Texas Instruments SR-51 (Second Design)

With the SR-51 Texas Instruments started a series of scientific calculators that added statistical functions and conversions to the "usual functions". You'll find similar performance and keyboard layout in the line SR-51 -> SR-51A -> SR-51-II -> TI-55 -> TI-55-II -> TI-55-III. But none of them was engineered as perfect as the SR-51. It was built like a tank to survive decades of use as its close relative SR-50. The SR-51 featured the first time a new keyboard design with white, grey, yellow and orange keys, a color scheme used with many Texas Instruments calculators introduced in the Seventies.

Dismantling the featured SR-51 Scientific calculator with Date code 205 and manufactured in May 1975 in Dallas, Texas reveals a perfectly engineered design looking very similar to the SR-50 introduced on January 15^th, 1974. The internal construction of the SR-51 is based on a sandwich of two printed circuit boards (PCBs) separated with a plastic frame holding the electric contacts for the rechargeable BP1 Battery Pack using three AA-sized NiCd cells and mounting hardware for the PCBs and the bottom shell of the housing.

At first glance the smaller of the two PCBs seems to use the same "Calculator Brain" setup as the SR-50 but a closer looks shows two TMC0522 and TMC0523 Scanning and Read-Only Memory Chips mounted on top of each other in a piggy-back manner next to the TMC0501 Arithmetic Chip instead the SR-50's single TMC0521 Chip. Other apparent changes are the gold-plated contacts for the PC-100 Printer Cradle (which is officially not supported by the SR-51) and a very complex circuitry for the clock signal generation of the calculator. The three calculator chips and a potentially connected PC-100 are using two non-overlapping clock signals PHI 1 and PHI 2 with a frequency of up to 250 kHz. On the PCB of the original SR-50 we located a fully transistorized clock circuitry that slows down the clock frequency of the calculator to optimize the power budget while the calculator is just displaying results and scanning the keyboard and not performing actual calculations.

This feature is gone with the SR-51 and we noticed a complex arrangement of two transistors as free-running oscillator, a SN74LS74 D-Type Flip Flop dividing the signal by two and four SN74LS02 2-Input NOR Gates generating the control signals of two level-shifters composed of four transistors to generate PHI 1 and PHI 2. Looks at first glance like an overkill but supports together with the Printer Cradle contacts the idea that Texas Instruments was already actively working on the SR-51P, better known as SR-56 Programmable. Learn more about the subtle changes on the Main-PCB that prevent some SR-51 from actually printing with a PC-100.

With the BP1 battery pack having a nominal voltage of around 3.7 Volts but the calculator chips manufactured in a 8 um metal gate PMOS process requiring two voltages of -10.0 Volts and -15.8 Volts, does the SR-51 include a transformer based DC/DC converter designed with discrete components.

The larger of the two PCBs contains the display of the calculator composed of 14 discrete 7-Segment LED modules with an attached magnifier lens and two SN27882 display drivers but most of the real estate is dedicated to the 40 snap action switches of the keyboard. Tracing back to the reliable Klixon™ switches and using double shot injection molding keys, the SR-51 might be together with the SR-50 TI's most reliable calculators ever produced. Time will tell.

Don't miss the rare SR-51 Clear-Case Prototype and a very early SR-51E, also known as SR-51 (Engineering Sample).

To reduce manufacturing costs and to give a similar appearance to the SR-52 and SR-56 calculators the SR-51 was replaced within few month with the SR-51A. The featured calculator was manufactured in May 1975, earlier models used bold printed characters for the shifted [2^nd]-functions and a different cancel exponent entry label (above the [EE] key). Compare them here.

Comparing the Constant ROM Content with the programmed constants frequently used with computing algorithm of trigonometric functions like sine, cosine, or tangent of an SR-50 manufactured in May 1974 still using the original TMC0521-2 SCOM Chip with the TMC0522C-5 Chip of the featured SR-51 with our TMS0500 Platform after recording their ROM Images showed no differences. Looking into the constants programmed into the TMC0523A-5 Chip reveals as expected values like 2.54 (inches to centimeters), 0.3048 (feet to meters) and 0.9144 (yard to meters).

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

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