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Gantry CNC - Dual Y's - Missing Steps

16 Jun 2021 12:31 #212196 by andypugh
Very late to the party here but...

Many years ago, I had just built my first CNC machine (using a parallel port and a cheap triple-TB6560 board from Amazon. I could not get it to work properly so I went to the IRC.

Seb Kuzminsky suggested that I try inverting my step pins (but not the dir pins, FWIW where you went wrong was in also inverting the DIR pins) and suddenly everything started to work perfectly.

The likely explanation is that the parport sinks current a lot harder than it sources current. For the step pulse timing that I had a negative-going pulse was better-detected by the driver than a more rounded positive-going pulse.

So, contrary to what everyone else has said, I will agree that with a parallel port system it can be necessary to invert the sense of the step pulses.

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16 Jun 2021 18:18 #212209 by BeagleBrainz
Ok that makes sense. If I think the input to the Bob didn’t conform to the ieee 1284 recommended circuitry. As that has a pull-up resistor then a 39r current limiting resistor between the pull-up resistor and the input of the first chip on the board. Which could be the case with the cheaper bobs.
Tho I can not confirm this due to lack of a scope.

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