasingly concerned that something else was wrong, because I could cause an axis to move in the + direction for about 180 steps and then it would simply stop!
After which none of the X+ or X- buttons had any effect. I later discovered that HomeX allowed a further X+ steps (again about 180) before it would stop.
I had no idea what I was doing wrong, fortunately some of the people of the reprap forum pointed out that I was probably activating my endstops and these were preventing further movement.
After tricking the board into thinking it had a normally closed endstop (simply shorted the terminals), I could now cause the motor to move in both directions (home was obviously a bit confused).
Here we see the full test setup, with MicroSD card, Bluetooth module, endstop cheat and stepper motor, power is supplied by 19V laptop brick.