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 Message was sent through the cbm-hackers mailing listReceived on 2012-07-31 17:00:05
Archive generated by hypermail 2.2.0.