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The circuit on page 4 is for a 16K byte RAM extension board to plug onto the ZX81’s rear connector. Connecting the board disables the ZX81’s internal 1K RAM, but it is compatible with the Pseudo ROM and 24 line I/O circuits given earlier.
4116 dynamic RAM ICs have been used for low cost, but they do require additional – externally generated – supplies of +12V at 150mA and -5V at 5mA.