Sinclair ZX80 / ZX81 Forums

[sirmorris]

You're absolutely right.

1 ---> J2
2 ---> J1
4 ---> J4
8 ---> J5
C ---> J3

You have 1 connected to J2 which is toggling the A16 line, not what you want really as your ROM doesn't have this. This means you would see only images in even (by index, odd by number) slots. Which matches observations.

You should have 1 -> J5 (a13)
You should have 2 -> J4 (a14)
You should have 4 -> J1 (a15)
and common to J3.

The rom crc failure is probably down to a real difference in the rom or the verification program.

C

[nama]

sirmorris,
Thanks you very much for confirming my mistakes. One quick question, are you saying that what I have now this is correctly wired up for a EPROM that would take 16 8k images (1Mbit)? If I rewire as per your suggestion, but at a later date get an 1Mbit EPROM, can I then just wire up 8 -> J2 and everything will still be ok, or do I have to rewire as per my present setup?

Thanks for your time
Phil

[sirmorris]

It needs to be rewired like so:

Common to J3.

(For 512 Kibit, 8 banks you need)

1 -> J5 (a13)
2 -> J4 (a14)
4 -> J1 (a15)

(For 1 Mibit, 16 banks, you also need)

8 -> J2 (a16)

I can't wait to see this work

Next up - sd/mmc adapter!

[nama]

One final question.
If I'm using a 512 Kibit EPROM, then should I NOT connect J2, or does it not actually matter if it's connected?

Phil

[sirmorris]

It shouldn't matter. Has your board been trimmed of the extra 4 holes? The 27x512 is a 28 pin package while the '010 comes in a 32 pinner.

If the board hasn't been trimmed then you may choose to upgrade to a megabit chip in the future Of course that would require ROMs ...

[nama]

Hello again,
I have rewired the adaptor as per your sugestion. The following list is now what I get:

0 ??????
1 ASZMIC
2 Husband Forth
3 Tree Forth
4 ???????
5 ASZMIC
6 Husband Forth
7 Tree Forth
8 Basic SG81
9 ???????
a Basic OLD
b Basic NEW
c Basic SG81
d ???????
e Basic OLD
f Basic NEW

At least now it kind of makes sense!!! Starting at 'f' the list goes BACKWARDS for 4 banks, then repeats again for the next 4 banks, then starting at '7' it continues through the list, then repeats again for the last 4 banks.
Interesting :-/
Is this correct?

The board has not been cut down, so I still have the option of later buying and installing a 1mbit eprom, but as you say, there really isn't that many alternate ROM images available, at least not what I've been able to find.

Thanks again

Phil