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Texas Instruments SR-51 (First Design)

Date of introduction:	January 1975	Display technology:	LED modules + lens
New price:	$224.95, DM 748.00	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:	0010563
Batteries:	BP1	Date of manufacture:	wk 05 year 1975
AC-Adapter:	AC9200	Origin of manufacture:	USA
Precision:	13	Integrated circuits:	TMC0501, TMC0522, TMC0523
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.

To reduce manufacturing costs and to give a similar appearance to the SR-52 and SR-56 calculators the SR-51 was replaced within a few months with the SR-51A. Logically it's quite rare.

Nevertheless there are two different variants known: This model from an early production lot uses huge yellow characters for the shifted 2^nd-functions while later SR-51 sports a much finer printing. (Kirk B. Muri provided the picture on the right. Thanks!)

Update: Fellow calculator collector Christine B. reported an SR-51 (Version 2) manufactured in wk 05 year 1975 like the featured SR-51 (Version 1) with serial number 0011312. We can safely assume now that the shift to the finer printing happened during this week after only about 1,000 calculators manufactured. Makes the SR-51 (Version 1) rarer than the TI-150!
Update: And fellow calculator collector Bob Wolfson reported an SR-51 (version 1) manufactured in wk 05 year 1975 with serial number 0013251. Makes us confused and we assume that either Christine B.s calculator was repaired or TI mixed up parts during production.

Don't miss the rare SR-51 Clear-Case Prototype and a very early SR-51E, also known as SR-51 (Engineering Sample).

Fellow collector Gary Snyder reported in September 2010 that on the earlier versions the cancel exponent entry label (above the [EE] key) has an overline over the EE text. On the later models the label has a slanted line striking through the text.

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 7442 printed on the TMC0523 SCOM (Scanning and Read Only Memory) Chip storing part of the program of the calculator.

Interesting to know, this calculator works sometimes with the PC-100 "Printer Cradle" developed for the SR-52 and SR-56. But you need to verify that your have the latest Main-PCB (printed circuit board) evolution in your SR-51. Learn more here.

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