RISC PC Disappearing Memory

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BeebMaster
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RISC PC Disappearing Memory

Post by BeebMaster » Mon Oct 21, 2019 1:33 pm

This is a new one on me. I noticed a few days ago that the amount of applications RAM on my RPC 700 had reduced considerably, and re-starting it I got a "RISC OS 66MB" message instead of the usual "RISC OS 130MB".

I don't know where the other 64MB has gone! I've checked down the back of the sofa and everything, even asked the dogs if they'd eaten it, but no.

It has a (presumably 2MB) VRAM module and one main (128MB) RAM module.

I've removed and cleaned the main RAM module's contacts and tried it in both sockets and still only get 66MB showing up.

The only thing that has "happened" recently is that I was doing an Econet remote peek (reading &300-&37F from a remote station, which on a Beeb is the VDU variables) which, when I used the RPC as the remote station, generated an odd error on the Desktop which I can't remember. I'm just wondering if I have set some internal bit somewhere which sets maximum RAM to 64MB.

I've done a CMOS reset several times, including disconnecting the battery for a while, but it hasn't brought my RAM back.

Could the RAM module itself have gone faulty, and would the RPC still work if the RAM is on the blink?
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Re: RISC PC Disappearing Memory

Post by geraldholdsworth » Mon Oct 21, 2019 1:42 pm

I seem to remember, back when I worked at Atomwide, that they turned a 32MB (I think) module into a 16MB module by desoldering all the chips on one side of the board (usually by hot air gun). So, could one of the chips on one side gone faulty?
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Re: RISC PC Disappearing Memory

Post by BeebMaster » Mon Oct 21, 2019 1:58 pm

Very possibly, there are chips on both sides. I wonder how I would test that. I didn't really think it would be a RAM fault as I thought POST would find it, but the Technical Reference Manual mentions a CMOS bit which skips a long RAM test for machines > 64MB RAM, I could do with re-enabling that, if indeed it has been disabled. I was actually looking for a full listing of the CMOS RAM contents (further to my 64MB RAM limit idea) - still looking - when I came across this info.
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Re: RISC PC Disappearing Memory

Post by BeebMaster » Mon Oct 21, 2019 3:13 pm

The only other 72-pin RAM module I have is a 4MB one, and installing that gives me a 70MB RAM message with the two modules in either slot. So that rules out my 64MB max theory. I think it also rules out faulty sockets on the board, although the 4MB module only has chips on one side, so it could still be a socket fault on the other side.

Come to think about it, I have had a few unexpected errors with the RPC over the last few weeks, such as no reply errors and other odd messages when running the Advanced L4 server, and those broken directories I wrote about a week or two back, so it is possible that the 128MB RAM module has been dying for a while.
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Re: RISC PC Disappearing Memory

Post by geraldholdsworth » Mon Oct 21, 2019 5:59 pm

I also seem to remember that some modules have a parity check chip (?) on them, in addition to the RAM chips - again, one per side.
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Re: RISC PC Disappearing Memory

Post by BeebMaster » Wed Oct 23, 2019 11:51 pm

I just had the RISC PC on again, and noticed it was showing 130MB, but then it crashed during the early part of the boot sequence. Pressing reset was the only way out and then it gave me address exception errors and and a supervisor prompt. Ctrl-Reset set it back to 66MB and working again.

I'm wondering if it's a bad contact on the RAM module. When I took it out I noticed a couple of the contacts were worn or tarnished.

It's possible this has been going on for some time but only noticed recently, as I don't always have the monitor on until after the Desktop is loaded.
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Re: RISC PC Disappearing Memory

Post by BeebMaster » Mon Mar 09, 2020 11:01 pm

I gave the contacts on the 128MB RAM module a good clean at the weekend but it hasn't helped. I'm a bit frit that if this module dies completely, I'll have to fall back to my 4MB module, which isn't going to be a lot of use, so I am after a replacement.

I've read the RISC OS FAQ and various other items about the RAM spec for the RISC PC. FAQ says definitely use FPM, other sources say EDO has been used and works. I've seen these which appear to be reasonably priced, one FPM, one EDO:

https://www.memoryx.com/32mbx326e.html
https://www.memoryx.com/ahs.html

Anyone in the know confirm if they would work?
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Re: RISC PC Disappearing Memory

Post by myelin » Mon Mar 09, 2020 11:43 pm

I have a 128MB SIMM that registers as 128MB in my Risc PC but only 64MB (or sometimes nothing at all) in my A7000... I assume bad contacts somewhere!
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Re: RISC PC Disappearing Memory

Post by sP1d3r » Mon Mar 09, 2020 11:55 pm

:mrgreen: Hi

Can you post pictures of the SIMM, both sides?
You can sometimes tell if a particular chip is faulty because the plastic should look uniform and matt, but signs of heating can be apparent.
Rather than chuck the SIMM, replacing the chips could be a solution, 128MB FPM SIMM memory is uncommon these days.

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Re: RISC PC Disappearing Memory

Post by BeebMaster » Tue Mar 10, 2020 11:57 am

Replacing the chips will be beyond me, but here goes:
IMG_6258.jpeg
IMG_6260.jpeg
I'd be interested to know why this RAM fault doesn't give me a POST RAM fault code!
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Re: RISC PC Disappearing Memory

Post by Boydie » Tue Mar 10, 2020 6:31 pm

At a guess, because the machine is only findng 64Mb, all of which checks out okay?

When the RPC was launched, the product literature claimed to take up to 128Mb SIMMs, with the small print to the effect “when this is available”. So it wasn’t ever tested as working with 128Mb during design, because they couldn’t.
BITD, the machines were incredibly picky about which modules they would and wouldn’t play nicely with. What worked in one machine would fail in another, two modules which worked well in isolation would fail when placed together. I whiled away many a happy hour, trying to work out what would play with what, even just with 4 and 8 Mb modules.

