by Thomas A Hender
Objective: design a "shirt-pocket" calculator package for minimum factory cost, with reliability equal to or better than that of the HP-35 family. HP quality standards must be maintained.
Shirt-pocket size was achieved by reducing the number of keys from 35 to 30 (one less horizontal row) and by spacing the keys closer together. Spacing is the minimum deemed comfortable for the majority of users. Also, the display was reduced from 15 to 12 digits, and decimal points share positions with their digits.
The HP-21 uses only two rechargeable size AA Nl-Cad batteries instead of the three required in the HP-35 family. This feature saves almost thirty grams of weight. The total weight of the HP-21 is 165 grams. Apart from the obvious weight saving accompanying its smaller size, the HP-21 package contains fewer parts: no backbone support, no key-spacing grid and no display window welding frame. Structural rigidity is designed into the monocoque or box shape of the battery compartment in the bottom case, and in the heat-staked egg-crate configuration of the top case and keyswitch printed circuit assembly (plastic posts on the top case fit through holes in the keyswitch printed circuit assembly; heat is then applied to deform the ends of the posts and rivet the two parts together).
Lower production costs of this package are mainly due to minimal assembly time, including testing. Only two screws fasten the HP-21 together—a reduction of ten from the HP-35. The display is an integral plug-in assembly. Modular construction eases handling and any necessary touch-up operations; for instance, there are no electronic components or soldering on the keyswitch printed circuit assembly.
The battery pack case doubles as part of the calculator's bottom outside surface, eliminating a separate battery-retaining panel. The battery jumper spring provides the force that holds the pack in the calculator, so latches are not needed. Battery terminals are automatically assembled into the logic board during fabrication. This feature eliminates the manual wiring and terminal fastening required in the previous generation's design. The ac terminal pins are mounted similarly. Electrical integrity is provided by a flow-soldering operation which connects all electronic components to the logic board. All keys except the blue prefix key are molded in two clusters, which are mechanically separated during loading into the keyboard bezel. This reduces the number of parts handled from thirty to three and minimizes assembly operator errors and fatigue. That this innovation works is largely because of the creative efforts of the plastic mold designers and craftsmen of the HP Manufacturing Division, whose continuing high standards of excellence contributed much to the success of the HP-21. The over-center breakaway tactile feel of our former calculator keys has been retained, and the molded design of the key-strip actuating surfaces on the undersides of the keys eliminates the control bumps needed on earlier models.
Acknowledgments
I want to express my thanks to Tom Holden and Craig San-ford for their fine assistance in the design and documentation of this package and to Denny Thompson and his group and to Bill Boiler of the HP Manufacturing Division for seeing to it that necessary things were done on time. Finally, recognition is due to the cooperative people in production and to Gabe Bonilla and Cliff Planer of the model shop for their painstaking efforts, particularly during the concept phase of the project.
Fig. 2. Arithmetic, control, and timing chip.
ply and recharger, Ed Liljenwall for the industrial design, and Ernst Erni and Chung Tung for their support throughout the project.E
Fig. 2. Arithmetic, control, and timing chip.
Richard E. Whicker
Rich Whicker, project leader for the HP-21 series, graduated from the University of Illinois in 1966 with a BSEE degree. For the next six years he did MOS logic design for a semiconductor company, then continued in that specialty after joining HP in 1972. Born in San Francisco, Rich now lives in Santa Clara, California. He's married and has three children, a daughter and two sons. For a change of pace from the job Rich plays piano, builds radio controlled models and, like a bus ' who goes for a drive on his holiday, works out elec-ideas of his own.
Cathode Driver
In designing the cathode driver circuit for the HP-21 calculator, the main objective was to have a circuit with extremely low power consumption, a prime requirement for the HP-21 since only a two-cell battery pack is used. Functionally, the custom designed bipolar driver chip consists of a 12-bit shift register, twelve cathode drivers each with a current limiting feature, low-battery detection circuit, input buffer, and timing control gating. The 12-bit shift register turns on the twelve cathode drivers one at a time.
The LED digit display drive technique used in the HP-21 is different from that employed in the HP-35. In both cases the segment drivers (anode) and the digit drivers (cathode) are scanned one digit at a time, one segment at a time, but the HP-21 does it by switching dc voltages while the HP-35 uses an inductive charge-discharge method. When they are on, the anode drivers of the HP-21 are dc sources for the individual LED segments, while the anode drivers for the HP-35 drive each LED segment indirectly by first charging an inductor which then discharges through the LED segment. The HP-21 method requires significantly fewer components.
