Texas Instruments SR-51-II

Date of introduction:  1976 Display technology:  LED-stick
New price:  $79.95, £39.95 Display size:  10 (8 + 2)
Size:  5.8" x 3.1" x 1.4"
 148 x 78 x 36 mm3
Weight:  6.5 ounces, 185 grams Serial No:  9402206
Batteries:  BP6 Date of manufacture:  wk 36 year 1977 (LTA)
AC-Adapter:  AC9131 Origin of manufacture:  USA
Precision:  12 Integrated circuits:  TMC0501E, TMC0581, 2*SN27882
Logic:  AOS - 5 Pending Operations, 9 () Displays:  DIS273D
Memories:  3    
    Download manual: (US: 10.9M Bytes)

Texas Instruments introduced the revolutionary Algebraic Operating System (AOS™) in September 1975 with their flagship Programmable Calculator SR-52 but the SR-40 released in June 1976 and its sibling TI-30 brought it to the masses.

From a marketing point of view it might be very difficult to explain why both SR-50A and SR-51A, the centerpieces of the Portfolio of Scientific Calculators in 1976 still relied on the ancient Sum-of-Products logic, but the lower positioned SR-40 features the modern AOS. From a technical point of view the discrepancy was easy to explain: The SR-50A and SR-51A trace back to the SR-50 introduced already in January 1974 and using the TMS0500 Building Blocks for Scientific and Programmable Calculators which needed additional Register Chips for the AOS implementation while the more recent SR-40 uses a single-chip calculator circuit solution with a total of nine Registers.

We assume that the nightmare for the Portfolio Manager started when it was obvious that the successor of the SR-51A, introduced in Fall 1977 as TI-55, was delayed due to the complexity of its TMC1500 single-chip calculator circuit. The solution of the dilemma was found with key components mainly developed for the TI-58 and TI-59 Programmable Calculators allowing a more powerful and yet smaller "Minimum Configuration" of a TMS0500 system and Texas Instruments could fill the gap between SR-50A/SR-51A and TI-55.

Looking into the available real estate of an SR-40 for the electronics and display of the calculator and comparing it with the SR-50A raises immediately a question: How could they do it? Not an easy task that took obviously a lot of efforts for a product meant to be just an interims soultion with an expected lifecycle of just about 12 to 24 months. Please do not ask for a Return-of-Invest calculation, this is about the rivalry between Texas Instruments and Hewlett-Packard started with the introduction of the groundbreaking HP-35 in January 1972.

Let's find out how Texas Instruments squeezed an SR-51A with AOS into the Majestic housing of the SR-40 and tear down the featured SR-51-II with Date code LTA 3677 and manufactured in September 1977 in Lubbock, Texas. Removing the Battery Pack BP6 from the calculator shows as expected just the backside of the keyboard assembly and we removed the rear part of the housing and indeed could locate the MAJESTIC MOLD description on the inside of the part.

The opened calculator reveals the backside of a printed circuit board (PCB) roughly the size of the SR-40 PCB but using four Integrated Circuits in a dense arrangement and unusual small footprints. Removing the calculator assembly completely from the Majestic housing shows a compact arrangement with both the 12-digit LED display module and power supply module mounted on top of the Main-PCB.

Desoldering both the display and power supply module shows the design of the Main-PCB with just four components:

TMC0501ESP: Texas Instruments introduced the TMC0501 Arithmetic Chip with the original SR-50, the TMC501E Enhanced Arithmetic was optimized for AOS and marks the final development of the device. The SR-51-II makes use of the smaller 0.4” wide 28-pin SPDIP (Shrink Plastic Dual In-line Package with a 0.07” / 1.778 mm lead pitch) instead the original standard 0.6” wide 28-pin DIP (Dual In-line Package with a 0.1” / 2.54 mm lead pitch).

TMC0581: The TMC0521 SCOM (Scanning Read-Only Memory) Chip of the SR-50 included 1k*13 Bits Instruction Memory and two 16-digit Registers for the TMC0501, this DSCOM (Double Scanning Read-Only Memory) includes 2.5k Bits Instruction Memory and eight 16-digit Registers for the TMC0501E, allowing both the feature set of the SR-51A and addition of AOS capability in the dramatically improved "Minimum Configuration" of the TMS0500 Building Blocks.

