Texas Instruments SR-50A (Version 1)

The SR-50A was introduced shortly after the SR-50 to to reduce manufacturing costs. The only advantage compared to early SR-50 calculators is the higher calculating precision. Please find the comparison in the Calculator forensics.

Dismantling the featured SR-50A calculator with Date code 2276 RCI and manufactured in May 1976 in Rieti, Italy reveals a surprise - the printed circuit board (PCB) is still using the original "Version 1" layout while calculators manufactured in the US during the same time used already the design known here at the Datamath Calculator Museum as SR-50A (Version 2). Compared to the first generation SR-50 introduced already in January 1974 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-50A is still centered around the "Calculator Brain" composed of the two TMC0501 Arithmetic and TMC0521 Scanning and Read-Only Memory Chips and 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.

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-50A 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-50 used a clock generation circuitry composed of multiple discrete components, utilizes the SR-50A (Version 1) a SN97227 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-50 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-50A a small 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.

The appearance of the SR-50A previews already the design language of the later SR-52 Programmable Calculator. 

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-50A.

The SR-50A 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.