Acknowledgments
The authors would like to thank the following people for their contributions to this project: Bosco Wong for the design of the cathode driver chip, Les Moore for the assembly and debugging of the breadboard, Mark Linsky for the design of the power sup-
Michael J. Cook
■B^^ff""»»-*. chip for the HP-21 series. He 4L| ! 1 joined HP in 1973 with an exten-m ...i1!m s've background in the design of fc^nji —m MOS LSI circuits. Born in Watford, Aft* ~ Hertfordshire, England, Mike earned his BSc and MSc degrees • *•'■ in electrical engineering at the
.jJ University of Southhampton in
1963 and 1966, then came to the United States to work for an aircraft company as a systems designer. Later he joined a semiconductor company, designing more than 50 MOS LSI circuits and serving briefly as MOS applications and marketing manager for that company in Germany. He speaks German and French as well as English and is a student of comparative linguistics. Mike is married, has three daughters, and lives in Cupertino, California. His interests include classical music, color printing, and sketching.
Correction
The article entitled "Active Probes Improve Precision of Time Interval Measurements" (Hewlett-Packard Journal, October 1975) understated the accuracy achieved by the Model 1722A Oscilloscope In time interval measurements. The accuracy of the Model f 722A for main time base settings between 100 ns/divand 20 ms/div is specified conservatively as ±0.5% of measurement ±0.02% of full scale for measurements less than 1 cm, and ±0.5% of measurement ±0.05% of full scale for measurements greater than 1 cm. Typical measurement accuracies are more than three times better than this. The time base calibration period has not been specified because it has not been a significant contributor to inaccuracy. Experience shows that yearly calibration may be sufficient for instruments maintained in a laboratory environment. The time base temperature coefficient is specified as ±0.03%/°C and short term stability is better than 0.01%.
FEATURES AND SPECIFICATIONS
HP-21 Scientific Calculator
PREPROGRAMMED FUNCTIONS
ARITHMETIC: +, -, x, * LOGARITHMIC: ex, In x, log x, 10x
TRIGONOMETRIC: sin x, arc sin x, cos x, arc cos x. Ian x, arc tan x OTHER: y , Vx7 1/x, w, rectangular/polar coordinate conversion, degrees-
radians mode selection REGISTER ARITHMETIC: addition, subtraction, multiplication, or division operations can be performed on data in storage register. NUMERIC NOTATION FLOATING POINT: 10 digit mantissa and sign
SCIENTIFIC: A sign and integer followed by up to seven possible decimal places. The exponent consists of a sign and two digits. MIXED FLOATING POINT AND SCIENTIFIC: Mixed numeric notation may be entered as data. After performance of any operation data reverts to floating point or scientific notation as applicable. ROUNDING TO LAST DISPLAYED DIGIT: Internal operations are calculated to within 10 digits. DISPLAY
NUMERIC AND DECIMAL POINT: Eight-segment, light-emitting diode (LED). Digit and decimal point are contained within a single eight-segment LED digit.
SIGN: Eight-segment light-emitting diode 12-digit display including two sign digits MAXIMUM DISPLAY NUMBER: ±9.9999999 x 10±99 DISPLAY FORMAT: Fixed notation and scientific notation as specified SPECIAL INDICATIONS: Overflow: All nines (9.9999999 99)
Underflow: Zero in scientific notation. In fixed notation automatically reverts to scientific notation for small numbers that would otherwise appear to be zero. Low Battery: Inverted decimal points for 30 seconds to Vi hour be fore display blanks. Improper Operation: "Error" written on display. DYNAMIC RANGE: 9.999999999 x 10" to 1. x 10~" and 0. NUMBER OF KEYS: 30, 1 on/off switch, 1 degree/radian switch MEMORY REGISTERS: six total Four working registers in an operational stack One storage register
One hidden register for trigonometric function computation DATA ENTRY Exponent entry Negative number entry (CHS) PACKAGING
High-impact, contoured beige plastic (ABS) calculator case All solid state electronics Light-emitting diode (LED) display SPEED: a one second maximum for all preprogrammed functions (200 kHz clock speed) POWER
RECHARGERS: European, 103-127 and 206-254 Vac 50-60 Hz; U.K. Desktop, 206-254 Vac, 50-60 Hz; United States. 90-127 and 180-254 Vac. 50-60 Hz. 5 watts, plastic box. Recharger warm to the touch in normal operation BATTERY: 350 mW derived from 2-cell quick recharge nickel-cadmium battery pack. Operating time 3 to 5 hours. Approximately 6 hours to recharge completely discharged battery pack when calculator is not in operation. Approximately 17 hours to recharge completely discharged battery pack when calculator is operating under maximum load (all 8's displayed). Battery pack must be in place for calculator to operate. WEIGHT
CALCULATOR WITH BATTERY PACK: 6 oz (170 grams) RECHARGER: 5 oz (142 grams) SHIPPING WEIGHT: 1V2 lb (682 grams), approximately. CALCULATOR DIMENSIONS LENGTH: 5'/e in (13.02 cm) WIDTH: 2 11/16 in (6.83 cm) HEIGHT: 1 3/16 in (3.02 cm) PRICE IN U.S.A.: $100.
