The Naked Atari 800

[rubber_jonnie]

So, basically, someone used this as a parts board because they thought it unfixable. That's what it sounds like.

A good challenge. :-)

I think thats the case, yes, and I like a challenge :)

One thing thats bothering me though is that having done some of the clock checks, the table looks wrong.

At first I thought maybe I'd printed the table for the 400, but no, the 800 is identical. Then I wondered if the 400 and 800 have different CPU boards. They don't, but what is different is that the PAL and NTSC boards have different IC's on board:

This is NTSC:

Screenshot 2025-12-12 at 11.17.59.png

And this is PAL (Mine):

IMG20251212111833.jpg

Aside from the 3 main IC's, which are labelled the same and match in position, the NTSC board has an extra 2 ICs and these are labelled differently to the PAL board, so the table of test points doesn't look right for my machine.

Since all the add in boards were working in another machine I can only really suspect the mainboard, CPU and GTIA.

The Antic was borrowed from a working machine, the GTIA was working IIRC and was just one with a bug, however I will test that next. The CPU should be fully functional, however I will test it in something other than this 800 just to verify.

I do like a good project :)

[stephen_usher]

6502s can go bad in subtle ways, so trying a known good CPU would be a very good idea.

Of course, if those PALs are bad and the CPU can't access memory then it won't work given the reliance on the zero page.

P.S. I've just noticed the 74LS244 on there! I'd check that for a start! They go bad all the time.

[rubber_jonnie]

OK, so I've managed to test the CPU, GTIA and Antic over lunch. Antic I already knew was good as it came from a working machine, GTIA works in my 600XL. Its a buggy GTIA, but otherwise works fine.

The CPU I had to test in my spare Electron and it didn't appear good, so I have a CPU that worked in the Electron installed in the 800 now. I couldn't test the CPU in the 600XL since that uses a Sally rather than a standard 6502.

Trouble is I'm a bit short on 6502's now so better get some spares. It's a shame as the possible dead CPU I had in there was period correct with proper markings. I now have 3 probably dead 6502's.

@stephen_usher Yes, the LS244 caught my eye too and I have those in stock, so easy to test and swap out, though the CPU card was working when it came out of the 400.

I think I can only test the CPU board in the 400 since the ROMs are on the mainboard in the 400. Also I have a modified 1MB RAM card so that may affect use of the original 16K boards, but I think I should try.

BTW there are no PALs in the 800, so not a problem from that POV.

[rubber_jonnie]

OK, so the CPU card test failed in the 400. I thought it might as the 400 has been modded for a bigger RAM card, though it did fail with a brown screen with odd graphics on it, rather than a black screen.

The 400 is modded for a SCCC as well as the RAM so that may be

I'm kind of tempted to try with the 800, but that has been modded for Incognito Gold so may have similar issues to the 400.

Annoying.

[rubber_jonnie]

Hmm, OK.

I seem to have RAS/CAS at the RAM but the ROMs have no chip select. I need to figure out where the signal originates and track it back.

[rubber_jonnie]

OK, so some more time spent with the 800 today and though I am not really any further along, I have done quite a bit if testing.

First off I want to talk about the reset circuit, which led me on a merry dance!!!

Checking pin 40 of the CPU (/RESET) power comes on and pin 40 goes high - so normal. However pressing the reset button showed no change on pin 40 at all. The schematics have been tricky because they are all separate on account of the CPU, ROM & RAM boards all being plugged in.

So, I got my larger display plugged into my laptop, and that did make the process of figuring out how the reset switch was wired rather easier. What happens when reset is pressed is that the RNMI signal on ANTIC pin 6 is pulled low. Antic then generates an NMI which is sent to pin 6 of the 6502 causing a reset. I've verified this by scoping the pins on reset so I'm fairly clear on this now.

What I can say is that there is a separate reset circuit that does head to pin 40 of the CPU, and this doesn't seem to be working and needs more looking into. On the subject, I believe the CD4050 used in the circuit is bad so have replaced it, though it made no difference, but the new IC tests ok.

Having checked the address and data busses I can see activity on both, and I see activity at the CPU, ROMs and RAM, as well as the RAS and CAS. I thought that there was no chip select happening at the ROMs, more over complicated circuitry, however I have found that CS is working at boot. Also pin 7 of the CPU shows activity when code is running on the CPU AFAIK and it does show a brief period of activity on power up but then stops.

