Retro Review: Hewlett Packard HP 20S and 21S
Company: Hewlett Packard
Type: Scientific ProgrammingMemory: 9 Registers, 99 Programing Steps, 6 Preloaded programs
Years: 1988? – 1993? (21S); 1988 – 2002(?) (20S), original price was around $50
Operating System: Algebraic
Batteries: 3 x LR44
HP 21S: The Rarer Cousin of
the HP 20S
The HP 21S is
an algebraic, keystroke programming calculator.
I paid $35, which is not bad considering the keyboard and the display
are in supreme quality, the keys are a pleasure to touch, and the learning
curve is easy to operate the calculator.
The HP 21S was originally released by Hewlett Packard along with the more familiar cousin the HP 20S. Both had a great dark brown keyboard with white font for primary labels, orange and blue for shift key fonts (my favorite!). This design has been used several Hewlett Packard calculators from the late 1980s/early 1990s, including the HP 32SII and the HP 48 SX. Sometime during the 1990s, the HP 20S garnered a purple/green shift font scheme, to match the HP 48 G series.
What the 20S and 21S Have in Common
Both are
algebraic programming calculators. Each
calculator has 10 memory registers (R0 through R9) with storage arithmetic
available (STO+, STO, STO*, and STO÷).
Both models have the standard array of scientific functions including trigonometric,
logarithmic, exponential, combination, permutation, integer and fraction parts,
and decimal/hoursminutesseconds conversions.
However, there is no fraction mode and all numbers are real
numbers.
One thing is to
consider is that the factorial function (n!) only accepts positive integers.
The INPUT Key
Rectangular to
Polar Conversion: x, [INPUT], y, [left
shift], [STO] (>P)
(θ is
displayed, r is stored)
Polar to
Rectangular Conversion: r, [INPUT], θ, [right
shift], [STO] (>R)
(y is
displayed, x is stored)
Combinations/Permutations: n, [INPUT], r, [right shift], [ 0 ]
(combination) or [ . ] (permutation)
Percent Change:
HP 20S: old, [INPUT], new, [left shift], [ 1/x ]
(%CHG)HP 21S: old, [INPUT], new, [right shift], [ 5 ] (%CHG)
Statistics
Both the HP 20S
and HP 21S have one variable, one weighted variable, two variable statistics,
and linear regression. There is no
separate mode to be entered, just enter data with the [ Σ+ ] and go. Upon entering and clearing data though, the
following registers become holders of statistical sums:
R4 = n
R5 = Σx
R6 = Σy
R7 = Σx^2
R8 = Σy^2
R9 = Σxy
What is really
nice is that both models have the sums indicated in gray at the bottom right
hand corner of the keys as reminders.
Programming
Both the HP 20S
and the HP 21S are keystroke programmable.
Both allow for labels A – F, 0 – 9.
In addition there are two tests:
x=0? and x≤y?.
The x≤y?
Test: x, [INPUT]*, y, [right shift], [ 7
] (x≤y?). A way to remember this
command is: hidden value ≤ displayed
valued?
* According to the manuals, any other
arithmetic/pending operation can be used such as [ + ] and [ ÷ ] are also
allowed.
Each digit
takes a step. For example, 250 takes
three steps, one for the 2, the 5, and the 0.
Differences between the HP 20S and HP 21S
Scientific Operations
HP 20S

HP 21S

Hyperbolic functions,
base conversions (binary, octal, hexadecimal, binary), metric/US conversions
(kg/lb, °C/°F, cm/in, l/gal)

Upper tail
areas and inverse (z, t, F, Chisquared), random numbers, seeding

Preloaded Programs
The HP 20S and
HP 21S have sixpreloaded programs. The
can be loaded by the key sequence [left shift], [ ← ] (LOAD). While the HP 21S programs can be loaded at
any time, the HP 20S must be in programming mode to load
programs.
Unfortunately,
usercreated programs cannot be stored into permanent memory which would have
been nice.
Preloaded
Programs HP 20S
A

Root Finder

62 steps

B

Numerical
Integration

58 steps

C

Complex
Arithmetic

75 steps

D

3 x 3
Matrices

98 steps

E

Quadratic
Equation

65 steps

F

Curve Fitting
(logarithmic, exponential, power)

77 steps

Preloaded
Programs HP 21S
A

One Sample
Stat Tests

82 steps

B

Two Sample
Stat Tests

82 steps

C

Linear
Regression Stat Tests (used with One Sample Tests)

86 steps

D

Chi Square
Tests

53 steps

E

Binomial
Distribution

39 steps

F

Time Value of
Money Solver (Finance)

99 steps

Final Verdict
Do I recommend
buying the HP 21S and/or the HP 20S? Assuming
the price is reasonable, Yes, Yes, and Yes! It is shame that the HP 20S and HP 21S are no
longer produced because these are nice calculators.
The original
price was around $45  $55.
Let’s get to
the features. I think it would be
appropriate comparing the features of both the HP 21S and HP 20S at the same
time.
Eddie
This blog is
property of Edward Shore, 2017
Hi, Eddie, thanks for your blog entries. I have a 20S and I really like it, but unfortunately it has a serious data input bug. Check it on HPMuseum, if interested: http://www.hpmuseum.org/forum/thread3658.html
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