Extract from /usr/src/linux/Documentation/networking/PLIP.txt, written by Donald Becker (becker@super.org):
Parallel Transfer Mode 0 Cable
==============================
The cable for the first transfer mode is a standard
printer "null" cable which transfers data four bits at a time using
data bit outputs of the first port (machine T) connected to the
status bit inputs of the second port (machine R). There are five
status inputs, and they are used as four data inputs and a clock (data
strobe) input, arranged so that the data input bits appear as contiguous
bits with standard status register implementation.
A cable that implements this protocol is available commercially as a
"Null Printer" or "Turbo Laplink" cable. It can be constructed with
two DB-25 male connectors symmetrically connected as follows:
STROBE output 1*
D0->ERROR 2 - 15 15 - 2
D1->SLCT 3 - 13 13 - 3
D2->PAPOUT 4 - 12 12 - 4
D3->ACK 5 - 10 10 - 5
D4->BUSY 6 - 11 11 - 6
D5,D6,D7 are 7*, 8*, 9*
AUTOFD output 14*
INIT output 16*
SLCTIN 17 - 17
extra grounds are 18*,19*,20*,21*,22*,23*,24*
GROUND 25 - 25
* Do not connect these pins on either end
If the cable you are using has a metallic shield it should be
connected to the metallic DB-25 shell at one end only.
I checked my cable twice. On both sides, pins are connected like this:
1 - 1 Yes
2 - 15
3 - 13
4 - 12
5 - 10
6 - 11
7 not connected
8 not connected
9 not connected
10 - 5
11 - 6
12 - 4
13 - 3
14 - 14 Yes
15 - 2
16 - 16 Yes
17 - 17
18 not connected
19 not connected
20 not connected
21 not connected
22 not connected
23 not connected
25 - 25 not connected to metallic shield
Unlike the cable described in the kernel document, my cable has pins 1, 14, 16 connected. It doesn't matter, apparently, since plip works fine for me.