Since the ROM, RAM and CPU boards are known good from a working machine and the CPU has been tested in my ELK running a diagnostic ROM I believe that is all fine. I have tested my other logic IC's on the mainboard too and they are all good.

I did find some damaged resistors and replaced those, but no change there either.

Bit of a head scratcher this one. I can't help but think I am missing something on the mainboard.

[ijor]

So, I got my larger display plugged into my laptop, and that did make the process of figuring out how the reset switch was wired rather easier. What happens when reset is pressed is that the RNMI signal on ANTIC pin 6 is pulled low. Antic then generates an NMI which is sent to pin 6 of the 6502 causing a reset. I've verified this by scoping the pins on reset so I'm fairly clear on this now.

More or less, but not exactly. Yes, contrary to later models, the reset button on the 800/400 doesn't actually triggers a hardware reset. It triggers an NMI interrupt via ANTIC. The OS then at the interrupt handler would detect the NMI source and would process a software reset.

But because this is not a true reset, it depends on the CPU not being halted (and also RAM must be working). Otherwise this "soft" reset won't work. Also it doesn't reset the hardware chips, at least not directly.

[rubber_jonnie]

So, I got my larger display plugged into my laptop, and that did make the process of figuring out how the reset switch was wired rather easier. What happens when reset is pressed is that the RNMI signal on ANTIC pin 6 is pulled low. Antic then generates an NMI which is sent to pin 6 of the 6502 causing a reset. I've verified this by scoping the pins on reset so I'm fairly clear on this now.

More or less, but not exactly. Yes, contrary to later models, the reset button on the 800/400 doesn't actually triggers a hardware reset. It triggers an NMI interrupt via ANTIC. The OS then at the interrupt handler would detect the NMI source and would process a software reset.

But because this is not a true reset, it depends on the CPU not being halted (and also RAM must be working). Otherwise this "soft" reset won't work. Also it doesn't reset the hardware chips, at least not directly.

And I think this is part of the issue I currently have, I think the CPU is halting since I see good signals (chip select for ROMs, address and data bus activity, RAS/CAS and data at the RAM) at boot but then it seems to just stop.

Now I know the 3 ram boards I have are good, they were pulled working from my other 800 when I added an Incognito Gold. Same for the personailty (ROM) board, that was working fine. The CPU board came from my 400 when that was upgraded to an SCCC card. The GTIA is a buggy one, but works in my 600XL and the Antic is borrowed from my 600XL and also known good.

Here is the thing though. I am assuming my 6502 is good, since it runs in my Acorn Electron, and 'appears to run in the 800', however I have 2 other 6502's that run in the Electron, but don't seem to do so in the 400 CPU board.

Of course I can't test the 6502 in my 600XL since that has a Sally CPU.

I think I'll have to borrow all the parts from the 400, since that has the ICs from the 400 that ran just fine in it. If it still doesn't run after that, I will at least know that the issue must be on the mainboard since I will have ruled out all the boards, the CPU, Antic and GTIA plus the POKEY and PIA are also known good items.

All of the logic IC's on the mainboard are tested and good, so I can at that point only see there being a mainboard issue I have missed. Time to open up the 400. Yes, again...

[rubber_jonnie]

OK, so the CPU, GTIA and Antic are all confirmed working in the 400. I had to switch the jumpers on the SCCC card in the 400 to make the CPU run, and oddly when I put the CPU from the 400 in the 800 (The CPU board it was originally from) I get no activity at all, so no PAL signal and just a blue screen, and no, not the Atari Memo Pad blue screen.

The part number for the CPU board is CO15500 rev 5, and supposedly it is meant for the 800 despite it coming from my 400. The CPU originally from the 400 is a 6502C, the one I have is a 6502B.

I might have to swap the 400 CPU back in as I know it worked in this CPU card.

Won't get much done today as I have a looooong work meeting now and then I'm off training tonight and in the office tomorrow. Fortunately my last day of the year is the 18th, so this Thursday, so I should be able to get some time in on this soon :)

[stephen_usher]

What "fun" (for certain values of "fun")! :-)

Did you find and check the hardware, power-on reset circuit in the end?