Posted By
Hubi
on 2021-01-25
13:38:09
|  C16 Black Screen, DIAG264 flashing
Hi all,
A couple of days ago, I got myself two C16s, both showing only a black screen. There is a video signal on the output and I can measure the LUM signal with an oscilloscope.
Switching the Kernal ROM for DIAG264 I get five brief, greenish flashes on screen that repeat after a second or so on one of the two machines. Does anyone know what that means? The DIAG264 manual does not describe this particular error message. Well, at least it seems the TED puts out a picture, for what it's worth. Also the voltages seem to be ok.
When I switch the CPUs of the two machines also the other shows the five flashes with DIAG264. So I believe the CPU that does not show any flashing is possibly damaged.
U12 is a MOS4404 in both machines, which I read are unreliable. I desoldered that IC, socketed it, and replaced it with a new 74LS02. No change, still five repeating flashes.
One of the two C16s has a 64K RAM extension that can be selected via a switch. The mod seems done well, but again the behaviour is the same, no matter whether the switch is in the 16K or 64K position.
I'd greatly appreciate any help and advice. I'm a beginner and not particularly familiar with the C16.
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Posted By
unclouded
on 2021-01-26
05:08:41
| Re: C16 Black Screen, DIAG264 flashing
Sorry I can't be of much help but I found it easier to identify the faulty part by switching parts from a broken C16 in to a working machine until it broke. It turned out to be the PLA in my case and that machine now has a shiny new PLA from MCes.
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Posted By
crock
on 2021-01-26
06:03:12
 | Re: C16 Black Screen, DIAG264 flashing
Hi Hubi, I created Diag264 so hopefully I can help.
The documentation at https://www.inchocks.co.uk/commodore/Diag264/ does explain what the flashes mean. To quote from the "LOW RAM TEST" section:
"The screen border will cycle through a sequence of colours as each test byte is used and, as the low-ram area includes the screen and colour memory, you will see the contents of the screen rapidly change. The same reporting mechanism is used if an error is found during this test, with the screen background being black. If a failure occurs at this point, the number of screen flashes will point very strongly to the defective DRAM. The table below shows which IC is the likely problem."
I tried to copy the table here but the forum software messes up the formatting by removing extra whitespace.
Flashes Bit Plus/4 IC C16 IC C116 IC
1 D0 U11 U5 U5
2 D1 U12 U5 U5
3 D2 U13 U5 U5
4 D3 U14 U5 U5
5 D4 U15 U6 U6
6 D5 U16 U6 U6
7 D6 U17 U6 U6
8 D7 U18 U6 U6
If you read down the table you can see that 5 flashes suggests a problem with bit 4 of the databus, which in a C16 is implemented by the RAM chip at U6. However, the fact that the same same CPU in another machine also gives 5 flashes is a bit suspicious as it would be a bit of a coincidence (but not impossible) for the same RAM chip in 2 separate machines to be at fault.
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Posted By
Hubi
on 2021-01-26
11:10:36
| Re: C16 Black Screen, DIAG264 flashing
Hi unclouded and crock,
Thanks a lot for your replies. This is very helpful.
@crock: Good to know that the Diag264 is doing what it's supposed to do. I guess I was confused because I didn't see any of the previous two steps of the low RAM test. Do I understand this correctly that there is no screen output if the first two tests are passed?
Indeed it is a bit intriguing that both boards show the same error with one of the two CPUs. I've ordered a 6510 -> 8501 adapter. So I will test the boards again with the C64 CPU in the coming days. Hope this reveals something.
Just two more questions:
1) Does the Diag264 care if you use an original 8501/7501 or the 6510 with an adapter?
2) How long should the low RAM test take if there's no error?
Thanks again for your support and for developing the Diag264. Great stuff!!
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Posted By
crock
on 2021-01-26
12:11:13
| Re: C16 Black Screen, DIAG264 flashing
The Low-RAM test has three parts, the databus, the addressing, and then the device/RAM test. From a reporting perspective, any failures in these 3 steps are reported via the screen flashes, because we can not make any assumptions about the screen memory working.
If all three steps of the low-RAM finish without problem, it reports back as OK on the main screen as "LOW RAM OK". To answer the additional questions:
1) It will work with a 6510 adapter *but* the serial and cassette tests will fail because the 6510 uses different port bits on the processor to control those signals. I could potentially add this to the to-do list if I had one to work with. Andy Challis did send me a copy of the 6510 adapter ROM and the ROM tests will correctly identify it.
2) The Low-RAM tests take about 7-8 seconds to complete.
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Posted By
Hubi
on 2021-01-26
13:55:34
| Re: C16 Black Screen, DIAG264 flashing
Thanks for your quick reply. This is awesome. I'll try things out with the 6510 and let you know.
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Posted By
siz
on 2021-01-26
16:48:17
| Re: C16 Black Screen, DIAG264 flashing
Slightly off-topic (okay, mostly off-topic) here but IMHO a 6502->7501 adapter is a better choice because those contain a proper $00/$01 CPU I/O port emulator too so all the serial and datasette port operations will work as expected.
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Posted By
Hubi
on 2021-01-27
02:05:58
| Re: C16 Black Screen, DIAG264 flashing
@siz: Interesting idea! Do you know where to get such an adapter? Or is it easy to soldier myself? Would that get rid of the need for a patched Kernal? Also would that work with the 65C02 that's still in production?
