Texas Instruments SR-50 (Version 2)

Texas Instruments introduced the SR-50 "Slide Rule" calculator in January 1974 as their answer to Hewlett-Packard's revolutionary HP-35, the World's first pocket sized electronic calculator performing both logarithmic and trigonometric functions. Being a full two years behind the HP-35 introduced already in January 1972, Texas Instruments seized the opportunity of Moore's law, the observation that the number of transistors in an integrated circuit (IC) doubles about every two years and improved HP's bit-serial architecture significantly. As a result did the SR-50 not only include an enhanced feature set including hyperbolic functions and a novel "Sum-of-Products Logic" but exhibited by far superior accuracy of the computing algorithm for transcendental functions.

While accuracy of the HP-35 dropped slightly after fixing the famous "exp(ln (2.02)) = 2" bug of early ROMs (Read-Only Memories), did the accuracy of the SR-50 improve over time. As of today we know four different SR-50 ROMs returning two different results with Mike Sebastian's "Calculator Forensics" test computing the expression arcsin(arccos(arctan(tan(cos(sin (9)))))):

Dismantling the featured SR-50 Scientific calculator with Date code 454 and manufactured in November 1974 in Dallas, Texas reveals a design identical to an earlier model manufactured in May 1974 and centered around the TMC0501 Arithmetic Chip and TMC0521 SCOM (Scanning and Read-Only Memory) Chip from TI's TMS0500 Building Blocks for Scientific and Programmable Calculators. The perfectly engineered calculator uses 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.

The smaller of the two PCBs is centered around the "Calculator Brain" and supported by power supply and clock signal generation realized with multiple discrete components. 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-50 include a transformer based DC/DC converter designed with discrete components. The two calculator chips use two non-overlapping clock signals PHI 1 and PHI 2 with a frequency of up to 250 kHz, to optimize the power budget of the SR-50 an innovative clock circuitry slows down the clock frequency of the system while the calculator is just displaying results and scanning the keyboard and not performing actual calculations. Looking closer at the TMC0521 of the featured SR-50 (Version 2) calculator you'll notice a marking "E7441-5" translating into a manufacturing date of the chip in week 41 of 1974 and using Revision "E-5" for its lithography masks used during production and testing while our featured SR-50 (Version 1) reads "B 7407-2" or Revision "B-2".

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.

Comparing the programmed constants of a SR-50 manufactured in May 1974 with the original TMC0520-2 SCOM Chip with a later SR-50 (Version 2) using the final version of the TMC0520-5 Chip with our DCM-0500 Platform after recording their ROM Images showed no differences, but we noticed some changes in the Instruction Memory.

To verify that the differences in accuracy between early SR-50 (Version 1) and later SR-50 (Version 2)/SR-50A calculators are tied to the SCOM Chips, did we actually remove a TMC0501 Arithmetic Chip from an early SR-50 and transplanted it into an SR-50A (Version 1). And yes, the "Franken-SR-50A"  stayed accurate.

To reduce manufacturing costs and to give a similar appearance to the SR-52 and SR-56 calculators the SR-50 was replaced within 18 month with the SR-50A. Don't miss the rare SR-51.

The SR-50 is featured in the Texas Instruments Incorporated bulletins SR-50_FL and CB-SR50 dated 1974.

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

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