JonB,
I just looked at my Model 4 and the 50 Pin connector has 2 thru 50 (GND) facing towards the back of the Model 4.
That makes Pin 1 facing the Front of the computer and Pin 1 is the one further away from the Power Cord.
Another thing you should verify is that Pin 1 is the same on the 50 Conductor cable, because some of the
Connectors are made different and if you used two different brands of Connectors for the ends of the cable
that can switch the signals. So, verify at least four pins on both ends of the cable are correct.
The Model 4 Schematic has RP3 shown as being used for a PULLUP resistor, but my Board does not have RP3 installed
for that 50 Pin connector.
I've had problems where a PULLUP resistor did not PULLUP the Line because of a bad Solder connection.
The other PULLUP resistors you might want to check are:
U73 = 74LS645/245 has:
U73 RP3 Address I/O Connector Pin
Pin 11 - 2 XD0 1
Pin 12 - 3 XD1 3
Pin 13 - 4 XD2 5
Pin 14 - 5 XD3 7
Pin 15 - 6 XD4 9
Pin 16 - 7 XD5 11
Pin 17 - 8 XD6 13
Pin 18 - 9 XD7 15
Pin 39 - R55 XIOBUSINT 39 is PULLED to +5VDC
Pin 41 - R55 XIOBUSWAIT 41 is PULLED to +5VDC
Pin 43 - R51 EXTIOSEL 43 is PULLED to +5VDC
And finally you might want to check the Chip enables (LOW) for U73, U74, & U75
If the Chip Enables are all working properly, and the Address lines aren't stuck, due to a Buffer IC,
I'd think it should work.
If you have TASMON or HTASMON you could execute it and write HEX Bytes to that Address, in a loop,
and chase the signals to the 50 Pin Connector with a O'Scope.
Larry