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

SR-40P_3.jpg (40075 Byte)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.

SR-40P_2.jpg (82715 Byte)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.  sr-50.jpg (38175 Byte)
October 1974 SR-16 First use of the TMS1000 single-chip calculator in the old SR-11 housing. sr-16.jpg (43859 Byte)
March 1975 SR-50A Cost reduction of the original SR-50. Main differences are the housing and arrangement of the printed circuit boards. sr-50a.jpg (37305 Byte)
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 ! Sr-40p.jpg (34259 Byte)
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. sr-16-ii.jpg (41277 Byte)
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. TI-2550-II.jpg (43265 Byte)

Interested in failed calculators? Don't miss the first TI-88 scheduled for release in 1982.SR-40P_1.jpg (10223 Byte)

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. +++

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If you have additions to the above article please email:

Joerg Woerner, January 18, 2003. No reprints without written permission.