SN27882: The two SN27882 7-digit Display Drivers are optimized to work at low supply voltages necessary with the design of the SR-51-II using a BP6 Battery Pack with just 2 AA-sized rechargeable NiCd battery cells.

Looking closely at the Main-PCB we are missing an important component, the so-called Clock Generator. The TMS0500 Building Blocks use two non-overlapping clock signals with 180° phase difference and a voltage swing between VSS and VGG. While the original SR-50 design introduced in January 1974 featured a discrete circuitry for the clock generation consisting of a free-running multivibrator using two transistors and an additional four transistors to define both shape and voltage levels of the two clock signals, used the SR-50A introduced in March 1975 an integrated oscillator SN97227 and a TP4011 Clock Buffer Chip manufactured in CMOS technology to condition the signals to the proper levels of the TMS0500 Building Block. Tracing the Main-PCB of the SR-51-II results in a surprising discovery, the TMC0580 SCOM features an integrated Clock Generator. The 120 kOhm resistor located on the left side of the power supply module of the featured SR-51-II results in a clock frequency around 120 - 150 kHz.

While studying the Main-PCB we noticed that it is designed with just two metal layers and rather wide signal traces, we expected based on the high density of the components a multilayer design to route all the electrical connections between the ICs, keyboard and display. To reduce manufacturing costs of the calculator, engineers traded a simple layout with software and hardware modifications:

• The 12-digit DIS273D uses a "non-standard" pinout with the segment connections moved for an easier and cleaner layout of the traces from the two SN27882 Digit Drivers
• The keyboard scanning order was changed from the usual Row 1 to Row 8 order to a more random - but easier to layout - R6, R5, R3, R4, R1, R8, R2, R7 order

Using our DCM-0500 Platform developed for Recording of ROM Images from all ROM (Read-Only Memory) Chips of the TMS0500 Building Blocks for Scientific and Programmable Calculators, we looked closely into the Constant ROM of the TMC0581 and discovered an unexpected difference to the two Constant ROMs TMC0522 and TMC0523 of the SR-51A. While the first half of the TMC0581 Constant ROM is identical with the TMC0522 and holds the usual constants used with the computing algorithm of trigonometric functions like sine, cosine, or tangent does the second half of the TMC0581 not hold the constants for the number conversions like millimeters to inches of the TMC0523. We assume that the second half of the Constant ROM Content is actually holding "Keycodes" similar to the TMC0540 Customer ROM used in the Solid State Software Modules of the TI-59 but we weren't able to decipher them yet.

We had more success with the SR-56 and located in its second SCOM Chip indeed the Keycodes for its "Special Functions", namely six programs totaling 127 program steps. We identified the routines for Polar/Rectangular Conversions [P→R], [R→P], Mean and Standard Deviation Calculation [Mean], [S.Dev.] and Data Entry [∑+], [∑−].

Another interesting detail of the SR-51-II is its battery pack BP6, it was shared only with early TI-57 models.

According to Texas Instruments the SR-51 II was manufactured till 1979, looks to us like someone from Finance asked for a Return-of-Invest calculation...

Find here an excerpt from the TI Learning Center leaflet CB-272 dated 1976:

SR-51-II. Advanced professional electronic calculator.

This powerful, full-functioning calculator is ideal for the advanced high school student to grow with into college and career. It offers solutions to simple arithmetic as well as trigonometric, logarithmic, and hyperbolic functions. It performs all the classical slide-rule functions and more: roots and powers, factorials, reciprocals, percent and percent-change, linear regression and trend-line analysis. Students can use the SR-51-II to help them with complex statistical problems as well: mean, variance, standard deviation and correlation among with seven unit conversions by direct key. It has three addressable memories with direct memory arithmetic and memory/display exchange.

TI’s AOS algebraic operating system allows complex mathematic expressions to be entered in the same order that they are written. Up to nine levels of parentheses are available to ensure proper and accurate interpretation of expressions. And, it handles up to five pending operations.

A fixed decimal option provides results displayed to as many as eight decimal places at directions of user. And, the SR-51-II can state numbers in scientific or engineering notation.

Calculating Better Decisions, a challenging and informative book, illustrates how the advanced, professional SR-51-II calculator can be a powerful decision-making tool – in school, business and everyday life

© Texas Instruments, 1976

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