---
title: "Simple RTTY TU Circuit"
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published_at: "2026-01-12T20:52:19+00:00"
modified_at: "2026-01-12T20:52:20+00:00"
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excerpt: "This simple RTTY terminal unit provides a TTL output that can be connected directly to the ZX81. Shown is a two position switch to allow for \"inversion\". but an additional position is recommended to keep it from messing up tape loading. The LED is used as a tuning indicator and works reasonably well on 170…"
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  - name: "Dallas Williams (WA0MRG)"
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pubdate: "April 1983"
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---

# Simple RTTY TU Circuit

This simple RTTY terminal unit provides a TTL output that can be connected directly to the ZX81. Shown is a two position switch to allow for “inversion”. but an additional position is recommended to keep it from messing up tape loading. The LED is used as a tuning indicator and works reasonably well on 170 Hz shift signals. It will go out when the PLL is locked on frequency. A scope should be used for tuning shifts greater than 425 Az.