Think I'm loosing my (beeb) memory

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steve3000
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Think I'm loosing my (beeb) memory

Postby steve3000 » Mon Jan 01, 2018 5:27 pm

I need some help tracking down the cause of a sudden memory problem which occurred today on my Issue 7 Beeb. I hadn't used my Beeb for a few months, and powered it up today to enjoy some 8 bit action before I have to return to work tomorrow... All was going well until I noticed the screen seemed to have random pixels out of place/corrupted in MODE 5...

A few minutes later and a bit of testing, and it seems the upper 16K of the 32k has is suffering corruption, see picture below - this shows what MODE2 looks like immediately after booting up.

20180101_161944.png
MODE 2 after boot up

The beeb still boots reliably all the time with S25 North (but the MODE7 boot screen is also corrupted.

The computer will not boot with S25 south, but removing S25 allows the computer to boot again with no problems, and no sign of memory issues - however obviously only 16kb... so I suspect the fault is with the CAS1 bank.

But (as you see from the picture) the corruption runs across the full 16kb, affecting the same bits for each RAM chip... so I'm guessing this means it is unlikely to be the RAM itself at fault? And probably a buffer IC? I tried replacing IC14 but this did not help, and all the other ICs are soldered in on this Issue 7 :(

So before I start desoldering things, any thoughts as to what might be the cause of this failure?

Thanks!
Steve

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Re: Think I'm loosing my (beeb) memory

Postby Kazzie » Mon Jan 01, 2018 7:48 pm

The individual RAM chips in a BBC don't cover a particular set of bytes. They respond to one of 16K address inputs, but only give one bit of output. Eight chips are used together to give 16K x 8 bits, or 16KB in a memory bank. As a result, it's perfectly possible for one faulty RAM chip to result in partial corruption throughout half your memory space.

Have you had a look through the Fault Finding thread? There are some test programs there to help you identify the faulty IC.
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Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)

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Re: Think I'm loosing my (beeb) memory

Postby DutchAcorn » Mon Jan 01, 2018 7:59 pm

What corruption do you see in mode 7?
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Re: Think I'm loosing my (beeb) memory

Postby steve3000 » Mon Jan 01, 2018 8:12 pm

In MODE 7 I see blocks of $$$$$ signs in some places where it should be black, suggesting certain bits are fixed 'on' in the upper 16k bank of RAM? Will post a photo later.

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Re: Think I'm loosing my (beeb) memory

Postby DutchAcorn » Mon Jan 01, 2018 8:26 pm

Yea, bit 1 specifically. So in terms of memory chips IC54 is suspect. But it can also be caused by other parts of the memory circuit (see this recent thread).

You can try the simple memory check program suggested in the same thread.
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Re: Think I'm loosing my (beeb) memory

Postby Kazzie » Mon Jan 01, 2018 8:29 pm

The $ is represented by a hex value of &24, whereas a space is &20 (in ASCII and Teletext). From that I'd guess that your faulty chip is bit 2 (2^2=4) of CAS1 stuck on. That should be IC55, if I've read the circuit diagram correctly.

Can you find any other faulty characters as you type? If they're also "off by four", that could support the above theory.

A picture of your screen in Mode 7 could help confirm this.

Edit: it seems DutchAcorn and I are in disagreement here :)
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Re: Think I'm loosing my (beeb) memory

Postby DutchAcorn » Mon Jan 01, 2018 9:35 pm

Kazzie wrote:The $ is represented by a hex value of &24, whereas a space is &20 (in ASCII and Teletext). From that I'd guess that your faulty chip is bit 2 (2^2=4) of CAS1 stuck on. That should be IC55, if I've read the circuit diagram correctly.

Can you find any other faulty characters as you type? If they're also "off by four", that could support the above theory.

A picture of your screen in Mode 7 could help confirm this.

Edit: it seems DutchAcorn and I are in disagreement here :)


Edit: You are absolutely right; my binary counting is obviously not up to standard.... :oops: :oops: :oops:


Well, for the most part we are in agreement. I was counting the 8 bits from 0 to 7 so "your" bit 2 is my bit 1. From the following table I concluded that IC54 may be playing up.

Data bits - IC numbers

Code: Select all

   /CAS0 /CAS1
D0= IC61  IC53
D1= IC62  IC54
D2= IC63  IC55
D3= IC64  IC56
D4= IC65  IC57
D5= IC66  IC58
D6= IC67  IC59
D7= IC68  IC60


If my reasoning is faulty please correct me! :D
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Re: Think I'm loosing my (beeb) memory

Postby steve3000 » Mon Jan 01, 2018 9:55 pm

Great work guys!

