Re: [CAD_CAM_EDM_DRO] Re: emc success

Wally K wrote:

> Hi Jon, i have a sugestion for cleaning up the signals did you try
> using a schmidt trigger to buffer the parallel port. Also you can
> try putting the schmidt trigger at the paralell port as a transmitter

Not so easy, as there is no +5 V power available at the parallel port
connector.

> and have another one located at the input to your circuit.

Well, I used a Schmitt trigger circuit on my hand-wired prototype, but
didn't have the right chip (need an SO-14) to fit the production board -
my goof! I used a 74HC04, which is obviously not as good. I will
have to get some 74HC14's soon.

> Make sure
> the schmidt triggers have bypass caps at least .1uf and also 1uf to 5
> uf tantalum caps if located a long ways from power supply.

I refuse to use Tantalum capacitors, due to the fact they burn up
in a rather nasty way. Multilayer ceramic caps are fine for the same
purpose, and even cheaper.

> If you
> are running long lead lenghts use ribbon cable with every other line
> a ground.

Well, ribbon cables may not be a good idea for this. Just using ribbon
cable on an IDC DB-25 plug won't work. The cable has to be made
with hand wiring to put the noise sensitive (strobe) signals on their
own twisted pair. This would be a pain, except that IEEE-1284
cables are already made to this spec! So, I'm using a 1284-compliant
cable. The real problem is that the drivers used are VERY weak, and
so there are all sorts of glitches, wiggles and slow rise-times at the
end of the strobe/ pulses. These can be deep enough to cause
additional pulses to be detected, which causes the address counters
inside the chips to bump twice, and get out of sync with the computer.

> only connect 1 of these ground wires to your circuit the
> others should only be connected on one end.

Well, that sounds right, if only one conductor is carring the signal.
Unfortunately, every signal wire has a fast-transition edge signal
on it at some time, and therefore needs a close, full-continuity
return path. So, you really need a separate ground on every fast
signal. In this case, I think they get away with a twisted pair ground
for all strobe signals.

Jon