White screen with STacy

[derkom]

Good to see that you got your scope. Even beyond this project, I'm sure you will find that a useful tool to keep in your arsenal.

Unfortunately, you've gotten to the point now where I don't personally know what to suggest. This is the kind of situation where I would be following the schematics and looking for signals, hoping to find something amiss. I did once manage to find a fault in someone else's Stacy board this way. There was a buffer chip between the MMU and the RAM board (and this chip seemed like it actually wasn't on the schematic!) where I found missing signals on the downstream side. Seems a poor quality factory soldering job had just broken due to board flex, and a reflow fixed it.

I wouldn't expect this specific thing to be your issue (but who knows?), but that gives you the idea of the kind of shot-in-the-dark troubleshooting I've ended up doing. Alas that kind of troubleshooting requires the hands-on approach and a lot of time.

However, I hope that some of the other people reading this thread, who know more than I do about what kind of proactive steps you can take, can give you some specific hints about where to look with your new scope. These are fairly simple computers, so with time and a few hints, most problems can be solved.

[exxos]

I've not been following much. But I would have a look at this https://exxosforum.co.uk/forum/viewtopi ... =78&t=2362

[Roberto]

I've not been following much. But I would have a look at this https://exxosforum.co.uk/forum/viewtopi ... =78&t=2362

Thanks Derkom, thanks Exxos for your advice.
I just wanted to remember (given the length of my posts) that my STacy worked, until I decided to clean it of all that mess that covered the motherboard. I washed it (as I did for all my Atari), but unfortunately I didn't dry it well; and when I connected it to the power supply, at first I saw the LEDs of the power supply flashing and then a big STack (just to be on the subject). A noise that left me foretelling a motherboard component breakdown after a short circuit.

Meanwhile, I wanted to post a photo where the DIAG cartridge is transmitting something, it is not clear what but let's say that my STacy is not really dead. I changed Serial Terminal (now I use TeraTerm, while before I used CoolTerm) and actually we see something confusing:

IMG_7161.jpg

In the meantime I have printed on paper all the logic diagrams of my STacy and also the pinout of the components.
I have run a test for all the components to verify the presence of the + 5V in the pins that require it. And actually ALL the components (even the LSxxxxx) have the + 5V. Except one pin of the Shadow chip, exactly pin 35 which is at 0V. Now I don't understand why in the pinout of the shadow scheme pin 35 is labeled Vcc, while in the logic scheme, pin 35 is labeled as GOUT2, furthermore in the logic scheme pin 30 should have + 5Vcc while the pinout of the component is labeled as A6 (the latter I have yet to measure it because I only realized it now).

the second test I did was to confirm the presence of the correct clock signal frequency for each component. I confirm, the signal is present and the frequency is correct, but testing the value in Volt PkPk and comparing it with an Atari ST, I noticed the following:

Code: Select all

                                  STacy         ST
GLUE         Pin 54 2MHz          5,3V          5,3V
             Pin 43 500KHz        5,3V          5,3V
             Pin 42 500KHz        5,3V          5,3V
             Pin 34 8MHz          3.5V          4V
     
68000        Pin 15 8MHz          3,3V          4V

STDMA        Pin 39 8MHz          3.3V          4V

ACIA         Pin 3 and 4 500KHz   5,3V          5,3V

STMCU        Pin 5 16MHz          2V            2.8V
             Pin 20 8MHz          3.8V          3.9V
             Pin 19 4MHz          5.2V          5.2V

MFP68901     Pin 35 4MHz          5,2V          5,2V
         
SHIFTER      Pin 2 32MHz          1,2V          (I did not find on ST)
             Pin 39 16MHz         2.50V         (I did not find on ST)

I wanted to understand if this is normal, having a slightly lower voltage in some components of the STacy for what concerns the clock.

The third test was to verify that the RESET, for each component (at least for where I was able to find the RESET label), goes high after about 1.5 seconds from power on. It is so.

I forgot; I have inspected under the magnifying glass, any broken tracks or burnt components, but unfortunately it is not found.

If you of course have any suggestions as to which test I could take I'm ready ...
Grateful to all of you.

[rubber_jonnie]

Ok, so my first piece of advice here is to stop what you're doing. If you're getting nowhere then stopping and taking stock is a valid thing to do.

You seem to be trying to check almost everything, but doing that will make it harder than if you focus on one thing at a time. I'd suggest focus on the ROM part of the board after a few basic checks. I know you may have done some of these before, but they can be worth reiterating.

First off, I'd suggest that you should look at YouTube for some videos on how to measure things with your scope. If it's anything like mine, things like frequency measurement need to be turned on per channel of the scope to allow you to get a proper view of those values. The scope can be your friend here, but it can also be your worst enemy if you don't understand what it's telling you.

