Note that Ports 3 and 5 uses pins 2-9 are inputs, so they are appropriate for expansion boards that have more inputs than outputs, like the C10D or C79.  For the M26 I can use Port 4, which uses pins 2-9 as output. 

I bought the M26 because it is noted as compatible with the C94 but don't quite know to what port the ribbon cable should be attached. P3 and P5 provide power and enable but is mostly marked as inputs.