Here's a picture of MODE7 on start up, note the pattern of 64 "$"s then 64 " "s.
20180101_210937.png

Clearly it should be saying "Acorn TUBE 65C102 Co-Processor" etc. but if bit 2 was set on, you get "Egovn$TUFE$65G546$Go-Tvogewwov" - almost what you see above, but the last 6 characters are not quite right on the screen. On screen they fall after where the 64 "$"s should end, so if bit 2 is working in the 'gaps' then it should show "cessor" but in fact they seem to have bit 2 forced off, which gives "casskr"?

So does this confirm faulty IC55?

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Re: Think I'm loosing my (beeb) memory

Postby Kazzie » Tue Jan 02, 2018 7:39 am

That's a very useful screenshot. The fact that bit 2 is forced on for 64 bytes, then forced off, makes me think it might not be the RAM IC after all, but something external (perhaps the memory refresh circuitry). I'll have a think about it once I'm back at a desktop machine and can look at a circuit diagram.

Edit: Right, I've had a think. That 64-byte interval could come from IC55 on its own, if by some means the data line is shorted to the address line that alternates every 64 addresses. Those pins (DO: pin 14 and A6: pin 13) are adjacent, so they could be shorted outside the chip, or inside.

I'd suggest taking a close look at those pins, perhaps doing a continuity check with a multimeter. If they're shorted, and you can't see/fix a solder short outside the chip, I'd be replacing the chip to see if that solves the issue.
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Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)

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Re: Think I'm loosing my (beeb) memory

Postby steve3000 » Tue Jan 02, 2018 6:06 pm

Interesting thought, yes, a short between address and data could certainly explain this... however I've just tested continuity between pins 13 and 14, and there's no indication of a short.

Also interesting, the effect now seems to be intermittent. Always there on start-up (at the moment) but sometimes disappears after a couple of mode changes...

I'll remove ic55 later tonight if I get time...

EDIT: not yet removed ic55 but now the issue has gone completely and Exile is running nicely...humm...

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Re: Think I'm loosing my (beeb) memory

Postby steve3000 » Wed Jan 03, 2018 6:53 pm

After 2 hours of Exile running without a problem, I switched the computer off and on, and the screen was back full of '$$$$'s as before.

Clearing the screen or changing MODE gradually improved the display, with '$$$$'s slowly disappearing with each CLS... Sometimes the computer would behave normally after this - with the memory between &4000 and &8000 reading and writing OK, however sometimes bit 2 was clearly just forced off rather than on (so the screen display looked OK as a blank screen, until anything with bit 2 set was written to it).

Most important point was that the fault seemed reliably reproducible immediately when the computer was switched on.

So late last night I decided to remove IC55...

20180102_224449.png
The offending IC55

20180102_230704.png
IC55 removed

I only had time to pop in a socket, in place of IC55, and try the removed IC55 back in the socket last night - and on testing everything was as before, with the same '$$$$'s present (so at least I hadn't damaged anything during the removal :) ).

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Re: Think I'm loosing my (beeb) memory

Postby steve3000 » Wed Jan 03, 2018 6:55 pm

So this evening I pulled my spare (but currently dead for various other reasons...) BBC B out of the loft and borrowed one of it's socketted RAM chips to test as a replacement for the suspect IC55...

...And it worked! No more '$$$$'s in MODE7, or green lines in MODE2. :D :D :D

Thanks both for your help tracking down my faulty IC55, it's great to have my beeb back functioning again :mrgreen:

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Re: Think I'm loosing my (beeb) memory

Postby DutchAcorn » Wed Jan 03, 2018 9:21 pm

That's good to hear!! =D>
Paul

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Re: Think I'm loosing my (beeb) memory

Postby tricky » Wed Jan 03, 2018 10:42 pm

Great to hear.

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Re: Think I'm loosing my (beeb) memory

Postby 1024MAK » Wed Jan 03, 2018 11:36 pm

Well done =D>

It's not unusual for DRAM chips to fail in strange ways.
If you have not yet binned the defective DRAM chip, would it be possible to send it to me please.
I'm after faulty DRAM chips so that when I have made my prototype DRAM tester, I have some real life example DRAM chips to test the tester with.

Mark
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Re: Think I'm loosing my (beeb) memory

Postby steve3000 » Thu Jan 04, 2018 12:43 am

1024MAK wrote:If you have not yet binned the defective DRAM chip, would it be possible to send it to me please.

Certainly Mark, happy to. But first might be useful for me to attempt to fix my other dead beeb with potential memory problems, as this may provide more dead DRAMs... It's been sat in the loft for almost a year now, so time I tried to fix it again. It has a fault not entirely dissimilar to the beeb in the recent thread about random pixels... However I'd already tried the fix reported on that thread (replace ic14) and that didn't fix it. I'll post more at the weekend.

