Re: [CAD_CAM_EDM_DRO] Re: 5V+ Supply for Gecko? Still confused!

Abby Katt wrote:

>Hmm.. myabe I'm having a brainworm day, but I just don't get this
>still.
>
>Okay, since we've established that the gecko has it's own internal
>bus which is powered from the 18-80V motor supply, and we know this
>powers the encoders....
>
>The Step/Dir pins and the 5V+ pin of the gecko make up two opto-
>coupled LED's emitters. From looking at the circuit diagram, it looks
>like the step-dir pins are triggered by negative voltage on the
>parallel port. Am I correct in assuming that when the parallel port
>pulses, it is negative?
>
>Now surely, if this is going to be negative, then there wil be a
>positive pin, or two in the parallel port, as I don't see
>lineprinters needing the PC power supply connected. All we need to do
>is light up two opto-isolator LED's from the parallel port - surely
>there is going to be enough power/connections for that?
>
>
>

Printers do not use opto-isolated inputs, and are referenced to the
ground potential,
which is available at pins 18-25 on the connector. But, due to the
electrical
noise produced by a high power switching circuit like the Gecko stepper
and servo
drives, they use opto-couplers on the inputs. These require current to
flow,
and in fact a LOT of current for a CMOS chip, about 10 - 15 mA. Because
of the assymetric nature of many driver chips and the fact that many PC
parallel ports now operate from a 3.3 V power supply, it is possible for
these
chips to sink more current INTO the chip than they can source FROM the chip.
So, the Gecko drives work best in these cases with the opto set up for
the sinking mode, where current flows from the +5 V source, through the
opto's LED, and into the parallel port pin.

I hope this helps explain why this odd hookup is preferred. If the Gecko
used a voltage-level input, this wouldn't be necessary. (But, it might
also compromise performance.)

Jon