interne 32K-RAM Erweiterung für ZX81
LowCostTuning ein Selbstbau Projekt in drei Stufen
Du brauchst einen ZX81 Schaltplan, etwas Bastelerfahrung und ganz wenige Bauteile,
die man fürs Taschengeld kaufen kann.
-
Step 1: Wer nur 16K-Byte will hat es besonders einfach:
- remove the original 1or 2k memory chip(s)
- solder a good ic-socket with 28 pins at the place of IC4
- fit some short wire at the place of LK2 (on the right side of IC4)
- take the 38Kx8 RAM-chip and bend pins 1, 2, 22, 23, 26 up, they are not allowed to
have contact with the pins of the socket
- put the ram (43256, 55257 or 62256 e.g.) into the socket
- connect pin 1(A14) with ground (pin 14 of IC4)
- connect pin 2 (A12) with A12 , this is the cathode of D3 on the ZX81 pcb
- connect pin 22 with ground
- connect pin 23 (A11) with A11, this is the cathode of D1 on the ZX81 pcb
- connect pin 26 (A13) with A13, this is the cathode of D5 on the ZX81 pcb
- if you want to run HRG8, connect pin 27 via a resistor of 10K ohms with 5 volts
- that's all
- Step 2: full 32K-RAM from 16K to 32K and 48K to 64K
- remove the RAM from the socket and bend pin 28 up (only nessary for step 3)
- put the RAM into the socket
- connect pin 28 with +5 volts
- remove the wire from pin 1 to ground and connect pin 1 (A14) with the following circuit
- build a simple circuit with only two NAND-gates from a 74LS00.
The function is as follows:
When the ULA is forced to output the display-file, this is indicated to the ULA with /M1 = LOW
and A15 = HIGH. Because the display-file is located in the 16-32K area, A15 for the RAM must be LOW
when /M1 = LOW. The two NAND-gates form an AND.
- connect the A15' output of the circuit with pin 1 of the 32K-RAM chip
- connect A15 (cathode of D8), /M1 (edge-connector 22A), 0 volts and + 5 volts with 74LS00
- Step 3: memory-backup
- only effective in 48K-64K area, as RESET will delete the 16K - 32K area
- replace the wire from RAM pin 28 to + 5 volts by a gemanium or a schottky diode
- connect a backup batterie of about 3 - 3.6 volts via a 100R resistor with pin 28