exxos's Atari, Amiga & retro forum

Happened on this article last night: https://atariage.com/forums/topic/31733 ... /#comments

Looks very similar to the problems I'm seeing, so better get reading!

Ok, so it 'looks' like the 5713's and U11, a 74LS02 are likely good. Pin 10 of U11 drives pin one on each 5713 (U2 & U3) and they all match the working drive.

What doesn't match is the output on pin 3 of U2 & U3. Based on the service manual there should be a pulse where the voltage jumps up above 12v and then stabilises, followed by the pulses that drive the stepper.

This first pulse is seen on the output, but not the following pulses, which I can see on the good drive.

The 5713s are driven entirely by the 6532 (RIOT) chip, so next I need to compare PB2-PB5 outputs on the good machine to the bad.

I think I'll also need to compare all of the 6532 inputs to see what is different between the good and the bad as at present the 6532 is a bit of a black box, because it may just not be getting the correct inputs.

Ok, so the 6532 to 5713 buzz out ok, so next I need to scope the outputs of the 6532 on boot to see what they look like compared to the good drive.

It's just like troubleshooting any old computer.

Oh, wait. It has a CPU, ROM, RAM and a PIA, it is an old computer!!

Ok, so moving down the chain to the 6532 and the 4 outputs that drive the stepper motor and again there is a problem.

The outputs are supposed to go high, then pulse, but they just go high. This matches what I saw at the 5713s, and having checked continuity from the 6532 to the 5713s, that's all fine.

So the 6532 is doing what it's told as far as I can see, so although I know the 6532 works (It's from a working 2600) I'm going to re-test them in the good drive to be sure.

There are very few inputs to the 6532 from the 2797 that can drive those stepper outputs, so I will scope those next after testing the 65632's.

If those match what I see on the good drive, and the 6532's are good, then I can concentrate on the CPU, RAM and ROM. I'll also double check all the address/databus continuity as it might be something as simple as that.

It's turning into quite an interesting round of troubleshooting.

The biggest problem... no diag ROM and console output for the debug messages. :-)

stephen_usher wrote: 19 Nov 2021 10:18 The biggest problem... no diag ROM and console output for the debug messages. :-)

Or screen output!! Bloody annoying, but what can you do! From what I read on another post, the constant on/off pulses of the drive motor are actually a fault code! I have yet to find out what that code means anywhere!

A quick look at the 2797 pinout and I don't believe it has anything to do with the stepper pulses. This is backed up by the fact that there is no change when I remove it.

I don't believe the shorted chip I removed is connected either, as that is related to the SIO port and I/O. Also removing it made no difference. Had to go due to the short though, new ones should be here soon.

I know that the 6507, ROM and 6810 are good as I removed my Happy upgrade from the good drive and put the originals back, and I have swapped them out into the bad drive with no difference. I need to verify the 6532's now, then see what's going on at boot.

I do know that when the 6532 is plugged in the address bus down at the CPU, ROM and RAM goes bad on some lines, and there don't seem to be any buffers in there, so I suspect somewhere along the line there is something wonky.

Once I'm happy with the 6532 I can get more probing done. May even have to break out the logic analyser!

6532s all check out in the good drive...

Based on the address bus going wonky when the 6532 is plugged in, that's where I need to look next.

Ok, so I took a short break from work and did some chip testing.

It looks like all of the ICs are good, including the 2797 replacement for the 2793.

Also a new discovery. I thought the drive would start with the 2793(7) removed because the service manual suggests that it would based on the block diagram.

Turns out when I tested the 279X in the new drive, when it's removed, the drive just spins. It doesn't pulse like the dead drive, but it doesn't work properly.

So, that now becomes an area of interest, in particular the address and data lines, which I'm going to check continuity on next between all of the components on both buses.

It was a worry that the 279X wasn't going to work, but now I've tested it in the good drive and it powers up correctly, I at least now know that the CPU, ROM, RAM, RIOT and FDC are all good and that does reduce the places I need to look for the problem.

So, as I was in the mood I've scoped all of the 279X lines (All the data and address lines checked out continuity wise) during power up.

A rather crucial thing I found was that the chip select line does nothing on boot, whereas on the good drive it briefly pulses high then low.

This is supposed to come from U12 pin 6, which is a NAND gate, so I need to check that next, see if it's getting inputs, and if it is even outputting.

Scope on, GO!!

Well, without /CS not much is going to happen. :-)