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title: "Simple T/S 2068 Output Port (Part 1)"
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published_at: "2022-10-07T12:25:31+00:00"
modified_at: "2026-08-01T13:12:54+00:00"
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excerpt: "Before your T/S 2068 computer can be used to perform real tasks instead of playing games or to cary out simple calculations it needs some means for actuating external devices. Our simple Out Port provides the interface between the 2068 and whatever external item you wish to control (turn 'on' or 'off'). The IC used…"
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  - id: 43566
    title: "Simple T/S 2068 Out Port (Part 2)"
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    url: "http://localhost/article/simple-2068-out-port-part-2/"
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---

# Simple T/S 2068 Output Port (Part 1)

Before your T/S 2068 computer can be used to perform real tasks instead of playing games or to cary out simple calculations it needs some means for actuating external devices. Our simple Out Port provides the interface between the 2068 and whatever external item you wish to control (turn ‘on’ or ‘off’).

The IC used in the Out Port circuit is a 74LS374. This circuit accepts data signals only when it is addressed to the clock input. The ‘state’ (high or low) is retained until the next clock pulse is sent to the 74LS374 IC. The IC will then be latched ‘on’.

Addressing is accomplished by the use of two gates of a 74LS27 IC. Address 31 was selected because only one signal is required on A5 with `WR` (write) and `IORQ` (in/out request). One gate of the 74LS27 accepts and decodes the address decode signal and the second gate inverts the signal. Inversion of the signal is required to trigger the Port latch.

With the Out Port connected to the T/S 2068/Spectrum (it works with the Spectrum ROM) edge connector and 255 entered into location 64000, the following sequence would be carried out:

1. Basic Program: `10 OUT 31,PEEK 64000`
2. 11111111 is the binary equivalent of 255 which appears on the data bus and the appropriate pins of the 74LS374 IC.
3. Signals A5, WR and IORQ trigger gate 1 of the 74LS27 and direct it to latch all of the eight mono-stables (flip-flops) which outputs eight 1’s to the eight output lines on the 74LS374.
4. The next basic command will remove the signals to the 74LS27 allowing the program to continue with other activities.
5. Since the 74LS374 is in a ‘latched on’ state, all eight outputs (eight 1’s) will remain on until the next `OUT 31,0` command is given. (`OUT 31,0`) will command the ‘off’ state.

Testing the Out Port is simple. Solder a 270 ohm, 1/4 watt resistor to the anode lead of an LED (see diagram). The cathode lead of the LED is attached to the ‘0’ volt terminal (ground).

Connect the Out PORT to the 2068 edge connector (With POWER OFF). Turn the computer on. If the copywrite appears on the monitor screen, then everything is OK.

Attach the free end of the 270 ohm resistor lead to the 1’st output on the 74LS374 IC. Type: `OUT 31,1` and the LED should illuminate. `OUT 31,0` will extinguish the LED.

With eight LEDs and eight 270 ohm resistors attached; one to each Out Port and ground, a simple program will activate all eight ports: `OUT 31,255` or type:

```
10 FOR i=1 TO 255
20 OUT 31,(i)
30 PAUSE 10
40 NEXT i
```

This program will activate all eight ports and the LEDs will illuminate with the binary count (count 1 the first LED lights; count 2 the second LED lights; count three and the first and second LEDs illuminate … and so on through count 255; all LEDs light).

Parts required to build the Out Port:

- 1 — 74LS374 and 1 — 20 pin IC socket
- 1 — 74LS27 and 1 — 14 pin IC socket
- 1 — PC Board
- 1 — Edge connector, Zebra Systems, Inc (Zebra also offers an experimentors board; can be used in place of perf board)
- 1 — LED and 1 — 270 ohm 1/4 watt resistor for testing output

Part 2, next issue of L.I.S.T. will cover isolating circuits to interface the Out Port and the outside world (relays and assorted opto-devices).