Texas Instruments SR-10 Version 1

Texas Instruments introduced with the SR-10 their first scientific calculator just a few months after the famous Datamath or TI-2500. Today we wouldn't call it a scientific calculator, but it allowed to enter and display numbers in the scientific notation with an 8-digit Mantissa and a 2-digit Exponent. Texas Instruments targeted obviously slide rules, guess what the abbreviation "SR" in the designation stands for.

The first series of the SR-10 shared the poor readability of the TIL-360 display known from the Datamath calculators. 

Texas Instruments experimented with different solutions and created some prototypes with lenses attached to the 6-digit LED-modules.

Later models used different LED-modules with an additional magnification lens. Don't miss the rare SR-10 Clear-Case Prototype and compare it with the SR-10 Version 2. 

The SR-10 makes use of the TMS0120 single-chip calculator circuit derived from the TMS1802, better known as first "calculator-on-a-chip". The remaining components found inside an early SR-10 are well known from the Datamath, too. Two display-drivers each for the segments and digits of the LED-modules and a discrete power converter to generate the three different supplies inside the calculator.

TMS0100 calculator family and scientific calculator sounds like a contradiction, please keep in mind that the TMS0100 was developed for portable and desktop calculators with just the four basic functions +, −, ×, and ÷ with either Constant or Chain operation and displaying results as an 8-digit or 10-digit value in either floating-point or fix-point representation. Engineers at Texas Instruments crammed the algorithms for three additional functions (reciprocal value of number in display, square number in display, find square root of number in display) into the tiny 3,520 Bits Read-Only program Memory (ROM, 320 Words x 11 Bits) of the TMS0100 while dropping only two functions (constant operation, fix-point representation). One of the successors of the TMS0100, the TMS0600 single-chip calculator circuit introduced in 1973 with the SR-11, increased the ROM size by just 64 Words to 384 Words x 11 Bits and allowed in addition to the functions found with the SR-10 both a Constant switch and a [pi] key to enter the value of pi to 8 significant digits (3.1415927).

Comparing the feature sets of the TMS0106 (TI-3500), TMS0120 (SR-10), and TMS0602 (SR-11) shows the limitations of the TMS0100 and explains the move from Texas Instruments towards architectures with scalable ROM configurations like the TMS0200 Building Blocks for Desktop Calculators introduced in 1973 but most important to the TMC0500 Building Blocks for Scientific and Programmable Calculators introduced with the "Slide Rule" calculator SR-50 in January 1974 and leading all the way to the legendary TI Programmable 59 and the amazing SR-60A Prompting Desktop calculator:

With the TMS0100 originally developed for portable and desktop calculators supporting either an 8-digit or 10-digit output with a leading negative sign/overflow indicator, these chips consequently support only 11 digit outputs for the display but the SR-10 obviously uses a 12-digit display for:

Reverse-engineering the SR-10 schematics reveals a simple trick used by Texas Instruments to control its 12-digit display with only 11 digit outputs of the TMS0120: The unused "H" segment available on the TMS0120 was repurposed for the minus sign of the Exponent in the pictured (pictures on the left) way. Every time the "H" segment output of the TMS0120 is activated, some additional circuitry on the PCB will enable both the digit drivers for the negative sign of the Exponent and the segment driver for the "G" segments of the right TIL-360 display module while the "G" segments of the left TIL-360 display module are activated by the "G" segment output. Hence you'll find 21 connections between the Main-PCB and the Display-PCB:

The second version of the SR-10 with twelve individual 7-segment modules continued this approach but uses the "G" segment output of the TMS0120 for eleven of the display modules and the "H" segment output only with the display module used for the negative sign for the Exponent.

The Klixon™ type keyboard looks very similar to the Datamath calculator with some additional keys placed in the upper line. Later calculators like the SR-11 changed the style of the keys but kept the extreme wedge-style of the housing that was adopted for the scientific desktop calculators SR-20 and SR-22, too. The last portable scientific calculator with this wedge-design was introduced in October 1974 with the SR-16.

The SR-10 manufactured in Italy for the European market introduced a slightly different design of the housing.

The SR-10 was sold with different nameplates, compare it with both the Radio Shack EC-425 and the Montgomery Ward P300.

Don't miss the TI-150, the only basic calculator using the silver trim around the display.

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

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