Re: Homebrew STG card - 8254

[Jon Elson <jmelson-at-artsci.wustl.edu>]

bfp wrote:

> Hello all,
>  I just dropped in from [CAD,CAM,EDM,DRO] and caught the tail end of this
> thread ,I've been working on a similar idea myself and I had some random
> thoughts on the 8254 timer chip.
> I've done some work with these chips before on a hobby project and have seen
> a few others
> using them on the web, to stop the timer output you can use a buffer with an
> enable pin like
> the 74als244 or an AND gate & one bit from a digital port like the original
> pc speaker did
> I like the idea of using it to clock a stepper board,that got me thinking
> about another use for it,
> a company called Rutex in Australia makes an external board  that translates
> step+dir commands
> into a +-10v servo reference signal by using a PIC chip to generate a PWM
> signal and pushing it
> thru an integrating op-amp[LM358] to get the +-10v output for the servos
> it seems to me that using the 8254 to generate the PWM(mode1) would only
> require a software
> change,and an op-amp on the card or on a breakout board near the servo amp
> might provide
> a "clear upgrade path from steppers to servos"
> another possibility is to feed a pulse string (mode2)or square wave
> (mode3)into an  LM2917
> frequency to voltage converter
> I know its a bit unorthodox but does this sound reasonable?

No, it is TOTALLY INSANE!  (Sorry to be rude.)
I was not in favor of other stepper to servo conversions, but, now
that EMC is actually emulating a servo drive in software, and then
using that modeled behavior to generate step pulses, converting that
information to servo is really going off the deep end.  If you want
servos, you should either just pay the price for the servo-to-go
card, or roll a substitute either from scratch, or by combining a
commercial D/A card with an encoder counter card, and throwing
together a driver for it.  Either way, having the computer directly
communicating with the servo amp and encoders sounds like a
much better idea.

Stringing so may conversions together is bound to lead to
undesirable behavior.

Jon