ZX Spectrum 128 edge connector

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Lower Upper
A14 1 1 A15
A12 2 2 A13
+5v 3 3 D7
+9v 4 4 NC
0v 6 6 D0
0v 7 7 D1
!CK 8 8 D2
A0 9 9 D6
A1 10 10 D5
A2 11 11 D3
A3 12 12 D4
NC 13 13 !INT
0v 14 14 !NMI
NC 15 15 !HALT
NC 16 16 !MREQ
NC 17 17 !IORQ
NC 18 18 !RD
!BUSRQ 19 19 !WR
!RESET 20 20 -5v
A7 21 21 !WAIT
A6 22 22 +12v
A5 23 23 12v AC
A4 24 24 !M1
!ROMCS 25 25 !RFSH
!BUSACK 26 26 A8
A9 27 27 A10
A11 28 28 NC

The ZX Spectrum+ 128K expansion connector is a double sided card edge connector with a 0.1 inch spacing. The two rows of conductors are numbered from right to left looking into the rear of the computer. One pair of conductors are missing as there is an indexing slot cut out of the circuit board.

The ZX Spectrum +2

The ZX Spectrum +2 uses the same pinout as the UK Spectrum+ 128K with the exception of Lower pin 13 which is connected to the !IORQGE input.

Notes on Connections

CPU Clock

The !CK signal, sometimes referred to as PHICPU is available on Lower Pin 8. This clock signal is generated by the ULA and is interrupted during contended memory access. This clock signal is inverted by a transistor switch to provide a clean clock edge for the Z80.

This signal is NOT connected on the Investronica ZX Spectrum+ 128K.


Some schematics label Lower Pin 13 as !IORQULA or !IORQGA, however this pin is NOT connected on either the Investronica or Sinclair ZX Spectrum+ 128K. It is connected on the ZX Spectrum +2.

Key Slot

The key slot ensures correct alignment of a peripheral with the edge connector. This slot is the width of one conductor and lies between Pin 4 and Pin 6, i.e. Pin 5 does not exist.


  • Lower Pin 4 is connected to the +9 volt (nominal) DC supply from the ZX Spectrum PSU.
  • Lower Pin 3 is connected to the smoothed +5 volt DC output of the internal 7805 regulator.
  • Upper Pin 20 is connected to the -5 volt DC output of the internal power supply circuitry.
  • Upper Pin 22 is connected to the +12 volt DC output of the internal power supply circuitry.
  • Upper Pin 23 is connected to the oscillating voltage within the internal power supply circuitry. This is nominally 12 volts AC.
  • Lower Pins 6, 7, and 14 are connected to the 0 volt rail.

Removed Video Outputs

Unlike the ZX Spectrum 16k/48k the ZX Spectrum+ 128K ULA generates RGB and Bright video signals rather than intensity and colour difference signals. The !Y, U, and V signals are therefore not available so the pins previously used for them are left unconnected. Additionally the composite video signal has been removed.