S/XEGS RAMCART 128KB by C.P.U.
version G1.04 fix3+
HARDWARE DOCUMENTATION


I. DISCLAIMER

The authors take no responsibility for any damages, which may take place during building and/or using of this hardware. YOU BUILD AND USE IT AT YOUR OWN RISK!


II. DESCRIPTION

U1 is SRAM 128KB memory used for storage data and its power is backuped by 3V battery B1 while you switch the switch SW5 to "BATTERY ON" position. For make sure that the battery B1 isn't charged by Atari diode DI2 is used there. Diode DI1 prevents to power from battery can come to Atari computer. The pull-ups resistors R3,R4 helps to define high level signal for disconnecting memory after Atari power down. For storing of active bank number (bank switching) there is the latch register U2. If his outputs are disconnected then resistor network RN1 sets address pins A13-A16 to high level, so the last memory bank is used. The main logic is in GAL U3. To the GAL there are connected switches SW3, SW4 and buttons SW1, SW2 for control of the S/XEGS RAMCART cartridge. All switches are described in user part of documentation. Capacitor C4 and resistor R1 are used for shorting of write signal during write operation. Diode DI3 (with resistor R2) signalizes that S/XEGS RAMCART is enabled/disabled.

We are using the low power LED Diode with corresponding resistor (for example for 2mA it's 1.8kOhm), but you can use any other LED diode with proper resistor value.

Please, be sure your Atari power supply is sufficient for higher power consumption with this RAMcart cartridge!


III. BILL OF MATERIALS

   Part  Used   PartType             Designators       PCB side
------------------------------------------------------------------------

     1     1    SRAM 128kB 628128-70    U1              bottom
     2     1    74LS574                 U2              bottom
     3     1    GAL16V8                 U3              bottom
     4     1    battery 3V - CR2032     B1              bottom
     5     3    100nF                   C1 C2 C3        bottom
     6     1    470pF                   C4              bottom
     7     1    resistor net 4x10k      RN1             bottom
     8     1    220R                    R1              bottom
     9     1    1k8                     R2              bottom
    10     2    10k                     R3 R4           bottom
    11     2    diode 1N4148            DI1 DI2         bottom
    12     1    low current LED diode   DI3             top
    13     2    button                  SW1 SW2         top
    14     3    switch                  SW3 SW4 SW5     top
    15     1    battery socket                          bottom


IV. USED AND RELATED DOCUMENTS

  1) Atari, Atari Intern
  2) A lots of cart descriptions on the Internet
  3) ST Microelectronic, M68AF127B memory datasheet
  4) Lattice, GAL GAL16V8 datasheet
  5) RAMCART by Zenon/Dial
  6) MD-DOS 128KB SRAM by RADKOR