PIA for Electron

Despite its neat design and relatively low cost, the Acorn Electron corm puter suffers from an unfortunate lack of I/O support, which is remarkable, considering the fact that it is a relatively simple matter to add, say, two I/O ports to enable the com- puter to drive a printer, plotter, modem, or other peripherals by means of the proposed PIA (periph- eral interface adapter). The circuit diagram of the PIA-based extension shows that address decoding over the full 64 Kbytes is by means of two 8-bit magnitude com- parators Type 74LS688. Address selection is manual with switches Si . .S14, which provide a logic low level when closed; observe this when writing out the ones and zeros to arrive at the desired address in the I/O map. The PIA chip is enabled when the preset address matches that on the computer"s address bus; writing simple I/O drivers is there- fore mainly a matter of assigning the relevant address block to control words and PIA I/O data. Ti has been included to enable t...