sinclair zx-81


(click on picture)


Introduced March 1981
Discontinued ???
Release Price    $99.95


      The ZX-81 was the second in a long series of computers designed by Clive Sinclair, a follow-up of the popular (at least in Europe) ZX-80 computer. The ZX-81 had the distinction in the early 80's of introducing a large segment of the growing home computer market to their first computer experience, myself included. Although originally marketed only through mail order in kit form or fully assembled under the Sinclair brand name, the popularity of the machine soon caught the eye of the Timex Corporation who then repackaged it and sold it in retail stores nationwide under its own brand as the Timex/Sinclair 1000. 

       The ZX-81 was introduced in March of 1981. It has a membrane type keyboard which doesn't allow you to type in BASIC keywords, instead you have to push and hold a function key while hitting a specific letter key and the BASIC keyword is printed on the screen. The computer has no color or sound and comes with only 1k of user RAM in the basic unit. Despite these shortcomings the ZX-81 was a remarkable machine. It contained a Z-80A processor running at 3.5 MHz and 8K of ROM including a built in BASIC. On the left side of the computer are 4 ports. The first from the rear to front is a standard RCA type plug output from the built in RF modulator to a standard TV. Curiously the original mail order units from Sinclair output to a UHF channel not the usual VHF channel 3 or 4 that is seen on most all consumer video products sold in the US. This was changed when Timex started selling the computer under its own logo and the later ZX-81's all included a channel 3/4 switch on their underside. The next two ports are 8mm mini jacks for input and output to a standard cassette recorder for storing the programs to and from the computer. The process of loading a program into the computer was a difficult and slow process. As it was with most of the computers of that era that used a cassette as a mass storage device. The cassette recorder was generally slow and unreliable and greatly dependent on a number of factors such as the volume the recorder is set at to record or playback a program. The last port is an 8mm mini jack for the supplied 9 VDC power supply. The ZX-81 comes packaged with a very good owners manual with a very informative BASIC tutorial that takes the first time user step by step through the many features and uses of the ZX-81. All this was sold at $79.95 for the basic unit in kit form and $99.95 fully assembled and tested when equivalent machines at that time were selling at 6 times the cost. Although the basic unit was sold with only 1K of RAM an optional 16K RAM module was available for $49.95.

      This was my first computer. I bought this one in kit form on June 5,1982 along with a 16k RAM module. I had been interested in computers for a number of years prior to purchasing this one and had read a lot of books and magazines about them. I wrote my first BASIC program on it, a program to balance my checkbook. 

      I used a Radio Shack cassette tape recorder as the mass storage device and had the ZX-81 hooked to a Sears 12" Black and White TV. 



System Architecture Ports Disk Storage
PMMU: none
FPU: none
Data Path:  
L1 Cache: none
L2 Cache: none
2nd Processor: none
Slots: none
Bus type  
Data Bus width  
Address bus width  
CMOS real time clock  
USB: none
ADB: none
Video: none
SCSI: none
Parallel: none
Geoports: none
Ethernet: none
FireWire: none
Mic Type: none
AirPort Ready: n/a
Other Ports:  
Floppy Size:  
Int HD Size: none
Int HD Interface: none
Orig CD Speed n/a
Int CD Support: n/a
Internal drive bays  
Standard floppy drives  
Optional floppy drives:  
* 5 1/4 inch 160k none
* 5 1/4 inch 1.2MB none
* 3 1/2 inch 400k  
* 3 1/2 inch 1.44MB none
* 3 1/2 inch 2.88MB none
Hard disk controller included none
Memory Video and Graphics
Logic Board:  
RAM Slots: 0, n/a
Min - Max RAM:  
Min RAM Speed:  
RAM Sizes: n/a
Install in Groups of: n/a
System board memory socket type  
Number of memory module sockets  
Graphics Processor  
Screen size - columns & rows  
Video on board Built in monitor
Video RAM n/a
Max colors Mono - B & W
RGB output n/a
Composit Video Output none
Screen Resolution  
Sprites or Missiles none
Physical Specs. Software Power
Form Factor:  
Gestalt ID:  
Weight (lbs):  
Dimensions (in):  
Keyboard Specs  
Number of Keys  
upper/lower case  
Sound interface device  
Sound Generation  
ADSR capable no
Addressing Modes:  
Orig SSW:  
Orig Enabler: none
ROM Ver: n/a
ROM Size:  
AppleTalk Ver: n/a
Mac OS Supported:  
Max Watts:  
BTU per Hr:  
Voltage: 105-125
Freq Range: 50-60 Hz
Battery Type:  
Soft Power: n/a
Pass Through: n/a
Programming language  
Built in language none
Built in M L monitor none


System Architecture



Microprocessor Z80A   Standard on system board 1k
Clock speed 3.5 MHz   Maximum on system board 1k
Bus type Sinclair 8-bit proprietary   Maximum total memory 64k
Data bus width 8-bit   Memory speed and type ???
Address bus width 16-bit   System board memory socket type ???
Interrupt levels N/A   Number of memory module sockets ???
DMA channels N/A   Memory used on system board ???

Standard Features


Disk Storage

ROM size 8k   Internal disk and tape drive bays none
Optional math coprocessor no   Standard floppy drives none
Parallel port type no   Optional floppy drives: no
RS232C serial ports no   * 5 1/4 inch 160k no
Mouse ports no   * 5 1/4 inch 1.2MB no
UART chip used N/A   * 3 1/2 inch 720k no
Maximum speed N/A   * 3 1/2 inch 1.44MB no
CMOS real time clock no   * 3 1/2 inch 2.88MB no
CMOS RAM none   Hard disk controller included no

Video & Graphics



Graphics Processor ???   Sound Interface device none
Screen size - Col x Rows 32 x 24   Sound generation none
Resolution - Colors/High mono / 256 x 192   ADSR capable no
Resolution - Colors/Low mono / ???      
Max colors none   Programming language  
Sprites or Missiles none   Built in language Sinclair Basic
Expansion Slots     Built in M L monitor no
Total adapter slots ???   Keyboard Specs.  
Number of 8/16/32 bit slots ???   Number of keys 40
Physical Specs.     Upper/lower case yes/no
Dimensions:     Keyboard cable length N/A
* Height 1 1/2 inches   Environmental Specs.  
* Width 6 1/2 inches   Power supply output 9.75 VDC
* Depth 6 7/8 inches   Operating voltage @ 60 Hz 104-127VAC
* Weight 1 pound   Maximum current 650 milliamps