If you don't already have a schematic and pinouts of the main chips then you will need them. I prefer to print them on paper so I can note my results.

Check every single IC has 5v where it is expected and note down your results.

Do the same for 12v as some serial ICs on the ST require 12v, and if they don't have it, that may be why your serial results are corrupted. You also need a null modem cable for the serial output to work, and it is worth checking your console settings on whichever terminal emulator you're using to be sure they are correct, see here for more detail on that: Diag Cart Usage

If all voltages are good, then check all the clocks across the board and compare to expected values, note down the results.

I'd recheck the/HALT and /RESET activity, though it seems the results there seem fine, just be aware that /HALT is bi-directional and can be active either because the CPU says so, or something else on the board has stopped execution.

Then I'd look at the ROMs. Make sure that the /CE and /OE pins is active during boot and that you are seeing address activity across all address pins and all data pins at the ROMs. Your scope will help here.

If either /CE or /OE are failing then you probably won't be able to get past the white screen and I'd suggest that you should then trace back along each line to check any logic ICs along the way to be sure the inputs are giving the correct outputs on any of the logic ICs.

That should keep you busy a while.

One final piece of advice I would offer is that despite what is being shown on many YouTube channels, I would suggest that washing mainboards is a bad idea. I know it's too late for you now, but ti think it's worth stating the reasons.

Aside from the thermal stress from going into hot water, unless you are able to rinse with de-ionised water and be 100% sure all residual soap is gone and there is no chance of hard water deposits forming during drying, you are risking unwanted problems such as shorts. Additionally you can get water stuck under ICs that does not dry at the same rate as surface water on the board, so unless you are prepared to wait or have somewhere warm to place the board to accelerate drying, you are always taking a large risk.

My personal preference is to use compressed air at a relatively low pressure to blow off dust and spot clean with a conductive plastic brush or paint brush and IPA. IPA will dry much faster and cleaner than water.

I can't offer much else above what I've said, I don't have a STacy so can't specifically help with direct comparisons, but what I've said above has helped me fix multiple ST family machines, and also the same techniques flow though into other machines too.

[Roberto]

Then I'd look at the ROMs. Make sure that the /CE and /OE pins is active during boot and that you are seeing address activity across all address pins and all data pins at the ROMs. Your scope will help here.

If either /CE or /OE are failing then you probably won't be able to get past the white screen and I'd suggest that you should then trace back along each line to check any logic ICs along the way to be sure the inputs are giving the correct outputs on any of the logic ICs.

Hi rubber_jonnie, thanks for all the advice you have given me. I state that I calibrated the oscilloscope (and the probe) directly from the store where I bought it (I didn't get it on the internet). I have printed all the logic diagrams of my STacy on paper so that I can take note of all the measurements I make. So I followed your advice, which is to focus on one thing at a time. I started with ROMs. The following is the logic diagram of the ROMs:

ROM.jpg

It looks very bad but knowing the initials of the ROMs involved (TOSHIBA TC571001D-20) I went in search of the pinout and we have the following:

THOSHIBA.jpg

and here I find myself in some difficulty. In the sense that I measured the signal / CE (pin 22) in both ROMs (I saw from the logic diagram that they are connected in common) and, with the machine on, they have + 5V, so they are active. You also said that / OE should also be active for the two ROMs to work (I imagine that by active you mean high value), but / OE (pin 2) is not, indeed, judging from the logic scheme it seems impossible that it can be active, since it "seems" connected to the GND of the ROM (in the logic diagram it is badly seen but it seems that it is connected to the GND). Correct me if I'm wrong.

Could you also tell me what that sign above the / PGM, / CE and / OE labels means? Sounds like a way to say active low maybe?

NB: I forgot to tell you (I had posted this on my previous posts) that I bought the diagnostic cartridge together with its null modem cable and I first did a test on an Atari 520ST where it showed me the menu correctly, both on Video and on RS232 on a terminal, so I reported the exact configuration on my STacy. I also read your post where you said that the white screen (if you have a color monitor) is a characteristic ROM reading problem. So I changed the ROMs (derkom was kind enough to give me a couple created by him and tested them on his STACY), but sadly we saw that they are not the ROMs.

[rubber_jonnie]

@Roberto

PGM is only used for programming the ROMs so can be ignored.

When you have /CE or /OE the / means active low, in other words, when the signal is at 0v, that is when it is active. Sometimes you might find, as here, that /OE is tied to ground, and that make it active for output (It's a ROM so you want to read OUT) as soon as /CE becomes active.

