Termination and PC parallel port

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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

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

Thanks in advance.

-- Jay.

Re: Termination and PC parallel port
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The IEEE-1284 spec defines two types of ports. Level 1 is equivalent
to the original parallel port's hardware, and Level 2 uses impedance
matching to enable driving longer cables. The spec has been around for
some time, and I would guess that modern PCs have Level 2 ports.

The output impedance of a Level-2 transmitter should be 45-55 ohms, or
slightly less than the specified cable impedance of 62 ohms
(twisted-pair cable). An interface may or may not use a series
resistor to achieve this.

Quoted text here. Click to load it

The standard says the pull-ups are required "to ensure operation with
Level 1 and compatible devices," in other words, older hardware that
doesn't have impedance matching at the source.

Jan Axelson

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