How to have 4 Mb on a G
                      compiled by Philippe Teuwen
                      ---------------------------
                 Version 0.02      26th November 1996

             ! This document is given only for information
             and *NOTHING WAS TESTED* , so ANY WARRANTY  !
                                                

Remember: you need the HC00, HC174 and resistors for enabling port 2.
See other docs to know how to do that.

512k SRAM
---------
          -----------
     D3 -| D3    GND |- GND             * : different from a 128k
     D4 -| D4     D2 |- D2                  so you can solder all
     D5 -| D5     D1 |- D1                  the pins on the main RAM
     D6 -| D6     D0 |- D0                  or a 128k port 1 excepted
     D7 -| D7     A0 |- A0                  these 3 ones.
 * NCEn -| NCE    A1 |- A1              All the pins are common for
    A10 -| A10    A2 |- A2              all the 512k excepted the NCEn
    GND -| NOE    A3 |- A3          
    A11 -| A11    A4 |- A4           
     A9 -| A9     A5 |- A5          
     A8 -| A8     A6 |- A6          
    A13 -| A13    A7 |- A7         
    NWE -| NWE   A12 |- A12         
 *  A17 -| A17   A14 |- A14        
    A15 -| A15   A16 |- A16       
    Vcc -| Vcc   A18 |- A18  *
       32 ----/^\---- 1  

Just one 512k chip
------------------
NCEn to pin 6 or 10 of HC00   ( =N(BEN*N(AR18)) or =NCE2.2 )
Now HC00 must be powered all the time: disconnect pins 1 and 14
and wire them to Vcc (as pin 32 of main RAM)
Because if HC00 is turned off, the 512k will be turned on.

8*512k chips (->4Mb)
--------------------
Must use a decoder:

74HC138: 3-line-to-8-line decoder/demultiplexer (with lines inverted)

         1 ----\_/----
     A19 -| A     Vcc |- Vcc           Connect NCEn of SRAMs 512k 
     A20 -| B      Y0 |- NCE5.0        respectively to NCE5.n
     A21 -| C      Y1 |- NCE5.1        (n=0-7)
     GND -| NG2A   Y2 |- NCE5.2    
     GND -| NG2B   Y3 |- NCE5.3        Rem: CE2.2 = BEN*N(AR18)
   CE2.2 -| G1     Y4 |- NCE5.4
  NCE5.7 -| Y7     Y5 |- NCE5.5    
     GND -| GND    Y6 |- NCE5.6      
           -----------             

G1 G2* C B A | Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7    G2* = G2A+G2B
-------------|------------------------
X  H   X X X | H  H  H  H  H  H  H  H
L  X   X X X | H  H  H  H  H  H  H  H
H  L   L L L | L  H  H  H  H  H  H  H
H  L   L L H | H  L  H  H  H  H  H  H
H  L   L H L | H  H  L  H  H  H  H  H
H  L   L H H | H  H  H  L  H  H  H  H
H  L   H L L | H  H  H  H  L  H  H  H
H  L   H L H | H  H  H  H  H  L  H  H
H  L   H H L | H  H  H  H  H  H  L  H
H  L   H H H | H  H  H  H  H  H  H  L

Thanks to:
----------
Stefano Garavaglia              for a judicious remark 
(alter.ego@iol.it)              about powering the chips

Contacting the author
---------------------

        Philippe TEUWEN
        Belgium
        s952365@student.ulg.ac.be