Texas Instruments SR-11 Version 2

This SR-11 Version 2 can be distinguished from the original SR-11 Version 1 easily: The white cap covering the sliding switch for the Constant function was significantly increased to cover the gap in the faceplate of the calculator.

Dismantling the featured SR-11 manufactured in September 1974 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: The SR-11 makes use of the TMS0602 single-chip calculator circuit derived from the TMS1802, better known as first "calculator-on-a-chip" but with a Read-Only program Memory increased from 320 Words to 384 Words x 11 Bits.

Comparing the feature sets of the TMS0119 (TI-2500), TMS0601 (TI-2550), TMS0120 (SR-10) and TMS0602 (SR-11) shows the limitations of the TMS0100/TMS0600 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:

Display: With the TMS0100 and its successor TMS0600 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-11 obviously uses a 12-digit display for:

Reverse-engineering the SR-11 schematics reveals a simple trick used by Texas Instruments to control its 12-digit display with only 11 digit outputs of the TMS0602 and known already from the second version of the SR-10: The unused "H" segment available on the TMS0602 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 TMS0602 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 DIS115F 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:

The featured SR-11 manufactured in September 1974 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-11 manufactured in September 1974 makes use of a total of four Display Drivers. The two 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. We do not recognize the manufacturer of the SN27423 chips due to the missing company logos.

Clock: While the nominal clock frequency of the TMS0600 single-chip calculator circuit is specified with 250 kHz, uses the SR-11 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 250 kHz, we observed with the featured SR-11 manufactured in September 1974 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-11 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 TMS0602 chip and the clock oscillator. We observed in the featured SR-11 manufactured in September 1974 rather high output voltages of V_SS = 8.4 V and V_GG = -8.9 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 TMS0602, 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, [pi]) 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:

Keyboard: The Klixon™ type keyboard uses much smaller keys than the SR-10 calculator in the same 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 appearance of the SR-11 changed during the life cycle (around May 1974) of the calculator slightly and together with a change of the twelve individual Seven-Segment displays used for the DIS115 display module, we differentiate between three different SR-11 Versions manufactured in the United States between September 1973 and October 1975:

Here at the Datamath Calculator Museum we classify the featured SR-11 as Display Type 2 and Large Cap for Constant Switch.

The SR-11 was not only manufactured in the United States, but in Italy and Spain, too.

It took 12 years and a similar scientific calculator with just the [1/x], [x²], [sqr X] and [pi] keys appeared, don't miss the unusual TI-18 SLR.

Press the X-RAY button and view the internals of a SR-11 calculator.
 (Pictures provided by Edward Soudentas)

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

© Joerg Woerner, September 23, 2003. No reprints without written permission.