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.