Memotech Interfaces

[XorA]

Very nice ... What a haul! Do you think the seller had some connection with memotech or was he simply a fanboy or perhaps authorised service agent or somesuch?

It is possible, he was a repair agent for other computer systems, he did however also talk about trawling computer fairs in the UK for parts.

Its going to take me a while to get most of those interfaces cleaned and working, a job for the raining months of winter I feel. Love to get the HRG one up and running

G

[Moggy]

Looking at picture of it all that seems to be missing is the little push switch that fits just above the resistor on the bottom of the board.
This is needed to return from Hi-res mode to Basic.


Moggy

[XorA]

zx81 doesnt even boot when plugged in so I think its worse than that!

[gozzo]

Has anyone got a circuit of the HRG unit, or a reasonably detailed description of how it works (I do have an explanation of sorts but not really detailed enough) so I and anyone else could have a go at building one? Also any info on the ULA/PLD/PLA thingy in the 16K pack - anyone got any ideas how to test one or replace it (I have a duff 16k pack and this is the only part I can't test at the moment - I have a chip tester that can do all the logic chips and can probably rig up an adapter so it can test the 4116's)

[1024MAK]

Also any info on the ULA/PLD/PLA thingy in the 16K pack - anyone got any ideas how to test one or replace it (I have a duff 16k pack and this is the only part I can't test at the moment - I have a chip tester that can do all the logic chips and can probably rig up an adapter so it can test the 4116's)

I trust that you have read these two threads 16 Ko Ram pack common failure ? and DRAM control circuits
The ULA does at least three things. First DRAM chips need sequential control signals (/RAS and /CAS) so the ULA provides these, second the ULA acts as switch for the non-multiplexed Z80 address lines to the multiplexed DRAM address lines, third the ULA controls the "external" 74LS157 switch.
Now the generation of the /RAS, 74LS157 switch control and /CAS can be done by monostable delays, by a delay line, by using a state machine driven by the system clock & system control signal or by using gate delays.

So the ULA can be replaced by standard logic chips.

Mark

[gozzo]

Yes, I have seen both posts - i was hoping someone might have the 'bit pattern' data for the ULA/PLD thing, but if not I think I could probably rig something up so it at least would work as a 'straight' 16k rampack, not one that could be 'piggybacked' ..?? I don't yet even know IF that is whats wrong with it..?? the rampack doesn't work whether used as a master or slave - I get no display at all so it obviously stops the zx81 from working somehow - I am going to investigate further as soon as I get time.(I have scoped all the data address and control lines and all seem to be 'active' and not stuck at one level or another.??) All I know for certain is that the supply lines are ok !

[siggi]

Maybe Wilf's description "Shades of Memotech Hires" might also help:
http://www.user.dccnet.com/wrigter/inde ... motech.htm

Siggi

[gozzo]

That's the description I've already got...

[1024MAK]

Yes, I have seen both posts - i was hoping someone might have the 'bit pattern' data for the ULA/PLD thing, but if not I think I could probably rig something up so it at least would work as a 'straight' 16k rampack, not one that could be 'piggybacked' ..?? I don't yet even know IF that is whats wrong with it..?? the rampack doesn't work whether used as a master or slave - I get no display at all so it obviously stops the zx81 from working somehow - I am going to investigate further as soon as I get time.(I have scoped all the data address and control lines and all seem to be 'active' and not stuck at one level or another.??) All I know for certain is that the supply lines are ok !

Ahh, sorry. Because you referred to the

ULA/PLD/PLA thingy

I thought you were referring to the Sinclair 16k RAM pack
The chip in the Memotech 16k RAM pack does a different job (at least in the 16K type using the 4116's). The /RAC, /CAS and 74LS157 switching is done by 74LS TTL chips.
I assume the Memotech chip you are referring to is the "fast" PROM chip - it is used for decoding the address lines as well as making use of the DIL selection switch position. The output then selects the RAM chips and provides one of the address lines for the next "slave" pack (if fitted).
I have reverse engineered the circuit from my working unit. However, I have temporarily lost one of the sheets of paper with the details on
I have not looked at working out the bit pattern in the PROM, but it may well be possible to use a GAL (or PAL) to replace it. Or if you are not bothered about using in a Master/Slave configuration, a bit of extra 74LS TTL logic would do.

When I track down the diagram, I will post it up (but no holding of breath, as it may take a while as I may have misfiled it).

Mark

[1024MAK]

Ah-hah! Found it!

Mark