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Texas Instruments SR-40 Prototype

Date of introduction:	Never	Display technology:	LED-stick
New price:		Display size:	10 + 2
Size:	5.8" x 3.2" x 1.3" 147 x 81 x 32 mm³
Weight:	8.1 ounces, 231 grams	Serial No:	001P
Batteries:	BP1A	Date of manufacture:	wk 22 year 1975
AC-Adapter:	AC9130	Origin of manufacture:	USA
Precision:	10	Integrated circuits:	TMS1111
Memories:	1
Program steps:		Courtesy of:	Joerg Woerner

At first glance matches this rare SR-40 perfectly with the successful scientific calculators SR-50A and SR-51A introduced in March resp. June, 1975.

But this SR-40 was never released to the market, the pictured calculator is a rare prototype with the impressive serial number 001P. Nevertheless do we all know the final SR-40 - a sibling of the best selling TI-30 calculator introduced June 13, 1976.

Dismantling the SR-40 001P explains immediately the story behind this project:

• The housing is identical to the SR-50A and uses still the BP1A battery pack.
The later SR-16 II uses another arrangement of the battery contacts with standard batteries.
The TI-2550 II got the BP2 battery pack.
• Instead the powerful TMC0501 architecture of the SR-50A and SR-51A this SR-40 P-001 is
build up around the TMS1111. This chip doubles the memory capacity of the original TMS1000
single-chip calculator found in the SR-16.
The additional memory space was used for the trigonometric functions, unfortunately the
algorithm are worse compared to the TMC0501.
• The display is the standard LED-stick of the SR-50A.

Fellow collector Miroslav Krob discovered in November 2009 the real sensation of the SR-40 Prototype: Please look carefully at the keyboard, there are two keys with parentheses! The first ones on a calculator from Texas Instruments! Thanks, Miroslav!

The date code on the TMS1111 and the internal construction allows a precise placement of the SR-40 P001 in the history of Texas Instruments:

Timeline	Calculator	Comment	Link
January 1974	SR-50	First scientific calculator including trigonometric functions based on the TMC0501 building blocks.
October 1974	SR-16	First use of the TMS1000 single-chip calculator in the old SR-11 housing.
March 1975	SR-50A	Cost reduction of the original SR-50. Main differences are the housing and arrangement of the printed circuit boards.
June 1975	SR-40	Sandwiched between the SR-50A and SR-16 II. Expensive battery pack and colorful keyboard from the big brother combined with the poor algorithm of the smaller brother. Not the best choice !
August 1975	SR-16 II	Cost reduction of the original SR-16, makes use of the SR-50A housing, standard batteries and cheap black keys.
October 1975	TI-2550 II	Introduction of the BP2, a rechargeable battery pack with only 2 AA-cells instead the 3 AA-cells of the BP1A.

Interested in failed calculators? Don't miss the first TI-88 scheduled for release in 1982.

Running Mike Sebastian's "Calculator forensics" gives an unacceptable bad result of 10.271817 instead the expected value close to 9.0000. This places the SR-40 in the league of calculators like the Canon F-6 (12.199423) and Rockwell 61R (10.4328).

All other Texas Instruments calculators fit between the TI-35 PLUS (8.99999798135) and TI-45 (9.177087103).

+++ UPDATE: Fellow collector Marie Collas discovered a SR-40 Prototype with a "real" serial number of 0000030 in Mexico. +++

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