ZX81The ZX81 was the successor of the ZX80 and was sold in two forms as a kit and assembled version. The difference between the two was the introduction of the ULA chip which reduced the number of components on the motherboard and it supports slow and fast mode, ie while typing or displaying text the screen does not blink as the new ULA was taking care of the display does freeing the CPU for other usage. The ZX81 was a complete success in Europe and the US.

The only drawbacks recorded was the keyboard and its ram pack. The keyboard was considered to small and third party companies supplied a keyboard replacement for this problem, but the solution was not cheap. The ram pack had connection issues mainly because the ram pack was  not perfectly aligned with the computer while plugged in. This lead to disconnections while operating and caused the machine to crash. One solution was to place blue tack under the ram pack to keep it in place preventing it from moving freely.

ZX 80
KIT £75.95 (UK) $139.99 (US)
ASS £99.95 (UK) $179.99 (US)
Boxed: No Serial Number on both 4 Assembled and 1 Kit
Zilog Z80 CPU running at a Frequency of 3,5 Mhz.
8 Kbytes Rom and 1 Kbytes of Ram (some Machines had 2 Kbytes). Expandable to 64 Kbytes.
Very Small Black plastic case with a white membrane keyboard for low cost production. The Sinclair Logo is embossed on the far end of the case, and a ZX81 Logo above the keyboard in red. On the left side there are the connections for the Tape and power sockets. While the bus expansion port is located at the back of the machine. The Keyboard membrane is a 40 key non standard QWERTY style with all the keys the same size except for the Line Key and the Shift key. The keys have 5 different functions which represents a basic command for faster programming.
The Display is handled by the ULA chip ie the same pin as load and save operations. As a matter of fact while loading and saving on the TV you where presented with black zig zag stripes. This is due to the fact that to keep the Baud rate the screen frequency is being disrupted. On text mode it can display a resolution of 32×24′ and in Graphics mode 64×48. In both modes the CPU can only display 2 colors (Black and White). Some companies released modules that could transform the ZX81 into a color computer and a better graphic resolution.
The ZX81 does not have any sound but Sinclair release a sound module with 3 Channels of Sound. Some users managed to produce white noise by simply using the FAST and SLOW commands in their programs.
1 x Phono RF Connector.
2 x 3.5mm Mono Jack one for Save and the other to load from a normal cassette recorder.
1 x Edge Connector for upgrading memory, graphics and sound.
The power supply of the ZX81 is a common 9V DC 700mA. The only problem that was encountered with this type of power supply is that when you upgrade the machine, you had to change the power supply with a higher wattage rating.
The weight is only 350g. As for the dimensions the ZX81 measured 167mm x 175mm x 40mm (W x L x H).
Sinclair sold more than one million units around the world.

The Club currently owns 2 versions the UK and the US version. The UK version is in fully working state, boxed and complete with manuals, cables and power supply. The US version is without manuals and box but fully working state. The two versions are in very good state and clean from any scratches or wear.



3.5mm JackFemale at the Computer

Pin Name Dir Desc
2 GND N/A Ground


Expansion Port

46 Pin EDGE Connector
at computer
side Key way is Ref

Pin Name Dir Desc
1 +5V OUT +5V DC
2 +9V OUT +9V DC
3 KEY WAY N/A Seperator
4 0V N/A Ground
5 0V N/A Ground
6 /0 OUT Clock
7 A0 OUT Address 0
8 A1 OUT Address 1
9 A2 OUT Address 2
10 A3 OUT Address 3
11 A15 OUT Address 15
12 A14 OUT Address 14
13 A13 OUT Address 13
14 A12 OUT Address 12
15 A11 OUT Address 11
16 A10 OUT Address 10
17 A9 OUT Address 9
18 A8 OUT Address 8
19 A7 OUT Address 7
20 A6 OUT Address 6
21 A5 OUT Address 5
22 A4 OUT Address 4
23 N/C N/A Rom Channel Select
A D7 IN/
Data Bit 7
B /RAM CS IN Ram Channel Select
C KEYWAY N/A Seperator
D D0 IN/
Data Bit 0
E D1 IN/
Data Bit 1
F D2 IN/
Data Bit 2
G D6 IN/
Data Bit 6
H D5 IN/
Data Bit 5
I D3 IN/
Data Bit 3
J D4 IN/
Data Bit 4
K /INT IN Interrupt Request
L /NMI IN Non Maskable Intr
M /HALT IN Request Port
N /MREQ IN Memory Request
O /IORQ IN IO Request
P /RD IN Read
Q /WR IN Write
R /BUSAK IN Bus Aknowledge
T /BUSRQ IN Bus Request
V /MI IN Memory Interrupt
W /RFSH IN Refresh



3.5mm JackFemale
at the Computer

Pin Name Dir Desc
2 GND N/A Ground



35mm JackFemale at the Computer

Pin Name Dir Desc
1 +9V IN +9C DC
2 GND N/A Ground



Phono Jack
at the Computer

Pin Name Dir Desc
2 GND N/A Ground