PLIP Install HOWTO

8. Building your own cable

Extract from /usr/src/linux/Documentation/networking/PLIP.txt, written by Donald Becker (becker at 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.