PC85 - Joining the conversation

[CheshireNoir]

PAK5 should be called SHELL not MENU as there is a separate MENU ROM used in the PC85b, this ROM actually resides at PAK 10 position.
So even if you do hid correct decoding you can't access ROM 10 as it is reserved for the MENU ROM.
That's the reason the "PC85b eprom IC27 (PAK 5, aka the SHELL)" crashes is that it tries to switch in the MENU at PAK 10 position and doesn't find it there.

That all said as Chickenman stated, there is two versions the PC85 and the PC85b.
The PC85 can only address PAK ROMs 0 - 7. PC85B adds and extra data line to the HCF4042 Latch and this latched output is used to bank switch the PAKs to decode ROM PAKs 8 - 15.

The board revision that enabled this was Rev 6, you have the Rev 4 board, but still doable if required.
However I would get it working in it's present form first before launching into any upgrading.

Ernest

Thanks Ernest!

I was looking at the core board description here and it seemed to suggest this core board supported the 27C128 series ROMs. If this revision doesn't support the double density ROMs. that explains some of the issues I was seeing.

I found a fried signal diode near the corrosion and replaced it, but it still hasn't come good. My next plan is to socket the ICs near the corrosion so I can test them separately out of circuit.

What is getting me is that once it's stable, it's stable for quite a while. I've had no end of fun playing Kilopede and games off "tape", so once it gets into BASIC it works quite well.

Never been able to get the NET command to work right and it's pretty random as to whether it boots to the Shell (Most of the time, at which point BASIC and WordBee usually work), BASIC (At which point WordBee usually works) or just WordBee (At which point you're usually just stuck)
This makes me think it's an issue with the line select logic, which, looking at the schematics would seem to be mostly tied to IC28 and IC31, if I'm reading things right?

Anyway it's gone into the "naughty corner" until next weekend as I want to work on other stuff until then.
Thanks for everyone's help here! It kept me investigating for a lot more of the weekend than I initially planned :-D

Chesh