Texas Instruments SR-51A (Version 1)

The SR-51A was introduced shortly after the SR-51 to reduce manufacturing costs. You won't notice too much differences to the SR-51, the keyboard layout kept each key on it's place, only the colors changed. You have to open the calculator to understand the differences, the rigid inner keyboard frame of the SR-51 was removed, the LED modules replaced with a LED-stick and bubble-lens magnifier. 

In Summer 1975 Texas Instruments' Portfolio of Scientific Calculators included four different calculators:

The SR-16 was a relict from the past and soon to be replaced with the SR-16 II with slightly better functionality and packaged in a housing similar to the SR-50A, the SR-40 was using the platform of the SR-16 II but opted for a larger single-chip calculator circuit to accommodate not only more functionality but introducing the revolutionary Algebraic Operating System (AOS™). The SR-50A was a cost-down of the original SR-50 aimed against Hewlett-Packard's HP-35 and the SR-51A was competing against the HP-45. Earning money meant either high volume or high margin - Texas Instruments played both games and introduced within just 12 months not only the SR-52 and SR-56 Programmable Calculators using the design language of the SR-50A/SR-51A but with the second and final iteration of the SR-40 a true low-cost Scientific Calculator leading to the legendary TI-30 that brought AOS into the classroom.

From a technical point of view the SR-51A repeats the approach of the SR-51 and is directly based on the SR-50A but uses a second program memory for the additional statistical and conversion functions. While increasing the manufacturing costs of the SR-50A only by about $30, skyrocketed the margin of the SR-51A:

The lifecycle of the SR-51A was much longer than the unfortunate SR-51, next calculator in that line was the SR-51-II.

Dismantling the featured SR-51A calculator with Date code DTA 4675 and manufactured in November 1975 in Dallas, Texas reveals no surprises - the printed circuit board (PCB) looks familiar and is identical to the SR-50A (Version 1) layout. Compared to the first generation SR-51 introduced already in January 1975 the internal construction was changed completely, the electronics previously distributed over two PCBs is now consolidated on one larger PCB and the electric contacts for the rechargeable BP1A Battery Pack using three AA-sized NiCd cells are part of that PCB, too.

The Main-PCB of the SR-51A is based on the TMS0500 Building Blocks for Scientific and Programmable Calculators and uses the TMC0501 Arithmetic Chip with an accuracy of 13 digits and two TMC0522/TMC0523 SCOM (Scanning and Read-Only Memory) Chips containing 1k*13 Bits instruction memory, each and 16*16 digits constants, each. The second SCOM Chip is mounted in a piggy-back manner on top of the first SCOM Chip to allow reusing the SR-50A PCB. The compact "Calculator Brain" is supported by power supply, clock signal generation and display driver circuitry. With the BP1A 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-50A include a transformer based DC/DC converter designed with discrete components.

Stacking two SCOM Chips obviously doubles the features provided by just one SCOM Chip as seen in the SR-50A and we looked into the stack carefully. All pins are connected in parallel and the position of the two Integrated Circuits (ICs) within the stack doesn't matter. Usually one would find the TMC0523 soldered on top of the TMC0522 but some calculators have the TMC0522 soldered on top of the TMC0523 - as seen with the stack to the right from an SR-51A manufactured just one week after the featured SR-51. According to the "Data Sheet" of the TMC0520, it provides four functionalities to calculators based on the TMS0500 Building Blocks:

It is interesting to see how Texas Instruments used the additional capabilities gained through the second SCOM Chip to differentiate against the SR-50A using just one SCOM Chip:

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-51A 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). Comparing the Instruction ROM Content of the featured SR-51A with an SR-51 manufactured in March 1975 showed no differences.

The three 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-51A 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. While the original SR-51 used a clock generation circuitry composed of multiple discrete components, utilizes the SR-51A (Version 1) a SN96239 Clock Generator Chip in a small 8-pin DIP (Dual In-line Package with a 0.1” / 2.54 mm lead pitch) housing together with a TP4011A Clock Buffer Chip in a 14-pin DIP housing to reduce both real estate on the PCB and manufacturing costs. Please notice the small "190" mark on the TP4011A, obviously a reference to the TP0190N designation used with the SR-52.

While the display of the SR-51 is composed of 14 discrete 7-Segment LED modules with an attached magnifier lens mounted together with the two SN27882 display drivers on the Keyboard-PCB, uses the SR-51A a small Texas Instruments DIS234G Display Module soldered directly to the Main-PCB that includes the display drivers.

Texas Instruments even considered to extend its Product Portfolio of Scientific Calculators and developed a product to slot below the SR-50A that was never introduced to the market. Read more about the rare SR-40 prototype.

With the introduction of the SR-56 in Spring 1976 the hardware of both the SR-50A and SR-51A was redesigned to accommodate the TMC0501 Arithmetic Chip, one (SR-50A) or two TMC0530 SCOM Chips (SR-51A and SR-56) and none (SR-50A and SR-51A) or one TMC0599 Multi-Register Chip (SR-56). Read more about the revised SR-51A.

Learn more about Printing with the SR-51 / SR-51A.

The SR-51A is featured in the Texas Instruments Deutschland GmbH leaflets ER-1975 and ER-1976 dated 1975 resp. 1976.

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

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