If they are at 5v all the time then they are not going active, you should see a pulsing signal and will also see a pulsing signal on the address and data pins.

At that point it means they are not getting a correct signal if there is no pulsing and just a constant 5v or a constant 0v, in which case you need to trace this back to the next item in the chain, which may well be some logic ICs.

Even with a set of known good ROMs from @derkom, if the chip select/enable logic isn't working then the ROMs won't be able to be read. Given that the diagnostic cart doesn't work, this further suggests a ROM logic issue.

Please try not to think in absolutes of 5v or 0v when it comes to things like /CE, /OE and address/data lines, you will see a signal and whilst it will have a theroetical max/min of 5v & 0v, the signal as a whole will be pulsing.

Have a look at some of the videos here: Signals on an oscilloscope and you'll see what I mean by signals.

I will try and get you some screen shots tomorrow of /CE on one of my machines so you can see what I mean.

[rubber_jonnie]

As I mentioned here is a screenshot of the /CE line on a ROM. It's not a STacy, but the theory is the same. You can see the pulses that turn the enable/disable the ROM. Address and Data lines will look similar when working

PXL_20221005_134952382.jpg

If you aren't seeing this and it is staying high (5v) or low(0v) then you likely have an issue. If you do see something similar (You will probably need to adjust your scopes horizontal and vertical scales or if you have, use the Autoset feature) but don't see anything on address/data lines then you may well have another issue.

I would concentrate on the /CE logic to begin with.

[Roberto]

...
I would concentrate on the /CE logic to begin with.

Hi rubber_jonnie!
So, I think I've made some small progress. I read in one of your posts that the 0 and 1, with TTL technology, as a signal given by an integrated circuit, do not necessarily have to correspond to 0V and +5V, but there is a margin, that is (if I remember correctly):

0 Logic is between 0V and +0.8V;
1 Logic is between +2V and +5V;

The voltage values ​​between +0.8V and +2V identify an uncertainty zone that is not to be used.

So, I placed the probe of my oscilloscope on the pin /CE to see the signal, and as in the photo it is always 1 (for both the HI and LOW ROMs), therefore, since it is active low, those ROMs are not starting to function . I also supported the probe on the pins relating to the addresses and data and of the aforementioned ROMs and, despite some pins having a certain activity, the signal is always 1 (based on what I learned above).

/CE

CE.jpg

One of the addresses

ADDRESS.jpg

Based on this I therefore deduce that those ROMs will never work properly if that /CE remains at +5V.

Thanks to the logic diagram, I traveled the connection of pin 22 /CE of the ROMs; I have seen that they are connected in common and I have also seen that it is connected to pin 7 of the 60 pin female socket labeled J2.

ROM.jpg

I continued with the path of that connection to the pin 7 splice of the male connector labeled J24 on the motherboard. The link goes to the label named /IMROME.

foto2.jpg

I searched through the /IMROME schematics and saw that it is an output signal from pin 12 of the 74LS11 (physically it is placed under the CPU).

foto3.jpg

So I went to see the technical characteristics of that integrated. It simply has three 3-input AND logic gates:

74LS11.jpg

So I measured the input voltage of pins 13, 1 and 2, since they are the ones that determine the output of pin 12. I saw that they are all three others, therefore, theoretically that 74LS11 works well since the AND gate gives as output 1 if all inputs are at 1.

Looking again at the logic diagram, we see that the inputs of that 74LS11 are taken from outputs 20, 21 and 19 of the Glue chip. Also measured there all three, values ​​+5V.

Now, I'm sure that Glue works, because I replaced it with a Glue from an Atari 520ST and put the STacy glue on the ST.

and here I stop.

One last thing that apparently has no connection with all this: The STacy's Swadow chip has the RESET (active high) at pin 66. I have noticed that it is always +5V.

[exxos]

I feel I should point something out here..

Just because CE is not switching does not necessarily mean there is a fault with it..

You need to use your scope in single shot trigger mode to detect CE properly. Generally the CPU (if it is running correctly) will start from address zero where the GLUE logic will set CE low.. BUT.. If TOS fails to start then it will not get the next instruction to execute and basically the system will crash so you will never see CE go low again.

Basically you would see CE go low once ( maybe even several times in fact) Then the system will crash and you will see BERR / HALT go low and stay there.

This is why I mentioned earlier you need to test address zero.. Ideally by removing the CPU manually setting it because then if you are forcing address zero on the CPU, then CE will be low continuously. viewtopic.php?p=91608#p91608 IMHO You need to really do this first before anything else otherwise you will be chasing faults which may not actually exist!

[stephen_usher]

I take it that you've checked the clock on the CPU? No clock == no go.