HP-22 Business Calculator
FINANCIAL FUNCTIONS:
n Number of periods i Periodic interest rate PMT Periodic payment amount PV Present value of money FV Future value of money
12x Converts yearly periods to monthly periods
12 + Converts annual interest to interest rate per month
ACC Computes accumulated interest between any two time periods of a loan
INT Calculates simple interest
BAL Gives remaining loan balance at any point in time
%S Percent one number is of total
% Calculate percentage of a number a% Percent of difference between two numbers
BEGIN-END SWITCH: This switch is a special convenience which works in conjunction with the financial keys in calculating payments due at the beginning or end of the period for annuities, leases, loans and other transactions. STATISTICAL FUNCTIONS: 2+ Provides the number of entries and sums two variables £ - Adjusts data, corrects an incorrect 1 + entry L.R. Linear regression; linear functions between two points J Linear estimate x Mean or arithmetic average s Standard deviation MATHEMATICAL FUNCTIONS: In Computes natural logarithm (base e) of value in display ex Natural antilog; raises e to value in display yx Raises number in Y register to value in display Vx~ Square root of number in display ARITHMETICAL FUNCTIONS: Subtract + Add x Multiply + Divide
CHS Changes a positive number to a negative number MEMORY REGISTERS: 10 separate addressable memories with full register arithmetic 5 financial registers
4 operational stack registers with stack roll-down for review DATA MANIPULATIONS; DISPLAY CONTROL; STORAGE FUNCTIONS:
x=y Exchange contents of the X and Y registers RJ Rolls down the stack to review contents
CLX Clears display
CLEAR Clears display, stack and storage registers: resets financial status indication
RESET Resets financial status indicators and clears only statistical data ENTERT Copies number displayed in X register into Y register; also separates numerical entries by moving entries up in operational stack RCL Recalls a number to the X register from a storage register STO Stores displayed value into one of the 10 storage registers Gold shift key; selects functions printed in gold on keyboard
DISPLAY:
10 significant digits (8 + 2 digit exponent displayed in scientific notation). Fixed decimal notation with automatic overflow and underflow into scientific notation
Scientific notation with dynamic range of 10""®® to 10®®
Automatic decimal point positioning and selective round-off
Indicators for improper operations (Error in display) and low battery condition
(lighted decimal points) Light-emitting diode (LED) display recessed for better contrast in harsh lighting DESIGN SPECIFICATIONS: Operates 3 to 5 hours on rechargeable batteries (under 6 hours to recharge)
Specially designed recessed plug to prevent erroneous insertion of improper
Solid state electronics with all critical connections gold-plated
Tactile feedback keyboard. Positive contact action assures accurate entry of
Heavy gauge compact case contoured to fit the hand Ultrasonically welded impact resistant case
Plastic liquid-barrier shield under keyboard sealed to resist entry of moisture Keys are double injection molded to help prevent the legend from wearing off. PHYSICAL SPECIFICATIONS: CALCULATOR LENGTH: 5% in (13.02 cm) CALCULATOR WIDTH: 2 11/16 in (6,83 cm) CALCULATOR HEIGHT: 1 3/16 in (3.02 cm) CALCULATOR WEIGHT: 6 oz (170,1 g) RECHARGER WEIGHT: 5 oz (141,8 g) SHIPPING WEIGHT: approx 1V2 lb (680 g)
OPERATING TEMPERATURE RANGE: 32°F to 113°F (0°C to 45°C) CHARGING TEMPERATURE RANGE: 59°F to 104°F (15°C to 40°C) STORAGE TEMPERATURE RANGE: -40°F to 131°F (40°C to 553C) POWER REQUIREMENTS: AC: 100-127 V or 200-254 V. ±10%, 50 to 60 Hz. 5 watts • BATTERY: 2.75 Vdc nickel-cadmium rechargeable battery pack PRICE IN U.S.A.: $165
HP-25 Programmable Calculator
PROGRAMMING:
Program writing capability Single step execution or inspection of a program Pause (to display intermediate result) Program editing capability
