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The circuit given on the following page is for an add-on board that will plug into the ZX81’s rear connector and provide 24 bi-directional Input/Output lines. These could be used to control music synthesisers, model train layouts, or whatever you wish.
Because the ZX81 has 1K resistors in series with the data lines DØ-D7, which severely limit the possible DC and AC loading of these lines, a specialised MOS I/O chip has been used. This imposes virtually no DC load on the data lines and only a slight AC load. It is also an attractive choice because it is a very versatile IC, which can provide many types of I/O function.