I've done a little experimentation and have come up with the
connections required to construct a geoCable.  The table below shows
the pin connections for the User Port, the Centronics connector found
on parallel interface printers, and for the DB-25 connector of the
IBM-PC parallel port type.  I include the last because the cable
(Centronics to PC DB-25) is commonly available.

Instead of making a cable, I used a cable for a PC and made an adaptor
to plug it into the C64's User Port.  The adaptor consists of a User
Port connector and a DB-25S glued to a small piece of circuit board,
with wires run between the two connectors.

Signal Name      PC DB-25    
         User Port       Centronics
-STROBE      M       1       1
   D0        C       2       2
   D1        D       3       3
   D2        E       4       4
   D3        F       5       5
   D4        H       6       6
   D5        J       7       7
   D6        K       8       8
   D7        L       9       9
  BUSY       B      11      11
  GND    1,12,A,N 18-25  16,19-30,33

                Ken Wallace

Corrections made 17 May 89.