Re: resistor to test stepper pulses

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "karl_l_townsend"
<karltownsend@e...> wrote:

> Thanks for the help from yesterday. I ordered a logic probe from
> digikey to check for step pulses in the wiring.
>
> I've got a stepper drive that doesn't seem to be getting signals
> from the control and I'm trying to trace the problem.
>
> It occurs to me that I'll need a resistor to ground at the end of
> the wire to check for the TTL signal with nothing attached. Correct?
>
> Using V=IR, Knowing the voltage is 5 volts. And a wild ass guess of
> 25 milliamps for current gives a 200 ohm resistor. Is this the

right

> value?
>
> Karl

If you are driving this straignt out of the parallel port you will
probably not need any external resistors. If you do need a resistor
it will probably be a pull-up (tied to +5 NOT gnd) and you can use a
1K for testing. The signal lines are pulled up at the computer end
and go low (referenced to PC ground). Most parallel ports won't
source 25ma per pin. Sinking current (active low signals) so that
the low is driving the load works better. All of this assumes that
you have a common ground between the PC and motor driver logic ground
or in the case of using active low the +5 either comes from the PC
itself or if from another supply the ground is common with the PC
ground.
If the signals look good with no load and go funky with the load
attached see next paragraph...

If your inputs to the motor drive are optos it could be that they
require more drive than a parallel port can supply. A buffer stage
is called for. If you are comfortable with building your own look at
the 74LS541 Hex buffer chip. If you are looking for a plug and play
solution look at a buffered breakout card like the Sound Logic card.

One more thing. Some of the new on-board parallel ports only swing
between ground and +3.3VDC. These types have proven to be unusable
with most motor controllers. To check this find one of the direction
pins and toggle it so it goes high and check with a meter.