Docs: ZX81 expansion port

The expansion port on a Sinclair ZX81 is a 23 pin double sided (46 pin) card edge connector with 0.1 inch (2.54 mm) spacing.

ZX81's expansion port, and a memory expansion

ZX81 expansion port pinout:

Component side / A   Pin nr.  Underside / B
D7                   1        +5V
RAMCS                2        +9V
(key slot)           3   
D0                   4        0V
D1                   5        0V
D2                   6        Φ (CPU clock)
D6                   7        A0
D5                   8        A1
D3                   9        A2
D4                  10        A3
INT                 11        A15
NMI                 12        A14
HALT                13        A13
MREQ                14        A12
IORQ                15        A11
RD                  16        A10
WR                  17        A9
BUSAK               18        A8
WAIT                19        A7
BUSRQ               20        A6
RESET               21        A5
M1                  22        A4
RFSH                23        ROMCS

The expansion port edge connector as shown in "Sinclair ZX81 Assembly Instructions" book: 

For comparison: The ZX Spectrum has a bigger edge connector, with 28 (56) pins.

Comparison with ZX Spectrum expansion port: 23 pin vs. 28 pin

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Additional sources:
https://www.zx81keyboardadventure.com/2023/02/zx81-expansion-bus-cheat-sheet.html
https://sinclair.wiki.zxnet.co.uk/wiki/ZX80/81_edge_connector
Sinclair ZX81 Assembly Instructions, Sinclair Research Limited, 1981 (book)