Texas Instruments SR-51 (Clear-Case Prototype)

Date of introduction:  (January 1975) Display technology:  LED modules + lens
New price:  $224.95 Display size:  10 + 2
Size:  5.8" x 3.2" x 1.3"
 147 x 81 x 32 mm3
Weight:  8.5 ounces, 240 grams Serial No:  45499
Batteries:  BP1 Date of manufacture:  wk 46 year 1974
AC-Adapter:  AC9200 Origin of manufacture:  USA
Precision:  13 Integrated circuits:  TMC0501, TMC0522,
Logic:  Sum-of-Products    
Memories:  3    
Program steps:   Courtesy of:  Joerg Woerner
    Download manuals:   (US: 2.0M Bytes)
  (US: 12.4M Bytes)

With the SR-51 Texas Instruments started a series of scientific calculators that added statistical functions and conversions to the "usual functions". You'll find similar performance and keyboard layout in the line SR-51 -> SR-51A -> SR-51-II -> TI-55 -> TI-55-II -> TI-55-III. But none of them was engineered as perfect as the SR-51. It was built like a tank to survive decades of use as its close relative SR-50. Compared to the previous models the SR-51 got a keyboard design with white, grey, yellow and orange keys and gave an outlook to later TI calculators.

This SR-51 Clear-Case Prototype was most likely manufactured in November 1974 and is a strange hybrid sporting:

A rather high serial number SR-51 0045499
A very early SCOM chip manufactured in October 1974
A keyboard with the bold lettering of an early SR-51 but the slanted line striking through the EE text of the later SR-51

The SR-51 was the first calculator in TI's line using stacked Integrated Circuits (Piggy Back ICs). Please note on the right picture the two SCOM (Scanning and Read Only Memory) Chips.

The SR-51 was introduced in January 1975, nevertheless the firmware of the calculator was released already three month earlier. Please notice the Date code 7444 printed on the TMC0523 SCOM Chip storing part of the program of the calculator.

Recently we discovered an SR-51E (Engineering Sample) with a Date code 7434 on its TMC0523 SCOM Chip, adding another 10 weeks between software release and market introduction...

Interesting to know, this calculator almost works with the PC-100 "Printer Cradle" developed for the SR-52 and SR-56. Learn more about Printing with the SR-51 / SR-51A.

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If you have additions to the above article please email:

Joerg Woerner, November 18, 2018. No reprints without written permission.