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Posted By
Hubi
on 2021-01-28
16:45:09
| Re: C16 Black Screen, DIAG264 flashing
@crock: So today the adapter board arrived. Using a 6510 salvaged from an old C64 made one of the machines run again. The full Diag264 runs through and all other systems are reported to be ok. Together with a modded kernal to account for the differences of the 6510 vs. the 8501 I have a working C16 now. Yay! Thanks so much for your help and for writing this great piece of software!!
Looks like I had two dead/damaged 8501 CPUs indeed. The second machine still has a problem though, but there is also progress: With the 6510 CPU, it shows the Diag264 low RAM test now and enters the subsequent tests but the screen output isn't quite right and it crashes after a second or so showing just a blue screen. I made a screenshot just before the blue screen but don't know how to upload it here. Find it here:
https://www.dropbox.com/s/k5f6n1tdbhnvyug/IMG_1893.jpeg?dl=0
That's the only thing visible after the low RAM test. Any idea what this means?
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Posted By
siz
on 2021-01-29
02:57:38
| Re: C16 Black Screen, DIAG264 flashing
@Hubi: you can buy it for example from Lotharek in Poland: https://lotharek.pl/productdetail.php?id=257
(it also contains an 512k Hannes-compatible RAM expansion but you have to remove your onboard RAM to use that). It comes with a 6502 built in. (I have this one in my main plus/4). And yes, it allows you to use stock KERNAL.
As for 65C02: if my memory serves well it is pin-compatible with the original 6502 but the instruction set is not 100% compatible with the 6502/6510/7501 line of processors so software using illegal opcodes (mainly old "protections" and a lot of demoscene demos (and perhaps recent games too) are not guaranteed to work (I haven't experimented with 65C02 so I'm not sure)
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Posted By
crock
on 2021-01-29
05:51:32
| Re: C16 Black Screen, DIAG264 flashing
@Hubi - Hmm, that is indeed odd that it passes the Low-RAM then crashes. Looking at the screen shot with those random @ characters, I would suspect RAM still, though some of the other patterns are suggestive of an addressing fault.
Question: Can you tell if it makes any attempt to run the High RAM test? Upon completion of the LOW-RAM test, it should display the initial DIag264 screen with the title, my name, build number etc, then it will pause for about 1 second before blanking the screen and running the High RAM tests. Do you see a pause for about at second without the corrupted screen? I assume it does because you say it returns after showing a blue screen? Any chance you could shoot a small video and upload it?
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Posted By
Hubi
on 2021-01-29
08:02:28
| Re: C16 Black Screen, DIAG264 flashing
@siz: Thanks for the link. This looks really interesting, exactly what I need. I'm not familiar with the Hannes RAM expansion, but if it means that I can run all Plus/4 games on my C16 that would be great!?
Right, the 65C02 certainly misses the illegal op codes. Better to stick to the real 6502 then, I guess.
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Posted By
Hubi
on 2021-01-29
08:04:21
| Re: C16 Black Screen, DIAG264 flashing
@crock: Thanks for your patience and all the help! To answer your question, the screenshot shows the only thing that's visible after the low RAM test. But let record a short video tonight and upload it to dropbox. That should clarify things.
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Posted By
Hubi
on 2021-01-29
13:54:21
| Re: C16 Black Screen, DIAG264 flashing
@crock: Please find the short movie here https://www.dropbox.com/s/flchxt2za6o9g4u/Video%2029.01.21%2C%2019%2046%2005.mov?dl=0
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Posted By
crock
on 2021-01-29
14:57:43
| Re: C16 Black Screen, DIAG264 flashing
Thx Hubi. So the display suggests RAM issues, possibly with the Multiplexers (74LS257's) as there are groups of incorrect bytes. It is bothering me that the low RAM checks did not pick it up though. I would start there.
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Posted By
Hubi
on 2021-01-30
11:58:59
| Re: C16 Black Screen, DIAG264 flashing
@crock: It's solved! You were right on the money from the start. It was U6. For some unknown reason the connection between pin 6 of U5 and pin 6 of U6 isn't there on this board. See this picture:
https://www.dropbox.com/s/ptdejzf3gokb85g/Foto%2030.01.21%2C%2017%2053%2016.jpg?dl=0
I just soldered a short cable to bridge this defect and now it's working.
Thanks so much again for all your help. Very happy to report that I have two working C16s now, one with 64K RAM.
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Posted By
Hubi
on 2021-01-30
12:29:39
| Re: C16 Black Screen, DIAG264 flashing
BTW, I checked the 74LS257s first. They were fine as my EPROM programmer told me. But this only led me to checking all connections with the help of the circuit diagram. In hindsight I should have seen the missing circuit track earlier...
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Posted By
siz
on 2021-01-30
14:54:14
| Re: C16 Black Screen, DIAG264 flashing
> For some unknown reason the connection between pin 6 of U5 and pin 6 of U6 isn't there on this board.
From your picture it seems that it got ripped out when someone desoldered the ICs and put sockets in there. Sometimes it happens when you are not extremely careful (I did similar damage to one of my Amigas)
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Posted By
Hubi
on 2021-01-30
17:35:05
| Re: C16 Black Screen, DIAG264 flashing
@siz: Makes sense. Must have happened when the previous owner installed the RAM expansion.
Thanks also for your advice with the 6502 adapter board. I'm currently testing the 6510 inside the C16 and there are some incompatibilities indeed. Will play around a bit more and then probably go for your suggestion.
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