From: scharf@lrs.e-technik.uni-erlangen.de (Ron Scharf)
Subject: DX7 RAM expansion description
Organization: LRS
Date: Fri, 20 Nov 1992 09:47:37 GMT


Hi out there!


This is for all the guys that asked me to post the schematic for a DX7 RAM
cartridge. As I sold my DX7 long ago, it took me quite some time to
find the schematics. Sorry for that, and thanks for your patience.

First thing one should know is the pinning of the 28-pin cartridge connector.

PIN #	ROW A (front)	ROW B (back)
-----------------------------
1	5V		R//W
2	/CRT AVAIL	5V
3	/OE		A10
4	/A11		A9
5	A11		A8
6	D7		A7
7	D6		A6
8	D5		A5
9	D4		A4
10	D3		A3
11	D2		A2
12	D1		A1
13	D0		A0
14	GND		MEM PROTECTED

Standing in front of your DX7, PIN A1 is the front left of the cartridge
connector.
If you look at the connectors of a cartridge, you will notice that the front
leftmost and rightmost pins are slightly longer, and the back leftmost
and rightmost pins are slightly shorter than the others. This is to ensure
power supply connection when you insert the cartridge, before any other
connections are made. Also, the R//W and MEM PROTECTED pins are the last to be
connected to the DX7 internals.
The address and data lines operate "normally". Why address line A11 exists 
normal and inverted, I can't say. /CRT AVAIL flags the presence
of a cartridge and should be set to GND by the cartridge circuitry. If
MEM PROTECTED is 1, the DX7 won't be able to write to a cartridge (independent
of the DX7s software protection).

Now, with the connector pinning and some basic knowledge on digital circuit
design, there should be no problem connecting static RAMs or EPROMS.
Please note that the connector spacing is *NOT* 0.1", so you will have to
design your own connector PCB.
I first used a 43256 static RAM to build an 8 bank RAM expansion. Later, I
improved this design using a 74LS154 binary decoder to select one out of 16
43256, which were all connected parallel to the corresponding address, data
and control lines of the cartridge slot, giving me 128 banks of RAM. As the
16 RAMs didn't fit in a cartridge-sized case, I made an adaptor PCB, which
did fit in the cartridge slot and had a regular 2-row 1/10" spaced connector on
the other side to attach a "flat band cable" (that's the word-by-word
translation of the german word "Flach-band-kabel"--- sorry, don't know the
english name for it, but I'm quite shure you know what I mean!). The other side
of the cable went into a small case containing the RAM expansion circuitry, a
5V accu cell for backup power supply and the bank select switches, with some
magnets on the bottom to fix the case on the DX7.

As it takes some time to convert a hand-drawn schematic to ASCII, I will stop
here. The design of a DX7 multi-bank RAM expansion should be no greater
problem now. Nevertheless, if one of you absolutely doesn't know how to connect
standard static RAMs to the DX7, email me and I will ASCII-draw a schematic and
post it here. Sorry I can't offer you a PCB layout, cause I wire-wrapped the
whole thing (except the cartridge connector).

Hope this helps, and I'd be glad to get some response whether you succeeded or
not!

Bye
	Ron

P.S. Thanks to my colleague Claus Kuntzsch for slaughting his original Yamaha
DX7 RAM cartridge to find out the connector pinning.

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