Hi all!
There's those 16K RAM packs we all love... ehm, loathe.
On the interwebs is easily found a schematic that involves discrete 74xx logic and 4116 RAM IC's. And photos of a RAM pack that consists of 2 circuit boards.
Funny thing is: I've got 2 of those RAM packs, and both of them have a single circuit board inside. Which includes a small ULA type IC. Just wondering if there's a schematic out there for this particular type of 16K RAM pack?
Disclaimer: this is NOT a call to break open your RAM pack and check! Just wondering if a schematic for this particular type RAM pack is out there. Comparing with the other type should make it straightforward to 'decipher' what that small ULA does, and schematic not needed that much. But hey, pinouts / schematics do tend to make the life of hardware enthousiasts easier...
RAM pack ULA schematic
- RetroTechie
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- 1024MAK
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Re: RAM pack ULA schematic
No known full Sinclair schematic, but there is a pin-out plus a partial reverse engineered schematic (done by me) somewhere on this forum. I'm on a mobile at the moment, so searching is a pain. Search for repairs to Sinclair RAM pack or similar... If you don't find it, I will have a look later.
Update:
Link to information relating to which data bit is handled by which chip.
Partial schematic link
Mark
Update:
Link to information relating to which data bit is handled by which chip.
Partial schematic link
Mark
Last edited by 1024MAK on Thu Jul 23, 2015 2:31 pm, edited 1 time in total.
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- 1024MAK
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Re: RAM pack ULA schematic
Oh, and the early Sinclair 16k RAM packs were more complex than they needed to be. There is no need to include refresh circuitry, as the Z80 CPU does the refresh anyway despite the way the ZX81 uses the CPU to produce the screen display.
I have investigated three other makes/designs of ZX81 16k RAM expansions and they don't have any refresh address generating circuitry, just address decoding, a -5V inverter, two address multiplexer chips and the DRAM chips themselves.
Mark
I have investigated three other makes/designs of ZX81 16k RAM expansions and they don't have any refresh address generating circuitry, just address decoding, a -5V inverter, two address multiplexer chips and the DRAM chips themselves.
Mark
ZX81 Variations
ZX81 Chip Pin-outs
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ZX81 Chip Pin-outs
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
- RetroTechie
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Re: RAM pack ULA schematic
Thanks for those links, Mark! Exactly the info I was 'fishing' for.
I did notice something unusual though:
DRAM control signals require some (short) delays, often something like /RAS low -> delay -> switch address multiplexers -> delay -> /CAS low. In practically every DRAM-based Z80 system I've seen so far, those delays are made in one of 2 ways:
And perhaps technically better too, since IC-internal circuit behavior could be much tighter controlled than say, the # of nanoseconds delay a 74LSxx -> RC -> 74LSxx setup gives (as a function of component variations).
I did notice something unusual though:
DRAM control signals require some (short) delays, often something like /RAS low -> delay -> switch address multiplexers -> delay -> /CAS low. In practically every DRAM-based Z80 system I've seen so far, those delays are made in one of 2 ways:
- Using edges of the Z80 clock signal. Not present on any pin of this mini-ULA, not even via-via.
- Using some analog circuitry (simple RC-delays are common). Not seen hanging between pins of this mini-ULA either.
And perhaps technically better too, since IC-internal circuit behavior could be much tighter controlled than say, the # of nanoseconds delay a 74LSxx -> RC -> 74LSxx setup gives (as a function of component variations).
Agreed! Checking that schematic suggests that 3 pcs. 74LSxx IC's might be removed without any downside. If it were verified that works, it could be a nice repair option (for folks still messing with RAM packs, that is ).1024MAK wrote:Oh, and the early Sinclair 16k RAM packs were more complex than they needed to be. There is no need to include refresh circuitry, as the Z80 CPU does the refresh anyway despite the way the ZX81 uses the CPU to produce the screen display.
Re: RAM pack ULA schematic
This ULA is currently the bane of my existence. See: viewtopic.php?f=7&t=4266&p=43485#p43485
@RetroTechie: Did you get further with your analysis? I'm kinda wondering how to recreate one of these... shudder.
@RetroTechie: Did you get further with your analysis? I'm kinda wondering how to recreate one of these... shudder.
- 1024MAK
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Re: RAM pack ULA schematic
RetroTechie was last active on here in August 2019
Mark
Mark
ZX81 Variations
ZX81 Chip Pin-outs
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Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
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ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.