Re: external ram expansion modules for 16k 264 series machines

On 07/31/2012 03:38 PM, HÁRSFALVI Levente wrote:
> On 2012-07-30 18:14, Gerrit Heitsch wrote:
>
>>> I couldn't believe that the known external ram modules simply suppress
>>> CAS' by some ugly hacky way, but as it seems this is indeed the case.
>>
>> Might explain some of the dead TEDs...
>
> Arranged another quick test; connected a trimmer between Vcc and CAS'
> (using solder pads close to the expansion port), watched CAS' on the
> o'scope, and tuned the trimmer until obtaining similar-ish waveforms to
> what I had seen previously. Then measured the trimmer's resistance. It
> took around 64 ohms to suppress CAS', and a little bit less to obtain
> very similar waveforms. So, 1.) the cartridge might just pull the CAS'
> line up by a simple resistor, the strange waveform is just "normal". 2.)
> Supposed that we use a pullup of 56 ohms, and we make at least 2.4 V on
> CAS' (standard TTL V_IH minimum), that makes it 5-2.4=2.6V on the
> resistor, therefore, current is ~0.046A. That is, the TED has to sink
> around 46 milliamps of current on its CAS' pin, whenever it goes low.
> Not bad, obviously way above any absolute maximum ratings. Personally, I
> have no doubts that this could kill some unlucky TEDs.

You used a full 64K expansion for your tests. That one likely used 64Kx4 
DRAMs internally. So for that a fixed resistor would do. But there were 
also some 16K Expansions that only used 2 16Kx4 DRAMs, so it had to 
still use the internal DRAM.

Like those:

http://plus4world.powweb.com/hardware/C16_16K_RAM_Pack
http://plus4world.powweb.com/hardware/16K_Expansion_Kingsoft

Interestingly, the extra logic is only 3 logic ICs and differs between 
the 2 cards. The small ceramic capacitors on the Kingsoft card suggest 
some timing hacks, probably to generate your own _CAS. Also there is a 
transistor and a 47Ohm resistor connected to it. That might be the 
_CAS-disabler.

The extra logic will have to detect $4000-$7fff (A15 low, A14 high), 
disable the internal RAM in that range by brute force and generate _CAS 
for the external RAM.


>> These expansions I can only see working by letting the output drivers of
>> the RAM chips fight it out during a read cycle and hope that the
>> internal RAMs lose by using 41464 with stronger drivers than the TMS4416
>> posess. At least this expansion should only be able to damage the
>> internal RAMs or its own RAMs but not TED.
>
> The data outputs of industry standard 4*16k and 4*64k chips are both
> rated to drive 2 standard (74xx) TTL loads, no difference in fan-out
> specs, that is, factory data doesn't seem to indicate that 64k chips
> could/should overdrive 16k chips (safely, or at all) in this scenario,
> at least in a predictably large percent of cases. There must be some
> other trick here IMHO.

Well, the C16 and C116 used only TMS4416 DRAMs (at least all I ever 
seen), so you only had to find a batch of DRAMs, maybe from a different 
maker that has better output drivers and can override the internal RAM. 
I can see no other way since the multiplexer will only supply the 
multiplexed addresses, there is no _CAS generator, no resistor to force 
_CAS high and they didn't do SMD back then, so there is no hidden logic 
on the back.

  Gerrit



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