exxos's Atari, Amiga & retro forum

stephen_usher wrote: 07 Mar 2022 06:56 Given that it’s doing strange things on power up, how does the system reset to a known state before running the CPU? Just wondering about the reset circuit, if there is one.

Also, did you try looking at the LA results during multiple power-ups to see if it’s consistent?

I'm not sure, the service manual isn't great, I don't think there is any sort of reset circuit at least not an obvious one, but I'll revisit the schematic see what I can see.

I did a couple of starts, but the results weren't consistent. I also did one with Star Raiders plugged in and that was obviously different, but the game did start, so obviously at some point it's hijacking the boot process and ignoring the OS ROM.

stephen_usher wrote: 07 Mar 2022 09:07 Oh, just another thought. Have you checked the address and data line pull-ups? Having the behaviour change if a cartridge is inserted might point to iffy voltage on the buses.

Funny you should say that, but I had the same thought late last night, but it was too late for me to start looking, so going to have a look later on.

Tomswork wrote: 07 Mar 2022 07:26 That pattern of the 600 looks very repetitive across the address lines. The 6502 has a simple timer to reset maybe the cpu is sitting in a no op condition


Tom

It's doing something, I think stuck in a loop until I use the Star Raiders cart that hijacks the OS boot early.

Time for a small jigsaw puzzle 😁


My other printout is too fuzzy to read.

:)

And all done 😁:




Let's have a hunt for some pull ups shall we 😁

Hmm, interesting, I just discovered in the small print on the back of the Bounty Bob cartridge that it requires 16KB to run. That is suggesting that I have at least 16KB of ram and backs up the self test that passes the 16KB RAM test.

This is good.

The rest, not so much...

But carry on we must, this machine must run again!

@stephen_usher

Well, schematic shows pull ups for the data bus, all 10K and attached to 5v. Some other pull ups around the board, but nothing for the address bus it would seem.

I'd better check them anyway.

If there was an address bus problem, would that not also prevent the cartridge from running? Is this not more likely to be a decoding issue, stuff that the cartridge isn't reliant upon since the roms etc get bypassed?

BennehBoy wrote: 07 Mar 2022 15:00 If there was an address bus problem, would that not also prevent the cartridge from running? Is this not more likely to be a decoding issue, stuff that the cartridge isn't reliant upon since the roms etc get bypassed?

I agree, you'd think that, but I know all of the main ICs are good, MMU included, and all the other logic ICs are new and known good. I think it may be to do with the carts executing before the OS ROM, or at least very soon into the ROM. It's bloody strange.