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Re: Think I'm loosing my (beeb) memory

Postby JannievanZyl » Thu Jan 04, 2018 8:06 am

1024MAK wrote:Well done =D>

I'm after faulty DRAM chips so that when I have made my prototype DRAM tester, I have some real life example DRAM chips to test the tester with.

Mark

Mark, do you have more details on the RAM tester you're working on? Been thinking lately, it might be a a good tool to have.

Alternatively, do you know of any commercially available testers for these old RAM chips?

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Re: Think I'm loosing my (beeb) memory

Postby 1024MAK » Thu Jan 04, 2018 12:24 pm

It uses a PIC micro controller and the initial objective is to have sockets suitable to test common DRAM chips, such as 16k x 1 bit, 16k x 4 bit, 64k x 1 bit and 64k x 4 bit. That would include the very common 4116, the 4816 (used by the Beeb), 4164 and 4464 chips for example.

Then, it would be possible to check individual DRAM chips. Either testing 'spares' or to test suspect chips.

There are many failure modes. So it will take some time to develop code that will catch all defects.

I also now intend to add support so that some of the older/smaller SRAM chips can be tested. But I do not intend to support 8k x 8bit or greater SRAM chips (as some EPROM/EEPROM/flash programmers can already test these).

If/when I am happy, the next version may be expanded to test higher capacity DRAM chips.

Development has been paused over the last two months, as I've been having to repair or service various things (for example, a fairy light PSU/controller, the control chip must have glitched and turned on both outputs that drive the transistor H-bridge, as the two NPN transistors were both dead short circuit).

I don't know of any commercially available testers for a reasonable price to test these old RAM chips. Hence why I started work on my own system.

Mark
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Re: Think I'm loosing my (beeb) memory

Postby crj » Thu Jan 04, 2018 6:23 pm

Out of interest, how portable are your tests likely to be, and how easy might they be to apply to DRAM in situ via the CPU socket of a model B?

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Re: Think I'm loosing my (beeb) memory

Postby JannievanZyl » Thu Jan 04, 2018 7:12 pm

1024MAK wrote:It uses a PIC micro controller and the initial objective is to have sockets suitable to test common DRAM chips, such as 16k x 1 bit, 16k x 4 bit, 64k x 1 bit and 64k x 4 bit. That would include the very common 4116, the 4816 (used by the Beeb), 4164 and 4464 chips for example.

Then, it would be possible to check individual DRAM chips. Either testing 'spares' or to test suspect chips.

There are many failure modes. So it will take some time to develop code that will catch all defects.

I also now intend to add support so that some of the older/smaller SRAM chips can be tested. But I do not intend to support 8k x 8bit or greater SRAM chips (as some EPROM/EEPROM/flash programmers can already test these).

If/when I am happy, the next version may be expanded to test higher capacity DRAM chips.

Development has been paused over the last two months, as I've been having to repair or service various things (for example, a fairy light PSU/controller, the control chip must have glitched and turned on both outputs that drive the transistor H-bridge, as the two NPN transistors were both dead short circuit).

I don't know of any commercially available testers for a reasonable price to test these old RAM chips. Hence why I started work on my own system.

Mark


I'll collect faulty RAM chips and send to you at some point. Have a few 64Kx1 chips from a bunch of C64's I repaired. I'm sure the collection will grow....

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Re: Think I'm loosing my (beeb) memory

Postby 1024MAK » Thu Jan 04, 2018 9:34 pm

crj wrote:Out of interest, how portable are your tests likely to be, and how easy might they be to apply to DRAM in situ via the CPU socket of a model B?

It's intended to be a standalone tester for testing individual chips that are not connected to anything else. So would not be suitable for testing chips that are soldered to a PCB.

Picture in your mind, something like a EPROM programmer, but with (maybe up to) four small DIL sockets, some LEDs, some push buttons and a 2 line 16 (or maybe slightly larger) character LCD. The DIL sockets can be turned pin, or ZIF type.

For testing chips on a partly working BBC Micro, Tricky's test ROM is what I would recommend. Or use Dave's (Hoglet's) ICE T65 :wink:

Mark
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Re: Think I'm loosing my (beeb) memory

Postby 1024MAK » Thu Jan 04, 2018 9:35 pm

JannievanZyl wrote:I'll collect faulty RAM chips and send to you at some point. Have a few 64Kx1 chips from a bunch of C64's I repaired. I'm sure the collection will grow....

Thank you :D

Mark
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Re: Think I'm loosing my (beeb) memory

Postby crj » Fri Jan 05, 2018 3:26 am

1024MAK wrote:Or use Dave's (Hoglet's) ICE T65 :wink:

Ooh, is there a hardware test suite for that? Does it provide any out-of-band channel for diagnostics?

If the answer to both questions is yes, that could almost certainly be repurposed to my needs. (-8


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