ZX-Spectrum +3's expansion I/O socket

U 27  26  25  24  23  22  21           8   7   6   5   4   3   2   1
  .-. .-. .-. .-. .-. .-. .-.         .-. .-. .-. .-. .-. .-. .-. .-.
|^^^^^^^^^^^^^^^^^^^^^^^^^^^^ /////// ^^^^^^^^^^^^^^^|^^^^^^^^^^^^^^^^|
|____________________________ /////// _______________|________________|
  `-' `-' `-' `-' `-' `-' `-'         `-' `-' `-' `-' `-' `-' `-' `-'
L 27  26  25  24  23  22  21           8   7   6   5   4   3   2   1


                        EXPANSION I/O socket
PIN UPPER ROW (U) LOWER ROW (L)
1 A15 A14
2 A13 A12
3 D7 +5V
4 ROM 1 OE not used
5 D0 GND
6 D1 GND
7 D2 CK
8 D6 A0
9 D5 A1
10 D3 A2
11 D4 A3
12 INT not used
13 NMI GND
14 HALT ROM 2 OE
15 MREQ DISK RD
16 IORQ DISK WR
17 RD MOTOR ON
18 WR BUSRQ
19 not used RESET
20 WAIT A7
21 +12V A6
22 -12V A5
23 M1 A4
24 RFSH not used
25 A8 BUSACK
26 A10 A9
27 RESET A11

Source: [The Hardware Book]

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