How to upgrade to more than 256k ?

I suppose you have the 48gs256k.zip file (go to take it if not).
More than 256k means 3 chips or more but you can't use the PCB of Thijs and 3 DIP SRAM because it's too tick.
So you need at least one chip in SMD package.
(Another way is to don't use the PCB, I did it on one HP, but I advise you against such methods because you'll have 40 free wires in your HP and it's awful to do and to see)
Imagine your HP is opened, keypad on the ground, batteries towards you.
(see my pictures)
Replace the 32k Main RAM by a 128k SMD one.(take attention : pin 1 towards you like the 32k one)
Add a 128k in port 1 with the PCB (or if SMD on the main RAM)
See at the 48gs256k.zip to know how to wire it.(only port 1)
Add a 128k (or 512k, I'll try in few weeks) in port 2
To do that, solder the chip on the one for port 1 for all pins excepted the 30th that goes to the pin N21of the 2nd port (the pin that is different from port one in the middle)
You need some other components to drive port 2, they must be soldered on the main PCB near the batteries :
A 74HC00 : Solder it on the right, pin 1 towards the top.(It drives OE  for port 2 and CE for chip 2)
A 74HC174 : Solder it on the left, pin 1 towards the batteries.(It holds the 6 lines needed to adress more than 128k on one port, but it is needed for only 128k also because one of these line drives the CE through the 74HC00) 
Two resistors SMD : a 47kohms (473) and a 100kohms (104)
Attention : two differents PCB exist.
Look carefully at the 4 slots in square on the middle left to see if the resistors must be soldered vertically or horizontally.
On older PCB : 473 on the left ; 104 on the right.
On newer PCB : 473 on the top ; 104 on the down.

Have a good hack and I apologise for my poor english.
Phil.

Philippe Teuwen
s952365@student.ulg.ac.be

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