[sdiy] Homemade Memorymoog - update

Luís Marka luis.marka at terra.com.br
Mon Feb 18 18:17:40 CET 2019

Hey Grant!

I just linked to a part of the schematics because, well, they are huge! 
I can provide the full page board diagram if you want to take a look, 
just let me know.

On 17/02/2019 22:36, MTG wrote:
> The 27128 *maybe*. It's 4x the size of a 2732 and 1/4 of that seems to 
> leave your schematic via a signal called ROM*.  Need to figure out 
> what that is. See pin 12 on U9
The ROM signal out of U9 goes to a 40 pin connector that was made 
available for future expansion: sequencer/MIDI boards, etc. I am keeping 
this connector in the current board clone, but will get rid of it in the 
compact version I am working on, have no plans to implement the 
sequencer. Now, if I got it right, U9 will be gone and the OE pin in the 
27C128 (pin 22) will receive its signal directly from pin 1 in U12... is 
that correct, or am I missing some detail here?

> The SRAM you are OK with near as I can tell. You could use an Alliance 
> 6264 from Mouser. That logic is butt-ugly though. I'm not sure how 
> much you can really clean it up without a total redo. There are four 
> off-page RAM signals, looks like another 8k x 8. Where do those go out 
> of curiosity?
Those are data and address bus, plus RD and WR from the Z80 that are 
used by the DMUX and keyboard/panel scan circuits.

The Memorymoog implements a mechanism to protect data RAM chips at 
shutdown: it taps voltage directly from the rectifiers in the power 
supply, before the filter capacitor. At shutdown, voltage in this line 
drops a few milliseconds before the voltage in the main power bus due to 
capacitance. The "power shutdown" circuit then feeds a low signal to 
U22, which in turn disables the RAM chips before power is gone (hopefuly).

Still doing my homework on how to address the RAM chip should I go for 
one single SRAM IC, I have not touched this type of circuit since the 
early 90s....


> ---
> I don't suppose there is a new Z80 that's flashed based with some of 
> those peripherals inside? It seems on the 8051's got lucky that way.
> I would glance at the Kiwi Technics stuff and Tauntek (sp?) to see if 
> there are clever subs for the Z80.
> ---
> On 2/17/2019 3:55 PM, Luís Marka wrote:
>> Short update: got the digital board pretty much done, here: 
>> https://imgur.com/a/luI6OHl
>> Not as compact as I thought it would be, but was a good training in 
>> using Kicad. Now ordering the parts, and making the final revision 
>> before sending the board for fabrication.
>> Already started work on a more compact version of this board: SMD for 
>> some of the components, getting rid of the sequencer connector and 
>> replacing 3x 2532 EPROMS (already changed for the 2732 in my board, 
>> had a number of 2732 around and I already did manage to burn the 
>> firmware on those, so why not?) with one single 27C128. In the same 
>> line, would it be possible to replace the 4x 2k 6116 SRAMs with 1x 8k 
>> AS6C6264, for example? For reference, the Memorymoog memory control 
>> section is here: https://imgur.com/a/fnsf1qs
>> Any help in implementing the logic for this reduced number of memory 
>> chips is much appreciated! In the mean time, will attack the PSU and 
>> DMUX boards.
>> Cheers!
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