Model Forum / General / Models / April 2009

The following is a post on the Mach group. Some tests as indicated may
show your problem.

*****************************
An LED array is handy to have, one sits on my desk since 2001 , and I
use it ALLOT! , BUT you wont find it usefull in your case. This is
because the pulse width is only 3us. The LED will show nothing unless
your running at high speed, then it will glow at a % of average
voltage. Single pulses cannot be seen, just a display of a varying
brightness showing output speed. ( Still quite usefull in
diagnostics.) Steve Stallings sells an excellent stepper test board
for such things , it even has a small stepper on it to play with in
motion, probably a little expensive for most for testing but
invaluable for people who diagnose various computers as to driver
function.

  There is an easier way to know though, the machine coordinate DRO.
Purposely, and mentioned many times in threads, that DRO was designed
not to reflect where mach3 thinks it is, there are many variables
internally for that, the DRO's programming is very simple, it counts
output pulses. The code that actually changes a step pulse increments
or decrements that counter. The DRO display is ( #ofsteps / steps/unit
).

 So you can just reset the motor tuning to tell it that 1 step is one
inch. Retune the speeds so it runs and watch the Machine coordinate
DRO. It will advance 1 for each step taken. and decrement for each
step in the opposite direction. If it counts 1, then you can be sure a
step pulse was put out, no question about it. if it decrements 1, then
a step was put out with a reversed DIR line, no question about it.
   
  Thinking about it, the DIR prechange time sounds like your trouble,
picture what happens, the system tries to reverse, it changes the DIR
line, waits 3us , then sets the step line, the driver hasnt yet seen
the DIR line change so it outputs a reverse step. Why only in one
direction? It may be case of the opto on the DIR line taking awhile (
longer than 3us) to turn on, but having a much faster turn-off time.
Kinda like those annoying flourescent bulbs replacing incandecent
ones, they take awhiel to get glowing very brightly, but when you
flick them off, their instantly off, somewhet the reverse of the
incandecent which is fast to glow, but fades out when switched off. Be
pretty rare, but if your runnign on the edge of that opto's timing, it
could in theory happen. Mach3 holds the DIR line on or off permanently
until direction changes, it isnt a pulsed output, so this phenomenon
can only happen at time of reversal, and is the only way a slow opto
on the DIR input of the driver will be seen.
**********************************
Richard

On Apr 13, 10:12 pm, Richard Edwards <poundea...@blueyonder.co.uk>
wrote:

Hi Richard, I found the "Sherline Mode" -but it made things much
worse, with much bigger errors.  I'm not sure what drivers the
Sherline uses, but this mode sets the output pulse to 20
microceconds.  I think it is loosing steps at the start or end of a
move (or both).  But I haven't come up with any explanation other then
dropping steps due to not driving the motors at full power (losing
stepson acceleration).  But I haven't found any settings that it works
at, even going down to quite low speeds.

Regards
Kevin