use of schmitt triggers on step, dir and enable?

[philba]

I've been perusing the various open source driver schematics. One thing that I don't quite fully understand is why the use of schmitt trigger inverters on the inputs.

I understand why the direction input has a selection for active high or active low. Nice, convenient feature. Thus the use of inverters. Though that could be done with a cheap NPN transistor and 2 resistors.

Actually, I have several questions:
- why a schmitt trigger? Is the PC parallel port that mushy and/or noisy? Is this necessary if using an opto isolator?
- is the need for a driver (2 ST inverters in a row) to make up for the fact that the PC parallel port has low drive capacity?
- the use of dual inverters - is that simply because inverters were needed for the direction line and thus it made sense to to use the remaining 4 in the package for drive capacity?

sorry if I'm missing the obvious.

Thanks
Phil

[pminmo]

Hi Phil,

A schmitt trigger has hysterisis, they make slow transitions and low level noise less of an issue. Remember a stepper driver is a potentially electrically noisy enviornment. Some people run long cables from PC parallel port. Long cables lead to slow transitions. A stepper is usally open loop, you can't afford missteps. I can't imagine not going to a cheap filter to guard against these issues.
Opto isolator typically would be slow enough not to respond. But they do present their own issues.
Yes on the extra inverters, use them or tie them off. I decided to use them.

[philba]

Thanks, that helps me to understand. I've seen people use low pass RC filters for this as well. Not sure I understand which is better.

Where is most noise picked up? pc to breakout or breakout to controller or a little of both? I would guess that the short breakout to controller run sees less noise.

You mention optos have issues - are you refering to their speed or something else?

[pminmo]

There is actually an RC filter in front of the schmitt trigger, the schmitt squares the slow transition of the RC filter. Noise picked up depends on enviornment and physical layout, no definitive answer.

Optos have speed issues, wide range transfer characteristics, slow output edges in some situations.