HP-91

The HP-91 was HP's first battery powered printing calculator. The printer was a moving head thermal design that printed from about 1 to 1.5 lines per second. This relatively low speed was designed to conserve battery power. Thermal printing was chosen because it was small, light an quiet. (Compare this printer to the HP-46!)

Printer Functions

The printer was controlled by a three-position sliding switch. The positions were:

• Man: The printer remained idle except when the user invoked a print function.
• All: The printer printed each number entered, each function and each result.
• Norm: Like All, but the printer didn't print each result. Useful when doing long chain calculations.

In addition, there were a number of keys dedicated to printing:

• Print x: Printed the X register.
• List SUM: Printed the summation registers.
• List Stack: Printed the four level stack.
• List Reg: Printed all 16 registers.

Other Features

The HP-91's, mathematical capabilities were very much like those of the HP-45 with the addition of:

• Six more registers (R.0-R.5)
• H.MS+ and H.MS- (hours.minutesseconds arithmetic)
• Linear Regression and Estimate
• Additional clear keys (all registers or summation registers)
• Engineering display (exponents in increments of 3 only)
• R↑ to complement the HP-45's R↓
• An additional percent function

The HP-91 was based largely on 20 Series technology. In addition, it added LED drivers for the larger display, and a new Printer Interface Control & Keyboard buffer circuit (PICK). Unlike the handhelds, the desktop machines used a light feeling keyboard well suited for rapid data entry on the desktop.

Front view (~72K)
Three quarter view (~91K)

Manual Available

HP-91 features

Technology and Packaging

Dimensions and Weight
Length 9"
Width 8"
Height 2.5"
Weight 2.5lb

Price: $425

Introduction-Discontinuation: 1976-1979

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