[Coco] FD-500/501 EPROM?

[Robert Gault]

Chad H wrote:
> Well I did stumble upon a solution from Cloud-9, I think?
> http://www.frontiernet.net/~mmarlette/Cloud-9/Hardware/DOS%20Adapter.html
>
> Would this adapter accept the 2764 chips I already use in the FD-502 though?
> Otherwise I would have to acquire 27128's
>

There is another approach depending on the flexibility of your EPROM programer 
which permits using a 24 pin controller with a 28 pin EPROM in the socket.

The Disto EPROM programmer has a jumper to swap leads A11 and A12 when 
programming an EPROM. If this is done and the 28-pin EPROM programmed, it can be 
used in a 24 pin socket as follows.

1. Bend up pin 20 on the EPROM. Solder a short piece of wire from this pin to 
pin 22.
2. Solder pins 1, 26, 27, 28 together.
3. Solder a wire to pin 2 of the EPROM. Insert the wire into controller socket 
at pin 20. You already bent up pin 20 to permit this.
4. Insert the EPROM into the 24 pin socket such that pins 1, 2, 27, and 28 
extend beyond the end of the socket. Pin 3 of the EPROM will be in pin 1 of the 
socket.

I have done this and it works quite well. With room in the controller, rather 
than soldering directly to an EPROM I've used an intermediate 28-pin low profile 
socket and done the bending and wiring on the intermediate socket. Then the 
EPROM is preserved and can be reprogrammed if desired.
I don't know if commercial EPROM programmers will permit swapping the A11 and 
A12 lines. You might need to wire an intermediate socket to achieve it.