Re: Homebrew STG card - PAR-PORT

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

"Arne Chr. Jorgensen" wrote:

> Here is another comment:  I don't understand why people would like to have 8255
> or 8254 chips to run steppers.  You could just add another printer card.  Or
> just use some ordinary buffers.  These chips is not that nice ( IMO )  as
> people often think. A 6821 or similar on the other hand,  can set any pin as
> either an input or output,  then I would say there is a lot more value in it.
> Well, this was a side track :)
> But you could use board with 8255 chips as a bridge to en extended io system.
> That would make more sense.

The reason is to allow a step rate which has nothing to do with the
cycle rate of the routine on the main CPU which generates the steps.
The old program cycled at fixed time intervals, and could either
generate a step pulse, or not.  This leads to very large jumps in step
rate when you get near the maximum (ie. one step every cycle,
one every 2 cycles, every 3 cycles, etc.)

So, either you have the CPU interrupted at an insane rate, tens of
KHz, or you have large jumps in step rate.  The timer chip idea is
to allow steps to come in between the cycles of the rate generating
program.  It still has some difficult things to work out, so as not
to lose steps.

Jon