Sinclair Scientific

Sinclair Radionics Ltd founded on July 25, 1961 by Sir Clive Sinclair († September 16, 2021) in Cambridge, England, launched with the Scientific in August 1974 a highly unusual 12-function calculator. It displays only in scientific notation - 5 digit mantissa, 2 digit exponent. Because of the way its software is designed, it relies on RPN (Reverse Polish Notation) like the famous Hewlett Packard HP-35 introduced in July 1972. This unusual method of mathematical problem solving meant that, for instance, to add 2 and 4, one had to enter 2, then 4, then the [+] symbol. There is no [=] key.

Texas Instruments announced on September 17, 1971 with the TMS1802NC the first available standard calculator building block on a chip, it was later renamed into TMS0102. The chip integrates 3,520 Bits Read-Only program Memory (ROM, 320 Words x 11 Bits), a 182-bit Serial-Access Memory (SAM, 3 Registers * 13 Digits, 2 * 13 Bit-Flags) and a decimal arithmetic logic unit as well as control, timing, and output decoders but no drivers for the display. These function blocks of the chip add up to an overall complexity of roughly 5,000 transistors and marked in 1971 with its die size of roughly 230 mils * 230 mils (6 mm * 6 mm) the limits of a chip for commercial manufacturing.

While the 3-Register Serial-Access Memory of the TMS0100 series of single-chip calculator circuits is large enough to realize a basic four-function calculator with independent memory, couldn't the engineers fit the necessary code into the 320-Word Read-Only program Memory of the device and Texas Instrument started to manufacture in 1973 calculators like the Montgomery Ward P8M with chips from their competitor Western Digital.

With the the numbers of transistors in Large-scale Integration (LSI) chips roughly doubling every two years, Texas Instruments decided for the successor of the TMS0100 family to expand  into three different branches:

Dismantling the featured Sinclair Scientific manufactured in March 1975 in United Kingdom reveals a surprisingly compact design with just one printed circuit board (PCB) for main electronics and keyboard with the display module soldered directly to it and powered by four AAA-sized disposable Alkaline batteries. The Main-PCB is centered around a Texas Instruments single-chip calculator circuit and supported by a two support chips and a few discrete components:

Calculating Unit: The Sinclair Scientific makes use of the TMC0805 single-chip calculator circuit derived from the TMS1802, better known as first "calculator-on-a-chip" but with a Serial-Access Memory decreased from 13 Digit Registers to 11 Digit Registers while keeping the Read-Only program Memory of 320 Words x 11 Bits and integrating Segment Drivers. The first designs of the TMS0800 architecture could be found with the TI-1500 introduced in April 1974 and using the TMS0803 chip, while both Canon LE-84 and Sinclair Cambridge introduced in August 1974 are centered around the TMS0801 chip.

While Texas Instruments introduced with the SR-10 in November 1972 a "Slide Rule" calculator with just [1/x], [x²], [sqr X] functionality and scientific notation into the 320-Word ROM of the TMS0120, cramped the engineers of Sinclair in Summer 1974 eight trigonometric and logarithm functions into the 320-Word ROM of the TMC0805. Learn more about "How could they do it, how could they?".

Display: Texas Instruments introduced together with the TMS0100 calculator chip two pre-configured LED (Light-Emitting-Diode) modules (DIS40, DIS95) based on the TIL360 arrays and the corresponding segment drivers (SN75491) and digit drivers (SN75492). Most early 8-digit designs like the LE-80 made use of these parts exhibiting two disadvantages:

Texas Instruments consequently introduced with the SN75493 and SN75494 revised display drivers optimized for designs with 3 or 4 batteries but the true innovation could be observed with the technology how to manufacture the 9-digit LED displays used with early four-function calculators:

The featured Sinclair Scientific manufactured in March 1975 makes use of a Bowmar Optostic Nine-Digit display module with 9 Seven-Segment displays chips bonded onto a PCB and protected with a red plastic lens. The display module is connected with 17 short jumper wires directly to the Main-PCB.

Display Driver: The Main-PCB of the featured Scientific manufactured in March 1975 makes use of nine Digit Drivers for the LED display integrated into custom ICs IFC1 and IFC2 manufactured by Texas Instruments while the TMC0605 chip drives the Segments directly.

Power Supply: The Sinclair Scientific is powered by four AAA-sized disposable Alkaline batteries and uses a transformer and the internal clock oscillator of the TMS0800 single-chip calculator to generate the necessary supply voltages for the electronics.

Keyboard: The keyboard of the Sinclair Scientific uses short-travel keys to push small metal discs against two contacts etched on the Main-PCB A separator plate keeps the metal plate holding the discs isolated from the electronics of the calculator.

The Sinclair Scientific calculator was sold as a Build-It-Yourself kit, too. The Scientific Programmable was an advanced version launched in 1977, again using reverse Polish notation. It could handle programs of up to 24 steps and cost £29.95.

"How could they do it, how could they?": The TMS0800 family of single-chip calculators was developed with cost optimized Four-Banger calculators like theTI-1500 in mind and supported consequently only 3,520 Bits Read-Only program Memory (ROM, 320 Words x 11 Bits), a 154-bit Serial-Access Memory (SAM, 3 Registers * 11 Digits, 2 * 11 Bit-Flags) and a decimal arithmetic logic unit as well as control, timing, and output decoders plus segment drivers for the 9-Digit LED display resulting in just about 5,000 transistors on a die with a size of 205 mils * 200 mils / 5.2 mm * 5.1 mm. At first glance not enough resources for a 12-function Scientific calculator but Clive Sinclair used a secret weapon - programming whiz and math PhD Nigel Searle.

Ken Shirriff studied the United States Patent Application US3934233A describing the TMS0800 architecture and the program code used with the TI-1500 very carefully and was able to create a JavaScript Simulator for the TMS0800 simulating a (slightly modified) TI-2500-II.

Ken learned in 2013 about the Sinclair Scientific and when John McMaster was in 2014 able to successfully decap a TMC0805 single-chip calculator circuit and retrieving its 3,520 Bits of Program Code, the Miracle of the Sinclair Scientific was solved. Well, it took Ken countless hours to figure it out. Thanks!

Please read more on Ken's blog Reversing Sinclair's amazing 1974 calculator hack - half the ROM of the HP-35.

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, December 5, 2001. No reprints without written permission.