TI SR-50

The SR-50 was Texas Instruments' first scientific pocket calculator with trigonometric and logarithm functions. It enhanced their earlier SR-10 and SR-11 calculators, introduced in 1973, which had featured scientific notation, squares, square root, and reciprocals, but had no trig or log functions. The SR-50 was introduced in 1974 and sold for US$170. It competed with the Hewlett-Packard HP-35.

The SR-50 measured 5-3/4 inches long by 3-1/8 inches wide by 1-3/16 inches high (147 mm by 78 mm by 31 mm) and was powered by a rechargeable NiCad battery pack, built from three soldered AA cells. It had 40 keys, and flat sliding switches for degrees/radians and on/off. "SR" stood for "slide rule."

The SR-50 had a red LED display with a signed ten-digit mantissa plus a signed two-digit exponent for floating point numbers (negative values were indicated with a leading minus sign and positive values with no sign). Internally, calculations were performed with a 13-digit mantissa, providing much greater calculation accuracy than the 10-digit precision of most scientific calculators of the time. After the leading sign, digits consisted of a seven-segment display plus decimal point. A blinking display indicated an error, such as a calculation error or an overflow or underflow condition.

Like most scientific calculators, the SR-50 used ordinary infix notation, as opposed to the postfix Reverse Polish Notation (RPN) employed by its competitor, the Hewlett Packard HP-35. The SR-50 followed the standard order of operations by performing unary (single-argument) operations (reciprocal, square, square root, log, trig and hyperbolic trig functions) immediately, and multiplication, division, root, and power operations before addition and subtraction operations. As an example, the keypresses to calculate "3 x log(4) + 5" was entered almost as written, namely "3 x 4 log + 5 =". This is because the calculator would execute the log function before performing the multiplication operation, and complete the multiplication operation before executing the addition operation. It did so by having unary operations operate on the X register, addition and subtraction operate on the X and Z registers, and multiplication, division, power, and root functions operate on the X and Y registers in its operational stack.

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