EDO was claimed not to work, but I’ve got a single 64Mb EDO stick in each of two machines, and they’re quite happy and will play with (certain) other EDO or FP sticks.

Maybe try your 128Mb stick in another machine?

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Re: RISC PC Disappearing Memory

Post by sP1d3r » Tue Mar 10, 2020 11:14 pm

Hi

It's difficult to tell because the chips would need a clean with a bit of lint-free cloth and some ipa, but the two chips in the bottom corners of the lower picture look suspect, instead of a regular surface/edge they look more like cheese on toast on the lower edges/corner.

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Re: RISC PC Disappearing Memory

Post by BeebMaster » Fri Mar 13, 2020 10:42 am

Not sure about the cheese on toast thing, I've only seen a pizza oven in a RISC PC! I think the next step is to get new RAM, and I'd like a 128MB module (or two) so I'm replacing byte-for-byte. When I read the RISC OS FAQ it mentions the specs, including square-array RAM. I'm not sure how to tell that from the links I posted, or even whether it's crucial.
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Re: RISC PC Disappearing Memory

Post by sirbod » Fri Mar 13, 2020 9:47 pm

It sounds like you have a fault on one side of the SIMM.

You can use most EDO in place of FPM - there's very little difference between them at a technical level. They work in the same way, except the data is available for an extended period...hence the name Extended Data Output.

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Re: RISC PC Disappearing Memory

Post by BeebMaster » Sat Mar 14, 2020 10:32 am

I think I'll take a punt on that EDO module then, it's only worth the shipping if I get two of them.
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Re: RISC PC Disappearing Memory

Post by sP1d3r » Sun Mar 15, 2020 7:13 am

#-o It'd be dependent upon whether the problem with the SIMM is because of an incompatibility with the RISC PC, or whether the problem is symptomatic of 'wider' issues with the RISC PC.
I think that I'd try to guesstimate how many power-on-hours it's had and ensure that all the passive components are up to specification, e.g. use an in-circuit Esr tester to check the electrolytic capacitors on the motherboard and in the psu.
It'd be disappointing if you bought new SIMMs and the same problem happened again.

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Re: RISC PC Disappearing Memory

Post by stevebubs » Tue Mar 24, 2020 11:34 am

BeebMaster wrote:
Sat Mar 14, 2020 10:32 am
I think I'll take a punt on that EDO module then, it's only worth the shipping if I get two of them.
I could do with one if that makes shipping easier? No hurry for it to be delivered to me - happy to wait until we start to go back to normal...

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Re: RISC PC Disappearing Memory

Post by BeebMaster » Tue Apr 28, 2020 10:52 am

Definitely I'd put in a larger order if there are any other takers, I'll contact them to see if they really have stock.

I'm not guaranteeing this though, I don't think anyone has said definitively that this specific part will work in RISC PC, and I doubt the seller would be able to say, so it could all be a waste of time and money.

In my case it's also worth considering whether there might be an underlying fault, as has been said, which has caused the RAM module to go faulty, but I haven't noticed any other problems. It's just done 7 days' (and nights') service as a Level 4 file server rescuing all my corrupted Econet file server data, so I think that's a good sign that overall it's sound.
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Re: RISC PC Disappearing Memory

Post by BeebMaster » Tue Apr 28, 2020 8:14 pm

They only had 3 and not likely to get any more, so I've ordered them all. If they work in RPC when they arrive then I may pass on the third one, or keep it as a spare spare. Total inc shipping for all three was £88.51 but it's likely that VAT (and possibly handling charge depending on who brings them round) will be added.
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Re: RISC PC Disappearing Memory

Post by sP1d3r » Wed Apr 29, 2020 11:07 am

:o I don't know for sure but I'd hazard a guess that because the Risc Pc wasn't 'engineered' with 128MB simm's in mind, using them is reliant upon the ability of the simm socket to deliver the necessary voltages to maintain the integrity of volatile memory.
The power requirement of a 128MB simm is likely too high for a Risc Pc with deteriorated capacitors, possibly.

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Re: RISC PC Disappearing Memory

Post by BeebMaster » Fri May 08, 2020 11:38 am

Well, they just cancelled my order saying they had no stock after all, and refunded me, except that I've lost 4 quid into the bargain due to the incoming currency conversion!

So I'm back to square (array) one!
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Re: RISC PC Disappearing Memory

Post by philpem » Sat May 09, 2020 2:02 pm

BeebMaster wrote:
Tue Mar 10, 2020 11:57 am
Replacing the chips will be beyond me, but here goes:

IMG_6258.jpegIMG_6260.jpeg

I'd be interested to know why this RAM fault doesn't give me a POST RAM fault code!
The chips in the first photo look very short of solder... I wouldn't be surprised if there's a cracked solder joint on at least one of those pins.

For the contact pads, I had a dodgy module which had to be completely re-tinned. I covered the pins in flux, melted some solder onto a piece of desolder wick, then went down the pads one by one. Eventually the corrosion came off and left me with shiny fresh tin. One or two pins had to be scratched back to copper with a scalpel before they'd re-tin.

The RPC has gold-plated SIMM connectors, and from memory gold on tin tends not to work terribly well... the tin ends up tarnishing over time.

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Re: RISC PC Disappearing Memory

Post by Kazzie » Sun May 10, 2020 4:23 pm

BeebMaster wrote:
Fri May 08, 2020 11:38 am
Well, they just cancelled my order saying they had no stock after all, and refunded me, except that I've lost 4 quid into the bargain due to the incoming currency conversion!

So I'm back to square (array) one!
This news is strangely fitting for the thread's title. :?
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