Sothius' Home of Ancient Microcomputers - Sinclair ZX81 (ISSUE 3 PCB)

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Sinclair ZX81

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Sinclair ZX81

Sinclair ZX81 (51 KBytes)

The Sinclair ZX81 in all its beauty
(picture copyright by M.A.Grundke)

Specifications

Name Sinclair ZX81

Made by Sinclair Research Ltd., Cambridge

Released March 1981

Serial-No. none

Additional info sinclair
ZX81 Personal Computer
PATENT PENDING
MADE IN UK

Board-info
board's lower side:
sinclair
1981 ©
ISS 3
PCIB
board's upper side:
1981 ©
ISS 3
SINCLAIR ZX81
also see the board overview

CPU Zilog Z80A
the Zilog Z80 is a descendant of Intel's i8080
some machines made use of the (fully compatible) NEC D780 CPU

Speed
3.25 MHz
since in the ZX81's design the CPU also does the video output, it runs at about 900 KHz effectively ('SLOW mode'). There was a dedicated 'FAST mode', in which display output was disabled and the CPU could execute code at full speed

RAM 1 KByte in 2 pcs. NEC µPC2114LC-1R (2114-type 1Kx4 NMOS)
early ZX81's came with one 4118-type (1Kx8) NMOS RAM
expandable to 56 KB, see note below
the board can also be configured for other RAMs, such as 2Kx8 (as in the TIMEX 1000)

ROM/ Native OS Sinclair ZX81 BASIC 8K
is also used as character generator ROM
unlike the ZX80's ROM, the ZX81's contains a sub-interpreter for floating-point arithmetic

Keyboard/ Layout Membrane keyboard
40 keys/ US (QWERTY)
keys have up to five functions!

Graphics In the ZX81, the Zilog Z80A creates the graphics!
Max.Res.: 64x48 'pixels' (256x192 'HiRes' with tricks)
Text: 22x32 characters with graphical charset
Max. Colors: monochrome
this is a PAL-version; NTSC-operation possible by setting a jumper, i.e. soldering a link wire to a specific location (R30?)

Sound none

Media
standard Audio-cassettes (via standard cassette-recorder, 250 Bit/s)
the cassette recorder had to be connected via EAR and MIC (see I/O)
later, with the ZX Spectrum, Sinclair introduced the so-called ZX Microdrive, which used small tape cartridges for storing data. However, microdrives were not backward compatible and thus unavailable for the ZX81

Input/ Output 'Sinclair Computer Logic' ('SCL', a Ferranti ULA 2C210E)
44 pin male edge conn. expansion port ('bus connector')
RF TV-out
3.5mm jack socket headphones-out ('EAR')
3.5mm jack socket microphone-in ('MIC')
9V DC-in

Miscellaneous
Successor to the legendary ZX80 (from 1980) and revolutionary due to its 4 ICs design (making production cheap); however, mass-markets weren't reached until its successor, the ZX Spectrum, was launched in 1982

Was sold by the TIMEX Corporation as TS1000 in the USA (slightly modified, e.g. NTSC output), and later in Portugal (as PAL version)

The ZX81 used the MIC and EAR-Ports to attach a standard cassette-recorder for data-storage - simple, but slow (about 250 baud transfer speed)
When there was additional memory (like the ZX 16K RAM) present on the bus-connector, the internal 1 KB were disabled
Memory expansions with up to 64 KBytes were available from 3rd party, but due to the 8K-BASIC occupying address space, only 56 KB were usable

the ZX81 was also available as kit for self-assembly (cheaper)
My original ZX81 was lost in all these years - thanks to Roland Kalus for donating this one!

'1K-Breakout' screenshot (4 KBytes) '1K-Chess' screenshot (4 KBytes)

Two ZX81 games which would run on the unexpanded machine, i.e. with only 1K RAM! on the left, '1K-Breakout', a very simple Breakout version featuring only one level. The bricks are 'pound' symbols, changing to dollars when hit. When hit for a second time, they disappear, and the brick above would take the vanished brick's position. Gameplay, however, is quite smooth considering the machine's limitied capabilites. On the right is '1K-Chess', which was written by David Horne and published as a listing in 'Your Computer', issue February, 1983. It's probably the smallest chess game with functioning AI ever written - effectively, it only takes 672 bytes!

