Original JS ROM Set

[t0nyt]

I’ve only seen 48k Minerva ROMS, not come across a 64K

Chr$ board has a jumper to switch TK2 in/out at $c000

[t0nyt]

Would the 64K EPROM not prevent QIMSI and other ROM Port plug-ins working at $C000 though?

Many thanks

It will if the TK2 part of the 64k EPROM is enabled with the jumper.... with it disabled the 16k starting at $C000 is free to be used by whatever is using the normal ROM port 16k chunk.

Hi Chr$, I was referring to using a 64KB ROM internally in a QL socket (rather than on a board) which wouldn’t have a jumper

[Pr0f]

The Minerva ROMS are actually on a board, and they are in fact 64K Eprom's with the top 16K disabled, so as not to conflict the ROM socket.

For some crazy reason, Sinclair did not opt for the usual JEDEC pinout on the 2 ROM sockets, which are in fact wired identically - and this poses a problem when trying to use EPROMS rather than the Sinclair ROMS. Pin 1 of the ROM sockets is just tied to 5V, but should have been A15 address line, and the A15 line instead goes to the CE input on the socket - and is used to select either the top ROM or the bottom ROM in the memory map - because one is active when the line is low, and one is active when the line is high. ROMOEH is used to enable the output of the ROM onto the QL address bus, and this signal is the opposite of what an Eprom would want to see.

So basically 3 pins are not wired right for any Eprom on the ROM sockets, and to use EPROMS, an adaptor board must be used.

[Pr0f]

On the plus side, I appear to have JS ROMS in the issue 5 QL I am rescuing, I need to put the new membrane in and replace the missing mdv1 and mdv2 leds, and then test the Hermes chips I've programmed this afternoon.

[t0nyt]

I seem to of got a bit lost on this as it was JS ROMS I was originally wandering if they could be burned to a single 64kb (not Minerva) and whether it would knock out the $c000

Am sure now that this isn’t an option though

[t0nyt]

On the plus side, I appear to have JS ROMS in the issue 5 QL I am rescuing, I need to put the new membrane in and replace the missing mdv1 and mdv2 leds, and then test the Hermes chips I've programmed this afternoon.

That’s great news, all round, Pr0f

[stephen_usher]

You can put together a quick and "easy" solution for Minerva. You need:

A 28 pin, pressed contact socket.
A 27C512 EPROM
A 74LS00 chip

Bend up outwards pins 1, 20 and 22 of the 28 pin socket. Solder a wire to pin 1 and another bridging 20 and 22. Make both about 5cm long.

Bend all the legs of the 74LS00 out except pin 7. Trim the thin points from these legs. Solder a wire between legs 3 and 4.

Solder the wire coming from the pins 20 and 22 of the EPROM socket to leg 6 of the 74LS00.

Solder a 10 CM length of wire to leg 1 of the '00. Solder another 10cm wire to leg 2 along with the wire from pin 1 of the large socket. Solder another wire onto pin 14 of the '00.

Plug the large socket into the left ROM socket on the motherboard. Plug leg 7 of the '00 into the bottom left of the right ROM socket (Ground).

Bare the end of the loose wires. Place the wire from leg 1 of the '00 into socket 20 of the right ROM socket. The one from leg 2 into socket 27 (chip select) of the right ROM socket and the one from leg 14 of the '00 into socket 28 of the right ROM socket.

Plug the 27C512 EPROM programmed with Minerva into the new large socket.

How it works:

We want the ROM to be enabled if the system has activated the ROM and when A15/"chip select" is either low or high, but not when A14 is high at the same time as A15/"chip select".

The connections to legs 1 and 2 of the 74LS00 make this comparison, it outputs a low voltage on the output (leg 3) if they are both enabled.

This is output into the input of the next testing cell, which is then compared with the output enable signal from the QL, which is active high. Only if both are high does it output the correct voltage (low) to turn the EPROM on.

I hope that makes some sort of sense.

[t0nyt]

Am sorted for JS ROM's now, many thanks Pr0f!