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Minerva ROM on Issue 5
Posted: Tue Sep 19, 2023 7:19 pm
by drgusman
Hi!
I want to replace the original rom of my Issue 5 with Minerva and if possible also add Toolkit II to it, and I have some doubts.
I have a good bunch of 27C256 and I have read that on the Issue 5 there is a jumper that allows you to use these, is that true? And if it is, which jumper is it? Any info/picture is appreciated.
Also, I have no clue how Toolkit II is installed, I mean, as far as I see Minerva is 48Kb and Toolkit II is 16Kb, so, is Minerva installed on the first ROM (IC33) and then Toolkit II in the second (IC34)? Is that correct? Or both are combined into a single 27C256?
Sorry if this is something obvious or already been answered a million times, I swear I haven't found the correct answer and I'm very new to the QL world
Cheers!
Re: Minerva ROM on Issue 5
Posted: Tue Sep 19, 2023 11:45 pm
by janbredenbeek
Only earlier versions of the Issue 5 board were equipped to use Eproms. These can be identified by the presence of IC17 (a 74LS00) immediately left of the ZX8302, just above the trimmer capacitor.
Later versions had ROMs installed which were programmed to output only on a specific combination of the A14 and A15 and ROMOEH signals, avoiding the need for the extra address decoding by IC17.
The original Minerva was shipped as a 27512 EPROM on a small PCB with extra address decoding, which plugged in one of the ROM sockets. Only the first 48K was used; Toolkit II was shipped as an add-on module which plugged into the ROM port at the back (it occupies the memory space $C000-$FFFF).
You can replace the ROMs by 2x 27256 or even a 27512, but you must add your own logic for decoding the A14 and A15 and ROMOEH signals and feeding them to the CE and OE pins.
You can find more info on the QL Homepage (link at the top of this page), in the Documentation section there is also a PDF version of the QL Service Manual.
Re: Minerva ROM on Issue 5
Posted: Wed Sep 20, 2023 4:10 am
by drgusman
janbredenbeek wrote: ↑Tue Sep 19, 2023 11:45 pm
Only earlier versions of the Issue 5 board were equipped to use Eproms. These can be identified by the presence of IC17 (a 74LS00) immediately left of the ZX8302, just above the trimmer capacitor.
Later versions had ROMs installed which were programmed to output only on a specific combination of the A14 and A15 and ROMOEH signals, avoiding the need for the extra address decoding by IC17.
The original Minerva was shipped as a 27512 EPROM on a small PCB with extra address decoding, which plugged in one of the ROM sockets. Only the first 48K was used; Toolkit II was shipped as an add-on module which plugged into the ROM port at the back (it occupies the memory space $C000-$FFFF).
You can replace the ROMs by 2x 27256 or even a 27512, but you must add your own logic for decoding the A14 and A15 and ROMOEH signals and feeding them to the CE and OE pins.
You can find more info on the QL Homepage (link at the top of this page), in the Documentation section there is also a PDF version of the QL Service Manual.
Excellent! I think I get the idea, I'm checking the schematics and now I understand what the dashed parts that I wasn't able to find on my board are, is that circuitry. So, if I understood it then I could use two 27c256, one with the first 32Kb of Minerva which would go in the first socket without any mod needed and a second one with 16Kb of Minerva and 16Kb of Toolkit II but it will need the logic to be enabled only when A15 is set. Time to tinker!
Thanks a lot!
Re: Minerva ROM on Issue 5
Posted: Wed Sep 20, 2023 1:09 pm
by drgusman
Yup! It works!
Thanks for the help
Re: Minerva ROM on Issue 5
Posted: Sun Sep 24, 2023 2:49 pm
by jobdone
I see you have a 7400 piggy backed on top of the eeprom, on issue 5 PCB's you will find a socket or an empty space for an IC next to the jumpers, this is the space for the 74LS00.
Re: Minerva ROM on Issue 5
Posted: Sun Sep 24, 2023 2:57 pm
by drgusman
jobdone wrote: ↑Sun Sep 24, 2023 2:49 pm
I see you have a 7400 piggy backed on top of the eeprom, on issue 5 PCB's you will find a socket or an empty space for an IC next to the jumpers, this is the space for the 74LS00.
Yeah, I found the socket and the resistors in the PCB after Jan's response, but the logic that implements that circuit is different from what I have done.
The original logic activates only 16Kb on the second ROM and leaves the upper 16Kb of the 64Kb address space free for the cartridge port, but I "cheated" and have activated all the 32Kb of this second EPROM, I have merged in the EPROMs Minerva and Toolkit II so I don't need to plug an external ROM with the toolkit (it also simplifies the logic to enable/disable the EPROM, it's only a matter of inverting A15, that's what that IC is, an inverter gate, the 74VHC04).
If by any chance anyone wants to do the same I have programmed a tiny app that merges the two roms and splits it, so ping me if you need it
Cheers!