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Sothius' Home - Museum - Sinclair - Sinclair ZX81 - additional pictures

Name	Sinclair ZX81
Made by	Sinclair Research Ltd., Cambridge
Released	March 1981
Serial-No.	none
Additional info	sinclair ZX81 Personal Computer PATENT PENDING MADE IN UK
Board-info	board's lower side: sinclair 1981 © ISS 3 PCIB board's upper side: 1981 © ISS 3 SINCLAIR ZX81 also see the board overview
CPU	Zilog Z80A the Zilog Z80 is a descendant of Intel's i8080 some machines made use of the (fully compatible) NEC D780 CPU
Speed	3.25 MHz since in the ZX81's design the CPU also does the video output, it runs at about 900 KHz effectively ('SLOW mode'). There was a dedicated 'FAST mode', in which display output was disabled and the CPU could execute code at full speed
RAM	1 KByte in 2 pcs. NEC µPC2114LC-1R (2114-type 1Kx4 NMOS) early ZX81's came with one 4118-type (1Kx8) NMOS RAM expandable to 56 KB, see note below the board can also be configured for other RAMs, such as 2Kx8 (as in the TIMEX 1000)
ROM/ Native OS	Sinclair ZX81 BASIC 8K is also used as character generator ROM unlike the ZX80's ROM, the ZX81's contains a sub-interpreter for floating-point arithmetic
Keyboard/ Layout	Membrane keyboard 40 keys/ US (QWERTY) keys have up to five functions!
Graphics	In the ZX81, the Zilog Z80A creates the graphics! Max.Res.: 64x48 'pixels' (256x192 'HiRes' with tricks) Text: 22x32 characters with graphical charset Max. Colors: monochrome this is a PAL-version; NTSC-operation possible by setting a jumper, i.e. soldering a link wire to a specific location (R30?)
Sound	none
Media	standard Audio-cassettes (via standard cassette-recorder, 250 Bit/s) the cassette recorder had to be connected via EAR and MIC (see I/O) later, with the ZX Spectrum, Sinclair introduced the so-called ZX Microdrive, which used small tape cartridges for storing data. However, microdrives were not backward compatible and thus unavailable for the ZX81
Input/ Output	'Sinclair Computer Logic' ('SCL', a Ferranti ULA 2C210E) 44 pin male edge conn. expansion port ('bus connector') RF TV-out 3.5mm jack socket headphones-out ('EAR') 3.5mm jack socket microphone-in ('MIC') 9V DC-in
Miscellaneous	Successor to the legendary ZX80 (from 1980) and revolutionary due to its 4 ICs design (making production cheap); however, mass-markets weren't reached until its successor, the ZX Spectrum, was launched in 1982 Was sold by the TIMEX Corporation as TS1000 in the USA (slightly modified, e.g. NTSC output), and later in Portugal (as PAL version) The ZX81 used the MIC and EAR-Ports to attach a standard cassette-recorder for data-storage - simple, but slow (about 250 baud transfer speed) When there was additional memory (like the ZX 16K RAM) present on the bus-connector, the internal 1 KB were disabled Memory expansions with up to 64 KBytes were available from 3rd party, but due to the 8K-BASIC occupying address space, only 56 KB were usable the ZX81 was also available as kit for self-assembly (cheaper) My original ZX81 was lost in all these years - thanks to Roland Kalus for donating this one!


Two ZX81 games which would run on the unexpanded machine, i.e. with only 1K RAM! on the left, '1K-Breakout', a very simple Breakout version featuring only one level. The bricks are 'pound' symbols, changing to dollars when hit. When hit for a second time, they disappear, and the brick above would take the vanished brick's position. Gameplay, however, is quite smooth considering the machine's limitied capabilites. On the right is '1K-Chess', which was written by David Horne and published as a listing in 'Your Computer', issue February, 1983. It's probably the smallest chess game with functioning AI ever written - effectively, it only takes 672 bytes!