exxos's Atari, Amiga & retro forum

If you see no activity on the databus, together with having all clocks and activity on the address bus, the databus is likely to be shorted to ground, and the ROMs are most obvious to be blamed for such behavior. Databus is directly connected (in parallel) to both OS and BASIC ROMs.

tzok wrote: 21 Feb 2022 20:12 If you see no activity on the databus, together with having all clocks and activity on the address bus, the databus is likely to be shorted to ground, and the ROMs are most obvious to be blamed for such behavior. Databus is directly connected (in parallel) to both OS and BASIC ROMs.

I am going to be coming back to this soon, but I do see activity on the address and data buses, all clocks are good and all IC's have been tested good bar the PIA and Basic ROM, with all logic ICs tested with new.

I'm going to re-do and post up the results, probably this weekend, depends when I get a spare PIA.

Ok, so the W65C21S6TPG arrived yesterday, but made zero difference. so I am going to have to take a fresh approach, so I'll buzz out the address and data lines across the board whilst I wait for the 6520s from @PhilC to arrive.

Then I'll revert my 800XL by removing the Ultimate 1MB, so I can use it to test all the ICs from the 600XL. If I do that, then I will be able to socket the PIA and BASIC ROM in my 800XL which I know work and at least have some reference points.

I think the scope and LA are like to be out again :) I'm going to need to get a better schematic than the fuzzy one I have now too.

It'll be a deep dive, but it'll be fun :)

So, it's been an interesting day XL wise.

I pulled the 800XL board, removed the U1MB and socketed both the BASIC ROM and PIA chip, and was able to verify that the GTIA, ANTIC, PIA, POKEY, MMU, OS ROM and BASIC ROM from the 600XL are all good.

Interestingly, the 800XL boots just fine with the W65C21S6TPG, so good to know that seems to work, at least enough to boot at any rate.

Sadly SALLY was not well. However the replacement I had on hand was just fine in the 800XL. So all major ICs check out. Since I didn't know if the RAM was good or bad, I opted to do the 64KB upgrade now, since it's 2 chips and 3 wires, and super easy to revert if needs be, and not hard to troubleshoot if still not working.

So, new SALLY in (Oddly this made the brown screen to change to a totally black one!!) and RAM upgrade done, and I popped the Star Raiders cart in, since it supposedly bypasses the OS ROM, and I get this:


Other ROM carts like SUperSalt and SIDE3 don't work, but I presume they require the OS ROM to be fully operational.

So this is the most life I've had from this machine, and it happily proves that the main machine is good, and where we have a problem seems to be in the land of ROM.

Next up I need to get the ROM pinouts and buzz out all the lines to see if anything is missing, as this suggests the onboard ROMS aren't loading properly. To add to the theory that something is wrong in ROM land, holding down Option on boot immediately drops me into the ROM/RAM test routine, rather than the test menu, but later fails whilst running the tests.

I'm quite excited now, as I have been looking for a working 600XL for some time now, and I seem to be edging even closer :)

EDIT:

Seems the OS ROM is still needed for Star Raiders, it just starts a lot earlier in the boot sequence. Time to find some datasheets.

Ooooo! Progress! :-)

stephen_usher wrote: 26 Feb 2022 17:28 Ooooo! Progress! :-)

Yeah, and good enough to point the finger in the direction of the OS ROM, since Star Raiders boots when the BASIC ROM is removed.

When starting with Star Raiders inserted, it jumps to the cart sooner than if it continued to run the OS ROM. It's making me think there is a bad address or data line, since the call to the self test sort of works, but then fails.

Star Raiders is unusual in doing this, other carts don't. I think Asteroids does the same.

So, logic probe/oscilloscpe on the ROM address pins to test?

I take it that the pinout of the ROM is not compatible with an (E)EPROM?

stephen_usher wrote: 26 Feb 2022 18:49 So, logic probe/oscilloscpe on the ROM address pins to test?

I take it that the pinout of the ROM is not compatible with an (E)EPROM?

I managed to print out a clearer version of the schematics showing the OS ROM pins, so it'll be getting scoped, and I can compare to the 800XL now too.

:dualthumbup:

OK, the pinout for the OS ROM is the same as a 27C128 which should mean making a replacement is easy, as would be verifying the contents of the original.