---
title: "24 Line I/O"
type: "article"
slug: "24-line-i-o"
url: "http://localhost/article/24-line-i-o/"
markdown_url: "http://localhost/article/24-line-i-o.md"
published_at: "2026-01-16T20:22:32+00:00"
modified_at: "2026-01-16T20:22:33+00:00"
featured_image:
  url: "http://localhost/wp-content/uploads/2022/04/20230809-041332.jpg"
excerpt: "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…"
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volume: "10"
issue: "9"
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    title: "QZX v10 n9"
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    url: "http://localhost/issue/qzx-v10-n9/"
pages: "4-7"
pubdate: "August 1992"
archive_link: false
---

# 24 Line I/O

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.