Re: Blame the Cable
Way, way more than two. Does it use H/W control & if so which lines? (Memory says there were a couple of alternative conventions.) Then there are D25s & D9s. With D9s whoever laid out the board headers hadn't been told that the pin numbering on a header isn't the same as that on a D connector so a plain ribbon cable won't help*. And if all else fails a wrong gender connector can be thrown into the mix.
* Got caught out by that one in reverse back in the early days. Spotted that the pin numbers on a 26 way header didn't correspond to the numbers on the D connector & started to wire up a custom cable. Oh, look, if you just use a ribbon...