DATAMATH CALCULATOR MUSEUM |
Texas Instruments SR-50 (Version 2)
Date of introduction: | July 1974 | Display technology: | LED modules + lens |
New price: | $169.95, DM 520.00 | Display size: | 10 + 2 |
Size: | 5.8" x 3.2" x
1.3" 147 x 81 x 32 mm3 |
||
Weight: | 8.5 ounces, 240 grams | Serial No: | 0390432 |
Batteries: | BP1 | Date of manufacture: | wk 45 year 1974 |
AC-Adapter: | AC9200 | Origin of manufacture: | USA |
Precision: | 13 | Integrated circuits: | TMC0501, TMC0521, 2*SN27882 |
Logic: | Sum-of-Products | Displays: | 14*DIS279 |
Memories: | 1 | ||
Program steps: | Courtesy of: | Joerg Woerner | |
Download leaflets: |
US: 2.2M Bytes) DE: 1.9M Bytes) |
Download manual: | (US: 5.6M Bytes) |
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)))))):
• B - 2: 9.000005272880 • C - 4: 9.000005272880 • D - 5: 9.000004661314 • E - 5: 9.000004661314 |
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.
If you have additions to the above article please email: joerg@datamath.org.
© Joerg Woerner, December 5, 2001. No reprints without written permission.