****	How to connect a 72pin SIMM to your A5K    ****
	
	By Christian Ludlam		chris@recoil.org
	
	
	
	IMPORTANT:
	This information is believed to be correct, and is given in good
faith, but it relates only to my experience carrying out this modification
to my computer. I will accept no responsibility for any loss or damage 
caused by the use or misuse of this information. If you hopelessly destroy 
your computer by trying this, it will be YOUR problem. Always think carefully
before carrying out major modifications to your computer, and always observe
safety procedures relating to mains operated equipment and static sensitive
devices.
	
	
	
	
	Connecting a 72 pin SIMM to your A5k is a necessarily complicated
process. It will require a lot of soldering, and in my opinion a purpose
designed Printed Circuit Board. The process does not simply involve wiring 
the SIMM in; there are various compatibility issues which prevent this. 
I designed a PCB to do this, but I can't offer to make any more. This 
much of the process is up to you.
	
	This process as it stands will only increase the RAM in your
system to a maximum of 4MB, because that is the maximum that one MEMC
can handle. It is possible to add another MEMC to the computer, but I
haven't worked out quite how to do that.

	Start by connecting the power lines, VCC = +5v, VSS = 0v. Then
connect all the data lines up to the SIMM. Note that since SIMMs use 
parity notation (even non-parity SIMMs), the pin mapping is as follows:
		
		A5k			SIMM
		D0-D7			D0-D7
		D8-D15			D9-D16
		D16-D23			D18-D25
		D24-D31			D27-D34
		

	The address lines are just a straightforward pin to pin connection,
but the nRAS and nCAS lines need some fiddling to work. Each nCAS line 
controls one byte of the bus, to allow byte wide addressing. These need
to be connected in order - make sure you connect nCAS0 to nCAS0 etc.
The nRAS line on the Acorn must be connected to both nRAS0 and nRAS2
on the SIMM, due to the slightly odd structure of the SIMM.

	Now it gets harder due to incompatibility issues between the two 
systems - the last 2 lines need direct connection the the motherboard.
There are two jumper sockets on the motherboard just next to the memory
expansion card socket. One of them has 3 pins, and the other has 2. The 
2 pin connection must be made, and the 3 pin one must connect the 
centre pin to the pin closest to the 2 pin jumper. (Got that?)
The last address line on the SIMM, RA9, needs to be connected to the 
3 pin jumper connection that you just made. 

	The last pin, nWE, needs to be connected to the ARM line DBE
after being inverted. I used a 74HC04 for the inverter, but any CMOS 
NOT gate would do. The DBE line can be found on pin 1 of the 74LS138
chip next to the 2 pin jumper. Be careful not to damage the board when
connecting this (like me).

	One last thing which really confused me is that the A5k seems
to do something interesting with the address lines after they have been
decoded. I never did find out where these lines went, but they are used to 
detect how much RAM is fitted to the system. Consequently, if the jumpers
are removed to disable the on-board RAM, the system will fail its POST
even if the SIMM is working properly. So the only alternatives are to 
desolder and cut out all the ram on the motherboard, or (as I did)
leave both systems in and hope that there are no clashes where both RAMs
try to output data at the same time. This should not happen, however, if
RAM of the sam speed specifications is used. The RAM on my motherboard
is 70ns, so I used a 70ns SIMM (which I had lying around anyway).

	Note that I cannot give any more help than this file.
If you get something wrong, I cannot possibly offer any advice. 
However, if you do get it to work, I would love to hear that you did.

	Good luck!



Pinouts:

The pinout of a 72pin SIMM is standard, and con be downloaded from the 
developer area of any SIMM manufacturer's website.

The pinout of an A5k memory expansion socket (the one next to the internal
speaker) is as follows (looking from above):

	FRONT of computer (ie. closest to power on light)
	
	VSS		VSS
	VDD		VDD
	-		-	(these are tha a5k's bank select pins
	-		-        not used in this design)
	-		-
	D30		nCAS3
	D28		D31
	nRAS		D29
	D26		D27
	D25		D24
	D22		VSS
	VSS		D23
	D20		nCAS2
	D19		D21
	RA7		D18
	RA6		RA2
	RA4		RA5
	RA8		RA3
	RA0		RA1
	D17		D16
	VSS		VSS
	D14		D15
	D12		nCAS1
	D11		D13
	D8		D10
	D7		D9
	nCAS0		D6
	D4		D5
	D2		D3
	D1		D0
	VDD		VDD
	VSS		VSS
	
	BACK of computer (closest to podule sockets)