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"

Pin	Lable	Function	Properties
*TOP ROW*
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