For the control of my CNC lathe, I need to read the spindle encoder (index and rotation signal), and two glass scales. These encoders all provide TTL level signals, and so far it worked well, but there are cases were electric noise of the switching motor or other equipment in the workshop. So finally I decided to put a bit more effort and upgrade the interface.
So far, it is just a cable that connects the three sources (spindle-index, x, z) to a parallel port input (a PCI card).
For signal conditioning, the input signals are filtered with a 4n7 capacitor+330 R, followed by a 74LS14 inverter schmitt trigger.
Everything on a little PCB, and put in dust-proof case so that it will work in the dusty workshop.
Finally, put it to a test and it worked “out-of-the-box”.
A simple schematic: