Mods for 32K RAM and 16K ROM on Orton 3C

[Peter Onion]

As built the 3C has 32K of RAM but the top half is replaced by the 16K ROM board.  This left A15 free to be used for the rather "hacky" serial transmit line.

I use a couple of port bits on my 3CX board for my bit-banged serial interface which leaves A15 to be used an address bit just as Zilog intended :-)

There are two mods :

1) Replace A14 on D1 on the CPU board with A15 (which disables the RAM from 8000h to FFFFh) by lifting the cathode of D1 and wiring it to A15 (Pin 1) on the bus connector.  This is shown in the top two images.

2) Replace A14 on pins 8 and 9 of IC1 (74HCT02) with A15 (which enables the ROM from 8000h to FFFFh) by removing Pin 2 on the bus connector and adding a link to pin 1 on the back.  Alternatively you could cut the track from Pin 2 and connect it to Pin 1 instead (see later for 32K ROM mod). 

You now have access to all 32K of the RAM from 0h to 7FFFh and the ROM appears at 8000h with a duplicate at C000h.  If you wanted access to 32K of ROM, you could put A14 onto the center pin of JP 1 (and remove the link).

Hope that's useful to someone,

PeterO