ZX81 Expansion Bus Cheat Sheet

While designing or reverse engineering external interfaces for the ZX81 it's usually advantageous to have some quick reference materials to hand. This is for me as much as anyone, may it be of some use to all.

ZX81 Expansion Bus Connector

ZX81 Expansion Bus Connector, Viewed from Rear of Machine

Pinout Functions and Properties Summary Table

A quick summary of the Signal and Functions of each Pin / Pad available on the Expansion BUS. This is not an extensive description, for that I'd highly recommend reading the 1983 Melbourne House book "The Ins And Outs Of The TIMEX TS 1000 & ZX81" by Don Thomasson, in particular chapter "The External Interface"

TOP ROW
Pin	Lable	Function	Properties
1A	D7	Data Line	Active High - Bidirectional
2A	RAM C.S.	RAM Chip Select	Active Low - Pull high to disable onboard RAM
3A	Slot	Cutout / Keyed
4A	D0	Data Line	Active High - Bidirectional
5A	D1	Data Line	Active High - Bidirectional
6A	D2	Data Line	Active High - Bidirectional
7A	D6	Data Line	Active High - Bidirectional
8A	D5	Data Line	Active High - Bidirectional
9A	D3	Data Line	Active High - Bidirectional
10A	D4	Data Line	Active High - Bidirectional
11A	INT	Interrupt	Active Low - Input
12A	NMI	Non-Maskable Interrupt	Active Low - Input
13A	HALT	Halt CPU Sate	Active Low - Output
14A	MREQ	Memory Request	Active Low - Output
15A	IORQ	Input/Output Request	Active Low - Output
16A	RD	Read Request	Active Low
17A	WR	Write Request	Active Low
18A	BUSAK	Bus Acknowledge	Active Low - Output
19A	WAIT	Force CPU Idle	Active Low - Input
20A	BUSRQ	Bus Acknowledge	Active Low - Input
21A	RESET	Reset / Restart	Active Low
22A	M1	Machine Cycle	Active Low - Output
23A	RFSH	Refresh (dynamic RAM)	Active Low - Output
BOTTOM ROW
Pin	Lable	Function	Properties
1B	+5v	+5 Volts Regulated	Internal
2B	+9v	+9 Volts Un-Regulated	External Supply Voltage
3B	Slot	Cutout / Keyed
4B	GND	Ground 0 Volts	Shared Ground
5B	GND	Ground 0 Volts	Shared Ground
6B	Ø	Clock 3.25 Mhz	Active Low - Output
7B	A0	Address Line	Active High - Output
8B	A1	Address Line	Active High - Output
9B	A2	Address Line	Active High - Output
10B	A3	Address Line	Active High - Output
11B	A15	Address Line	Active High - Output
12B	A14	Address Line	Active High - Output
13B	A13	Address Line	Active High - Output
14B	A12	Address Line	Active High - Output
15B	A11	Address Line	Active High - Output
16B	A10	Address Line	Active High - Output
17B	A9	Address Line	Active High - Output
18B	A8	Address Line	Active High - Output
19B	A7	Address Line	Active High - Output
20B	A6	Address Line	Active High - Output
21B	A5	Address Line	Active High - Output
22B	A4	Address Line	Active High - Output
23B	ROM C.S.	ROM Chip Select	Active Low - Pull high to disable ROM mirrors