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

From: HÁRSFALVI Levente <>
Date: Mon, 30 Jul 2012 16:56:16 +0200
Message-ID: <>
On 2012-07-19 17:25, Gerrit Heitsch wrote:
> The thing is, there have been external memory expansions for the C16
> that must have done exactly that with the _CAS line from TED without
> killing it outright. The default is that the RAM in the C16 is mirrored
> 4 times and there is no way on the expansion port to disable it. The
> only way to add another 16 KB on an external board is to force _CAS for
> the internal RAM high at the proper address space and generate your own
> _CAS for the RAM on the board.
> All the 16KB expansions I know have an extra single transistor on board
> that would provide the power to do that:
> I'm still no sure how they were able to get the 64K RAM expansions to
> work on the expansion port with only multiplexers and the RAMs with the
> internal RAM still in place. I never bought one, I replaced the internal
> RAMs with 41464.

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.

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.

I took a stock C116 board, a 64k external memory expansion module (one
of these: ) plus an
oscilloscope and arranged a quick measurement. Here are the results.

The images show RAS' (top) versus CAS' (bottom), triggered by the
falling edge of CAS'; the stock unexpanded machine first, then with
cartridge attached.

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

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.


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Received on 2012-07-30 15:00:05

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