If there are multiple LPT ports on the PC make sure to choose the right one with
the +/- keys.
The X mark in the cable test program user interface means that the corresponding
LPT port input or output pin is in the logical high state.
The pinouts and logic functions of 7406 and 74LS86 ICs are shown in
the type 1 cable documentation.
You will most likely need a voltage meter for troubleshooting.
Commodore serial bus signals can be controlled from the C-64 by using the
following poke commands:
CLK
poke 56576, peek(56576) or 16
poke 56576, peek(56576) and 239
DATA
poke 56576, peek(56576) or 32
poke 56576, peek(56576) and 223
ATN
poke 56576, peek(56576) or 8
poke 56576, peek(56576) and 247
Look at the schematics and invent test cases like this:"If I put that
output to this state then I should be able to control that serial bus
signal with these poke commands." Check if your theory holds up. If not,
troubleshoot with your voltage meter.
Check CLK first and then those more complicated signals.