ZX-Key ZX81 Interface Guide Book

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The ZX-Key ZX81 Interface is comprised of 2 PCBs, the Interface Card and a ZX81 Expansion Carrier Board. The Interface Card holds all the logic an ICs, and is where all the 'magic' happens. The Expansion Carrier Board simply holds the Interface Card and could also be used for any number of other projects.

Powering the ZX81 & ZX-Key Interface

It is highly recommended that you use a modern +9v switch mode power supply when using the ZX-Key Interface with a ZX81. Older supplies from the 1980s are likely to be performing outside their original specifications and will have adverse impacts on the ZX-Key Interface and the ZX81.

Supplies such as the Switchmode Plugpack 9VDC 1.66A from Jaycar (Australian Electronics Chain) provide a variety of jacks that can be substituted as or if required. Similar Plug packs should be available just about everywhere. Ebay often seems to have packs marketed as being specifically for the ZX81.

  • Do not use the Power Supply that came with the ZX Printer, this has been known to cause problems with the ZX-Key Keyboard Interface Card.
  • Do not use Old Power Supplies unless they have been fully tested and found to be operating within specifications. A faulty supply will damage your computer and the ZX-Key Interface Card.

ZX-Key Interface Connectivity

ZX81 Keyboard IDC Header

  • Pin 1: 5v
  • Pin 2: Ground
  • Pin 3: Reset line. Connected to the Z80 reset lines and the Arduino Pro Micros on the keyboard PCB. If brought high the ZX81 and Keyboard are reset.
  • Pin 4 - 8: Column lines on the ZX81 keyboard matrix.
  • Pin 9 - 16:  Row lines on the ZX81 keyboard matrix.

ZX81 Expansion Bus Card

The main ZX-Key Interface PCB is mounted to a carrier board which holds a Double Sided Bus Connector for plugging into the rear of a ZX81. Additionally the mounting PCB presents all the necessary pass-through signal lines at the rear of the board for connecting further expansion interface cards (such as a ZXpand).

The Main Interface and the carrier board are affixed by double row right angle pin header strips. Thesw pin headers should be soldered to both PCBs. (See image below).

ZX81 Expansion Bus Connector - Front

  • 2*23 Pin Double Sided ZX Bus Connector
  • Pins 3a & 3b not connected, marked as SLOT
    • Ensure the Key Spacer is firmly in the ZX Bus Connector SLOT before connecting to a ZX81. If missing replace the Key Spacer, else damage may be caused to both the interface card and the ZX81 computer.

ZX81 Expansion Bus Connector - Rear

Pinout at the rear of the expansion interface is identical to that of the ZX81. Please refer to the ZX81 BASIC Programming guide for full details.

ZX-Key Interface Compatibility 

Computer Compatibility

The Interface and Keyboard are fully compatible with Sinclair hardware and software.

Compatible Systems:

  • Sinclair ZX80
  • Sinclair ZX81
  • Timex Sinclair 1000

Incompatible Systems:

  • Timex Sinclair 1500

Add-on Hardware Comparability

The majority of modern hardware should work with the interface, there is one know exception.

Compatible Add-ons & Expansions:

  • ZXpand
  • ZXpand Plus
  • UDG 4 ZXpand
  • ZX81 Chroma 81 SCART Interface
  • ZX81 ULA Replacement - By Andy Rea
  • All video Modifications will work without issue.
  • vLA81 : A replacement for the Sinclair ZX81 ULA (R1 to R6 must be 10k - Latest boards are sent out this way) 
Incompatible Add-ons:

  •   vLA81 : A replacement for the Sinclair ZX81 ULA (R1 to R6 at 4.7k - Some older Boards)

ZX81 Interface Card Assembly

ZX-Key Expansion Interface Main PCB Layout

Assembly List

Part Type
C1Electrolytic Capacitorvoltage 20V; capacitance 22µF
C2Ceramic Capacitorcapacitance 100 nF
C3Electrolytic Capacitorvoltage 16V; capacitance 47µF
IC174HC245DIP; chip label 74HCT245; pins 20
IC274HC245DIP; chip label 74HC2T45; pins 20
IC3AP1117E50G500mA; chip AP1117E50G; Voltage Regulator; variant sot223
J116 pin IDC HeaderShrouded Male Right Angle; Double Row
R110kΩ Resistorresistance 10kΩ Ω; tolerance ±5% (previously  4.7kΩ)
R210kΩ Resistorresistance 10kΩ Ω; tolerance ±5% (previously  4.7kΩ)
R310kΩ Resistorresistance 10kΩ Ω; tolerance ±5% (previously  4.7kΩ)
R410kΩ Resistorresistance 10kΩ Ω; tolerance ±5% (previously  4.7kΩ)
R510kΩ Resistorresistance 10kΩ Ω; tolerance ±5% (previously  4.7kΩ)
R610kΩ Resistorresistance 10kΩ Ω; tolerance ±5%
R710kΩ Resistorresistance 10kΩ Ω; tolerance ±5%
ZX BUS4 Pin HeaderMale Right Angle; Double Row; Before 'SLOT'
ZX BUS46 Pin HeaderMale Right Angle; Double Row; After 'SLOT'


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