Disabling Upper RAM

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HappyLittleDiodes
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Disabling Upper RAM

Post by HappyLittleDiodes »

Hi all

A link was posted recently to a trick to disable the upper RAM in a 48k.
Unfortunately the link is dead now :(
Can anybody remind me what the trick was?
I think it was a case of bending a leg out of the socket so disable RAM addressing or...something along those lines?

I'd like to avoid cutting legs at the moment

Many thanks

HLD
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1024MAK
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Re: Disabling Upper RAM

Post by 1024MAK »

You could use that nice empty search box on the top right corner ;)

Anyway, I’ll repost my comments from another topic...
1024MAK wrote:
Thu Apr 16, 2020 2:59 pm
To disable the "upper" 32k of RAM, take a piece of wire and connect pin 5 on IC23 (a 74LS32) to +5V. (link).

I suggest you make a temporary solder connection with an SPST on/off switch in series. Start with the switch set to on. Then if you get to the copyright screen and BASIC works, after that, you can turn the switch off so that some simple BASIC commands can access and therefore test the upper RAM.

This is done because the ROM code tries to see how much RAM there is at start-up. After this, the BASIC ROM will ignore any RAM that is not present (or in this case, is disabled)(well, unless you reset the limits using CLEAR n). But the BASIC POKE and PEEK commands have no limits and work across the whole Z80 address range.

Double and triple check that you have the correct pin on the correct chip, or you WILL damage something. This temporary modification will disable the "upper" RAM (when the switch is ON), as the CAS signal won't reach the RAM chips.

Note that if a faulty DRAM IC is jamming one of the data lines, this won't really help.
Mark

P.S. The World of Spectrum site is currently off line while server maintenance is being done. So the link above may not work until the site is back up. It’s expected to be back up next week.
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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

Thanks, I'll try it.

The handy search box took me to the link!

Cheers Mark
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WDeranged
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Re: Disabling Upper RAM

Post by WDeranged »

N00b question. Why would anyone want to disable the upper ram?
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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

I have a spectrum that won't boot. Very erratic, almost a different result every time. I'm eliminating various components... lower ram, upper ram, cpu. I think I'm now down to bad ROM, or IC27
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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

I've also done a fair bit of continuity testing and tested for shorts, checked the entire board for splashes. Voltages good and steady enough I think. It's a very interesting one
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1024MAK
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Re: Disabling Upper RAM

Post by 1024MAK »

WDeranged wrote:
Sat May 02, 2020 3:42 am
N00b question. Why would anyone want to disable the upper ram?
If you think that one or more of the ‘upper’ DRAM chips, or the multiplexers that supply the address bus to them may be faulty.

The ROM code carries out a test at power on or reset to see how much RAM is installed in the system. It is not designed as a full test to see if the RAM is working correctly. It’s job is just to work out how much RAM is available to the system.

If the upper RAM system is faulty, but the ROM code does detect that it is present, it will set the system up as a 48K system. So the Z80 stack will be placed high up in the ‘upper’ RAM area. But with faulty RAM (or a faulty multiplexer), the data stored on the stack will be corrupted. Hence as soon as the Z80 reads the data from the stack, the machine will crash.

If you disable the ‘upper’ RAM, then the ROM code will set the system up as a 16K system, so the Z80 stack will never be put in the ‘upper’ RAM. If the ‘lower’ RAM is fully functional and there are no other faults that prevent the Z80 and the ROM from working normally, BASIC should start up.

Mark
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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

Slight diversion from the original thread but I just noticed that whoever recapped this board used the wrong rating of capacitor in a few locations.

........might be a good place to start!
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1024MAK
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Re: Disabling Upper RAM

Post by 1024MAK »

HappyLittleDiodes wrote:
Sat May 02, 2020 7:05 pm
Slight diversion from the original thread but I just noticed that whoever recapped this board used the wrong rating of capacitor in a few locations.

........might be a good place to start!
Different capacitance? Different voltage? Higher value? Lower value?
In most applications, the next value up, either capacitance or voltage is not a problem.

Mark
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HappyLittleDiodes
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Re: Disabling Upper RAM

Post by HappyLittleDiodes »

1024MAK wrote:
Sat May 02, 2020 11:12 pm
HappyLittleDiodes wrote:
Sat May 02, 2020 7:05 pm
Slight diversion from the original thread but I just noticed that whoever recapped this board used the wrong rating of capacitor in a few locations.

........might be a good place to start!
Different capacitance? Different voltage? Higher value? Lower value?
In most applications, the next value up, either capacitance or voltage is not a problem.

Mark
Hi Mark

C80 - should be 22uF, was 1uF
C74 - should be 4.7uF, was 22uF
C27 - should be 1uF, was 4.7uF

Changed them all, hasn't resolved anything but needed doing at least for C80 as it's right in the middle of the power circuit

Cheers

Jim
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