DIY CNC

timo wrote:

I had issues getting a 100ppr encoder to work reliable at 600 rpm. The cheap BOB seemed the issue, the system was loosing signals.
 

You are right, this is PC817 optocoupler bandwith issue.
I did some test, and PC817 response time depends on input current, if current increases, the bandwitch decreases (is this saturation?).
However 1 kHz is very low and PC817 sure can more.
If you use 5V encoder, you can safely connect it directly to LPT or MESA pins.
Most of 24V encoders with NPN outputs can be connected directly too, but you must check outputs voltage before (some has builtin pullup resistor an thats can burn low voltage inputs.)  
MESA can more than encoder. Low cost optical encoders have bandwith 100 kHz, this mean max 2000 p/r at 3000 RPM.
LPT depends on jitter, but in practice 100 p/r is max at 3000 RPM

Just an idea of what you can do with a parallel port(s) with Linuxcnc. I had a double Z axis (Z and W), 6 spindle gang router (3 spindles on each Z), all the spindles with individual control using analog speed commands, individual +/- limits for each axis. All ran off of 3 parallel ports.

There are even some special things that you can do using a parallel port that you can't really easily do with Mesa cards and hardware stepgens. I had a machine that I retrofitted to Linuxcnc, that I wanted to keep the ability to plug in and reuse the original control. It had it's own driver box for the step motors and used individual commands to directly trigger the stepper coils rather than traditional step/dir commands. I was able to directly replicate those signals using Linuxcnc's software stepgen and output them to a parallel port.