Monday, February 27, 2023

ZX81 Expansion Bus Cheat Sheet

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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"

1AD7Data LineActive High - Bidirectional
2ARAM C.S.RAM Chip SelectActive Low - Pull high to disable onboard RAM
3ASlotCutout / Keyed
4AD0Data LineActive High - Bidirectional 
5AD1Data LineActive High - Bidirectional
6AD2Data LineActive High - Bidirectional
7AD6Data LineActive High - Bidirectional
8AD5Data LineActive High - Bidirectional
9AD3Data LineActive High - Bidirectional
10AD4Data LineActive High - Bidirectional
11AINTInterrupt Active Low - Input
12ANMINon-Maskable Interrupt Active Low - Input
13AHALTHalt CPU SateActive Low - Output
14AMREQMemory RequestActive Low - Output
15AIORQInput/Output RequestActive Low - Output
16ARDRead RequestActive Low
17AWRWrite RequestActive Low
18ABUSAKBus AcknowledgeActive Low - Output
19AWAITForce CPU IdleActive Low - Input
20ABUSRQBus AcknowledgeActive Low - Input
21ARESETReset / RestartActive Low
22AM1Machine CycleActive Low - Output
23ARFSHRefresh (dynamic RAM)Active Low - Output
1B+5v+5 Volts RegulatedInternal
2B+9v+9 Volts Un-RegulatedExternal Supply Voltage 
3BSlotCutout / Keyed
4BGNDGround 0 VoltsShared Ground 
5BGNDGround 0 VoltsShared Ground 
6BØClock 3.25 MhzActive Low - Output
7BA0Address LineActive High - Output
8BA1Address LineActive High - Output
9BA2Address LineActive High - Output
10BA3Address LineActive High - Output
11BA15Address LineActive High - Output
12BA14Address LineActive High - Output
13BA13Address LineActive High - Output
14BA12Address LineActive High - Output
15BA11Address LineActive High - Output
16BA10Address LineActive High - Output
17BA9Address LineActive High - Output
18BA8Address LineActive High - Output
19BA7Address LineActive High - Output
20BA6Address LineActive High - Output
21BA5Address LineActive High - Output
22BA4Address LineActive High - Output
23BROM C.S.ROM Chip SelectActive Low - Pull high to disable ROM mirrors

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