Hello all.
I have been asked to help fix a problematic situation that involves a new generation (P4) PC's parallel port driving a remote digital logic circuit(5V PAL) via 50 ft coax cable.
It sounds like a joke, but obviously the people who set this up didn't think about termination or buffering! There is NO buffering element NOR is there any termination between the parallel port output and the coax / remote circuit.
Anyway, I wanted to know, do all modern PC parallel ports include a 50 Ohm series terminating resistor ("Rs")?
According to this site:
it's mandated by the 1284 spec.
My own SPICE simulations show for source matched termination driving a
50 ft RG-58 coax transmission line (td = 77 ns, Z0 = 50 Ohms), the current transients will be -58 mA to 50 mA on the source of the series resistor (output of the parallel port circuit). I find that surprising...Also, do I need to install the 1.2k pull-up resistors on the reciever end, as shown in the picture on the website?
If it were up to me, I would re-do the entire setup with dedicated driver ICs, proper termination and perhaps even do a differential pair setup. Unfortunetly a quick band-aid is required in this situation.
I'm thinking Schottky termination to clip any ugly transients.
As usual, suggestions, comments and other useful information is welcomed!
Thanks in advance.
-- Jay.