After receiving my replacement A4988 driver boards, I wanted to verify that the driver board I took out of my printer had actually failed. After wiring up the minimum required connections, I measured the voltage at 1A, 1B, 2A, and 2B on the suspected driver board. VMOT is 16.6V (from a spare Linksys 12V power supply)
Regardless of the voltage applied to the STEP pin,
V1B = 12.96V
V1A = 0.75V
V2B = 11.13V
V2A = 12.97V
A forum post at Pololu’s site indicates that these outputs should be stepping in response to pulses on the STEP pin.
Let’s try the new board.
V1B = alternates between 0V and 16.65V on successive pulses of the STEP pin.
V1A = alternates between 0V and 16.65V (opposite of V1B)
V2B = alternates between 0V and 16.65V on successive pulses of the STEP pin.
V2A = alternates between 0V and 16.65V (opposite of V2B)
This seems to confirm that the previous stepper board that was driving the extruder motor is not functioning normally.
This does not give me further confirmation that plugging the new driver board into RAMPS is going to work normally though — and it doesn’t confirm that there hasn’t been any damage to the RAMPS board. I don’t really want to sacrifice a A4988 to figure this out. I reviewed the RAMPS 1.4 schematic; the stepper motor outputs of the Pololu board are wired directly to the stepper motor connectors on the RAMPS board.
Maybe the Makergear IRC channel is the next stop…