Texas Instruments SR-10 Version 1 (Display 2)

Date of introduction:  February 1973 Display technology:  LED modules
New price:  $149.95 Display size:  8 + 2
Size:  6.3" x 3.1" x 1.5"
 158 x 78 x 38 mm3
Weight:  9.2 ounces, 262 grams Serial No:  SR-10 64489
Batteries:  3*AA NiCd Date of manufacture:  mth 03 year 1973
AC-Adapter:  AC9200, AC9130 Origin of manufacture:  USA
Precision:  8 Integrated circuits:  TMS0120, 2*SN27422, 2*SN27423
Logic:  Chain Displays:  DIS115F (12*DISXXX)
Program steps:   Courtesy of:  Joerg Woerner
    Download manuals:   (US: 4.0M Bytes)
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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.

While the appearance of the SR-10 with its bold wedge-style housing changed dramatically from the smooth design with its rounded corners of the Datamath developed by Industrial Designer Fred M. Gore of Fred Gore & Associates, Inc, resembles the internal construction a striking similarity.

Dismantling this early SR-10 manufactured in March 1973 by Texas Instruments in the United States reveals a complex design with three printed circuit boards (PCBs) for main electronics, display, and keyboard powered by three AA-sized rechargeable NiCd batteries. The Main-PCB sports not only five familiar looking Integrated Circuits (ICs) but a myriad of discrete components in a from the Datamath well known arrangement:

• Calculating Unit - TMS0120 single-chip calculator circuit
• Display Driver - 2*SN27422 Segment Drivers and 2*SN27423 Digit Drivers
• Clock signal generation for TMS0120 with discrete components
• Power converter with discrete components and transformer
• 21-pin connector to the Display-PCB
• 15-pin connector to the Keyboard-PCB

Calculating Unit: The SR-10 makes use of the TMS0120 single-chip calculator circuit derived from the TMS1802, better known as first "calculator-on-a-chip".

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 +, −, Χ, and χ 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).

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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:

[+] [−]
[Χ] [χ] [+/−] [C] [CE] [CONST] [F/2/4] [EE] [1/x] [x2] [sqr X] [pi] Display
TMS0106 * *   *   * * *           E8888888888
TMS0120 *   * * * *     * * * *   E88888888-88
TMS0602 *   * * * * *   * * * * * E88888888-88

Display: 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:

• 1: Overflow indicator, negative sign Mantissa
• 2-9: 8-digit Mantissa
• 10: Negative sign Exponent
• 11-12: 2-digit Mantissa

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 driver for the negative sign of the Exponent and the segment driver for the "G" segment of Digit 3 (counted from right to left) of the DIS115 Twelve-Digit display module while the "G" segments of all other digits are activated by the "G" segment output. Hence you'll find 21 connections between the Main-PCB and the DIS115 Display-PCB:

• 12 connections for Digit 1 to Digit 12
• 1 connection for Segments G of Digit 3
• 1 connection for Segments G of Digit 1, Digit 2 and Digit 4 to Digit 12
• 6 connections for Segments A, B, C, D, E, and F of Digit 1 to Digit 12
• 1 connection for Decimal Point of Digit 1 to Digit 12

The first version of the SR-10 with two TIL360 Multi-Digit display modules used a slightly different approach, the "H" segment output of the TMS0120 enables both the digit driver for the negative sign of the Exponent and the segment driver for the "G" segments of the right TIL360 Multi-Digit display module while the "G" segments of the left TIL360 Multi-Digit display module are activated by the "G" segment output. Consequently the Main-PCB of the SR-10 was redesigned from Revision B to Revision D.

The featured SR-10 manufactured in March 1973 uses a DIS115F Twelve-Digit display module with 12 individual DISXXX Seven-Segment displays and an integrated magnifying lens.

Display Driver: The Main-PCB of the featured SR-10 manufactured in March 1973 makes use of a total of four Display Drivers. The two SN27422 (SN75493) Segment Drivers for four segments, each and the two SN27423 (SN75494) Digit Drivers for six digits, each are improvements of the original SN75491/SN75492 chips introduced with the TMS1802 but allow for operation at lower voltages.

Clock: While the nominal clock frequency of the TMS0100 single-chip calculator circuit is specified with 250 kHz, uses the SR-10 a slower pace to reduce overall power consumption of the product slightly. The astable multivibrator using two discrete transistors operates at a frequency between 150 kHz and 200 kHz, we observed with the featured SR-10 manufactured in March 1973 a clock frequency of 185 kHz.

Texas Instruments introduced in August 1973 with the TI-2500 Version 3 the approach of a dynamic switching of the clock frequency for the TMS0100 single-chip calculator circuit to conserve power between calculations. The astable multivibrator idles at a frequency of around 50 kHz but increases with the detection of a depressed keybutton for a short time to about 200 kHz to reduce execution time of the operations. Two diodes are connected between the keymatrix inputs KN (numbers) and KO (operations) and the oscillator to catch every entry of a number or function keys for a impressive reduction of power consumption.

Power Supply: The SR-10 is powered by three AA-sized rechargeable NiCd batteries resulting in a typical voltage between 3.0 V (completely depleted cells) and 4.5 V (while charging full cells). The Main-PCB hosts a power converter circuit centered around an astable multivibrator, step-up transformer and various diodes and capacitors to generate the supply voltages for the TMS0120 chip and the clock oscillator. We observed in the featured SR-10 manufactured in March 1973 rather asymmetrical output voltages of VSS = 7.2 V and VGG = -8.0 V for the electronics.

Battery Saver Circuit: To save battery power the LED display turns off automatically between 15 and 60 seconds after the last keyboard entry, except for the first digit (Digit D3 of TMS0120, LSD of Mantissa). If the display turns off while entering a problem, the display turns on automatically with the first keyboard entry. Depressing the [=] key brings back the last calculated display. Three diodes are connected between the keymatrix inputs KN (numbers), KO (operations) and KP ([1/x], [x2] and [sqr X]) and a simple monoflop to catch every entry of a number or function keys to keep the Digit Drivers enabled. If the monoflop time expires, the Digit Drivers sans Digit 3 are disabled for an impressive reduction of power consumption:

Mode Display Current
VBAT = 4.5 V
Clock Frequency
Calculating 0 63 mA 185 kHz
Power Save 0 63 mA 185 kHz
Calculating E88888888-88 132 mA 185 kHz
Power Save 8 75 mA 185 kHz

Keyboard: 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.

SR-10-V1_DISP.jpg (18223 Byte)The first series of the SR-10 shared the poor readability of the TIL360 based display with the early Datamath calculators. 

SR-10_Proto_DISP.jpg (27302 Byte)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.

Not only the appearance of the SR-10 and the used LED-modules changed during the life cycle of the calculator, a later cost-reduction redesign changed the Main-PCB completely and we differentiate between four different SR-10 Versions manufactured in the United States between November 1972 and June 1975:

Version Position of
SR-10 logo
SR-10 USA V1 Display frame 6-digit modules
 without lens
4 ICs
SR-10 USA V1D2 Display frame single modules
with lens
4 ICs
SR-10 USA V2 Keyboard single modules
with lens
4 ICs
SR-10 USA V3 Keyboard single modules
with lens
2 ICs

Here at the Datamath Calculator Museum we classify the featured SR-10 as Display Frame Version 1, PCB Type 2 and Display Type 2.

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.

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© Joerg Woerner, November 18, 2021. No reprints without written permission.