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

From: Gerrit Heitsch <>
Date: Mon, 30 Jul 2012 18:14:53 +0200
Message-ID: <>
On 07/30/2012 04:56 PM, HÁRSFALVI Levente wrote:
> There's some hint that in fact they had planned on providing a correct
> option for that. On an early Plus/4 schematic diagram, one could spot a
> (RAMEN) label beside pin Z of the expansion port (
> ). Indeed, it's been a mistery how these expansion modules could work at
> all, since there's no standard method to disable the internal dynamic
> ram chips ie. to avoid bus contention.

In order to implement a high active RAMEN-signal all you need is a spare 
gate from a 74LS08 wired between _CAS from TED and the _CAS at the RAMs 
plus a pullup resistor. On the plus/4 they did have 2 spare AND gates 
but didn't use them.

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.

> 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...

> CAS' appears to be pulled up by a somewhat strange way (note the
> waveform). I don't want to break this cartridge shell open because it
> doesn't appear that I can do that without causing some damage, so I can
> only guess. Fact is, CAS' doesn't appear to fall below around 2...some
> volts.

Which means the internal RAM never reads it as LOW and stays inactive.

> It'd be very interesting to take some measurements of, say, this module:
> (which, in turn, is likely a remake of this one:
> ) which only seem
> to hold RAM chips and multiplexers onboard. I'm wondering that while
> they need CAS' for the external ram chips, they need to suppress it so
> that the internal rams (in the machine) would never get selected. Which,
> doesn't appear to be impossible, after all, only if doing it without any
> extra components.

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.


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Received on 2012-07-30 17:00:10

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