1040 STF showing occasional rogue pixels

[sandord]

I think that the fact that the corruption is only visible in part of the screen and not across the entirety of it actually implicates the ram more. As the screen is being moved 16 bits at a time between the RAM and the shifter, the only thing that changes as it gets to the "bad" bit of the screen is which location in RAM is being accessed. If the problem was somewhere in the transportation of the data, I would expect it to either be random (totally different for each frame) or covering the screen completely at all times.

In support of the "bad ram" hypothesis, you said that sometimes the problem goes away when the problematic part of the screen is redrawn. Possibly this is because the problem has a 50% chance of not affecting a pixel (if it's a single stuck bit in the word), so 8 palette colours would look fine, and 8 would look wrong. So when the colour of a bad pixel is written over, every second time it'll make the problem go away.

Well, that makes sense. But the lack of randomness is very strange when RAM tests can't find any faults (except for one occasion, see first ram test picture in this thread).

[sandord]

I found this program, never used it, but looks like it does some bit tests on screen...

MEMTEST.zip


Blank_00027.png

Thanks! It's been running for 10 minutes now, back in 1mb configuration. No errors yet.

[troed]

So, there's a good chance this isn't about when the CPU reads RAM, but when the Shifter reads RAM, then.

[sandord]

I've made a GIF of Metrocross, which still exhibits the most interesting artifacts. By the way, I've verified that this game runs fine on other STs.

The picture is a little bit shabby because I captured it using my phone, which doesn't sync to the 50Hz flicker too well, causing the GIF to shimmer.

2iz316.gif

Don't pay attention to the variations in the horizontal bar as shown here, it's just a video capturing artifcat, IRL you just see the bottom version all the time:

Untitled.png

Here's a single screen shot:

x.png

There are pretty much two things going on here:

(1) Some bad pixels in the status area of the screen (bottom of the screen).

Untitled4.png

The amount of pixels slowly increases over time, as the ST heats up I presume:

Untitled3.png

(2) Some bad pixels in some of the game background sprites:

Untitled4.png

Now these guys are interesting because they scroll to the left, along with the level. I don't think Metrocross does its scrolling by shifting the video RAM to the left but rather redraw the background every frame. Otherwise, the artifact wouldn't be animating like it does. I think Metrocross keeps pre-shifted versions of its tiles in memory and some of them have become corrupted. As you can see in the GIF, the pixels are jumping vertically and this pattern keeps repeating consistently while I play the game. It's just that they appear only in certain areas of the levels.

The strange thing is though that the tiles are only corrupted on a specific range of display lines, so its just like a certain combination of source and destination adresses (during copy operations) is causing artifacts.

Any other artifacts that might attract your attention, are caused by the video capturing. On the real monitor, there's just the two mentioned artifacts, (1) and (2).

[exxos]

Maybe its a problem with the bus isolation buffers ?

[sandord]

Maybe its a problem with the bus isolation buffers ?

Are those the SN74LS244N chips?

[exxos]

Are those the SN74LS244N chips?

and 373.

Might be worth resoldering the shifter,mmu,244,373...

[sandord]

Are those the SN74LS244N chips?

and 373.

Might be worth resoldering the shifter,mmu,244,373...

I already had resoldered the MMU. I'll do the shifter and the 4 buffer chips as well, good advice.

What do you think of using freeze spray of each of them, one by one to see if the pixels disappear when restarting Metrocross? Or could that be dangerous to the ST?

[exxos]

Should be fine :)

[sandord]

I don't believe my eyes :roll:

I've just reflowed the shifter, buffers and the cpu while I was at it. I started Metrocross.

It's running in 2 frames instead of one. All juddery now. :shock:

How can this even be possible??