The discussion "Placing blame on IBM (RA4 inversion)" inspired me to do
a little investigation, since the parallel port control pins is my primary
means of interfacing PCs and PICs.

Question is: What ARE the pullups, usually?

According to the Parallel port-FAQ, the control ports are supposedly
pulled up by 4.7k resistors, if I remember right. I decided to
check this out on three PCs I had in my vicinity. This can be easily done
with the following procedure:

1. In DOS, run DEBUG and give the following command:

   o 37a 4

This sets the four control pins 1, 14, 16, and 17 high. The address
37a (hex) is the standard parallel port address. Replace with whatever
is the address for your system.

2. Use an ammeter to measure the current between GND (any pin 18-25)
and pins 1, 14, 16, and 17, respectively.

3. For a 4.7k pullup, this current should be about 1mA (=5V/4.7kOhm).

The result was interesting. For each single computer, the current
was the same over the pins, but there was an order of magnitude
difference between computers:
        Old Toshiba notebook (5 years): 0.44 mA
        Medium old Taiwanese desktop (3 years): 6.1 mA
        New Taiwanese notebook (1 year): 1.0 mA

I would be interested in knowing the results for anybody else. It seems
the pullups vary generally in the range 1k-10k.

Cheers,

 -- Martin Nilsson