[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