On 2012-07-30 18:14, Gerrit Heitsch wrote: > On the C116/C16 they could have used the spare 74LS125 gate together > with the spare 74LS02 gate and 2 pullup resistors to the same effect. > Very strange that they didn't since it wouldn't have meant any costly > extra parts. Yep. >> 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. > 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. Levente Message was sent through the cbm-hackers mailing listReceived on 2012-07-31 14:00:05
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