ZX81 Technical Details and Trivia


  • an expansion port for increased RAM or access directly to the Z80 microprocessor - 44-pin male edge connector
  • an RF output socket - standard TV aerial type
  • an earphone jack (used for reading programs from cassette) - 3.5mm jack socket
  • a microphone jack (used for writing programs to cassette) - 3.5mm jack socket
  • a power supply socket - 9V DC @ 1A - 3.5mm jack socket.

Technical Facts

  • The ZX81 BASIC ROM was also responsible for character generation. Unlike the ZX80's ROM, the ZX81 ROM contains a sub-interpreter to handle floating-point arithmetic.
  • The ZX81 does not have a dedicated graphics chip, so the Zilog Z80A has to do this work as well. Due to the low clock speed, the ZX81 provided a 'FAST' mode, which would halt all updating of the display while the CPU could get on with other tasks. In 'SLOW' mode, it cycled the CPU's tasks between updating the display and performing other tasks, e.g. calculations.


Hover your mouse over the circuit board for a description of the components

Note: The above PCB is an Issue 3 board

Component details reproduced with friendly permission from Sothius' Home, www.sothius.com



  • The ZX81 eventually got RAM expansion devices up to 64K, although whenever the expansion slot was in use, the 1K of onboard RAM was disabled.
  • The ZX81 was initially sold in the United States through a small Sinclair operation. However, soon, Sinclair struck a deal with Timex (who were manufacturing all the ZX81s anyway up in their Dundee, UK plant) to sell them stateside. They were permitted to rebrand the ZX81, so it was sold as the Timex-Sinclair TS1000.