8 relational tests: x<y, x=y, x^y, x<0, x»0, x=0, x^0 Conditional branching Direct branching KEYBOARD COMMANDS: TRIGONOMETRIC FUNCTIONS: 3 angular modes (degrees, radians, grads) Sin x
Cos x Arc cos x Tan x Arc tan x
Rectangular coordinates ~ polar coordinates Decimal angle (time) Angle in degrees (hours)/minutes/seconds LOGARITHMIC FUNCTIONS: Log x Ln x
STATISTICAL FUNCTIONS: Mean and standard deviation
Positive and negative summation giving n, Xx, Ix2, Sy, 2xy OTHER FUNCTIONS: Integer (gives only integer portion of number) Fraction (gives only fractional portion of number) Absolute (gives absolute value of x)
Register arithmetic in all 8 addressable registers
Addition, subtraction, multiplication or division in serial, mixed serial, chain or mixed chain calculations. DATA STORAGE AND POSITIONING OPERATIONS: Data entry Stack roll down x.y interchange Data storage Data recall Change sign Enter exponent MEMORY: 4-register stack "Last x" register 8 addressable registers Program memory for storage of up to 49 steps LIGHT-EMITTING DIODE DISPLAY: Displays up to 10 significant digits, 8 plus two-digit exponent in scientific and engineering notation, and appropriate signs. Three selectable display modes: fixed point (with automatic overflow and underflow into scientific), engineering, and scientific, with dynamic range of 10®® to 10-®®. Automatic decimal point positioning. Selective round-off; range 0-10 digits in fixed point; 0-8 digits in scientific; 0-8 in engineering notation. "Error" appearing in display indicates improper operation; low battery indicator. GENERAL SPECIFICATIONS: Operates on fast-charge battery pack or ac. (Battery recharges in 6-17 hours.) Tactile feedback keyboard. Polyethelene liquid-barrier shield under keyboard. Compact case of high-impact plastic with recessed display. Recessed recharger/ac plug receptacle. Solid state electronics. OPERATING SPECIFICATIONS: POWER: AC: 115 or 230 V, ±10%, 50 to 60 Hz, 5 watti Battery: 500 mwderived from nickel-cadmium rechargeable battery pack. WEIGHT: HP-25: 6 oz (170 g) with battery pack. Recharger: 5 oz (142 g). SHIPPING WEIGHT: Approx 1.5 lbs (.7 kg). DIMENSIONS: LENGTH: 5.1 inches (13.0 cm) WIDTH: 2.7 inches (6.8 cm) HEIGHT: 1.2 inches (3.0 cm) TEMPERATURE RANGE: OPERATING: 32°F to 113°F (0°C to 45°C) CHARGING: 59° to 104°F (15°C to 40°C) PRICE IN U.S.A.: $195
MANUFACTURING DIVISION: ADVANCED PRODUCTS DIVISION 19310 Pruneridge Avenue Cupertino. California 95104 U.S.A.
Thomas A. Hender
Tom Hender was responsible for the product design and packaging of the HP-21 series. Born in Cobourg, Ontario, Canada, he served in the Royal (British) Navy during the second world war, then attended British Admiralty College (Devonport Division), graduating in 1947 with a BSc degree in mechanical engineering. His engineering career includes work on punched-card machines, line printers, point of sale terminals, and related peripheral mechan..............isms. He joined HP in 1973. Tom is married, has three daughters, and lives in San Jose, California. He serves his church as choir director and enjoys photography, chess, and model railroading.
George M. Fichter
George Fichter designed the read-only memory for the HP-21 series. A native of New York City (Brooklyn), he graduated from Stevens Institute of Technology with a BS degree in 1965, spent six years as a U.S. Air Force meteorologist; and then returned to school at the University of Washington, earning a BSEE degree in 1972 and an MS in computer science in 1973. He joined HP in 1973. George is married and has a son and a daughter. An accomplished musician, he plays French horn and experiments with computer music using an HP 2100 Computer. He's also learning to fly and hopes to get his private pilot's license this year. The Fichters live in Los Altos, California.
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