Features in optional ROMs are listed as Option. 

Display and Keyboard  HP28S 
Display type  LCD 
Display format  4Line 
FIXed number format  Yes 
SCIentific number format  Yes 
ENGineering number format  Yes 
STanDard / ALL number format  Yes 
Alpha messages and prompts  Yes 
Softkeys labeled by mag. card or overlay   
Softkeys labeled by display menus  Yes 
Softkeys with neither labeling system   
Status annunciators  Yes 
Redefinable keyboard   
Logic  HP28S 
Logic system  RPN/RPL 
Stack levels  Unlim 
Last x  Yes 
Undo  Yes 
Precedence  Yes 
Parentheses  Yes 
Storage Addition (STO +)  Yes 
Storage Arithmetic (STO +, , ×, ÷)  Yes 
Storage Exponentiation (STO ^)   
Recall Arithmetic   
Named variables  Yes 
Multiple data types  Yes 
Directories  Yes 
Math  HP28S 
+, , ×, ÷  Yes 
1/x  Yes 
sq. root  Yes 
x^{2}  Yes 
LOG x  Yes 
10^{x}  Yes 
LN x  Yes 
e^{x}  Yes 
y^{x}  Yes 
pi  Yes 
%  Yes 
% change  Yes 
INTeger truncation and ABSolute value  Yes 
Trigonometric functions (SIN, COS etc.)  Yes 
Hyperbolic trig. (SINH, COSH etc.)  Yes 
# of trig. modes (degrees, radians, grads)  2 
Advanced Math  HP28S 
Solver / root finder  Yes 
Numeric integration  Yes 
Complex numbers  Yes 
Matrix operations  Yes 
Matrix writer   
Equation writer   
Symbolic operations  Yes 
Fractions   
Plotting and graphics (display)  Yes 
Plotting and graphics (printer / plotter)  Option 
Statistical  HP28S 
Mean, standard deviation (# of variables)  Multiple 
Summation of n, x, x^{2}   
Summation of n, x, x^{2}, y   
Summation of n, x, x^{2}, y, xy   
Summation of n, x, x^{2}, y, y^{2}, xy   
Summation of lists  Yes 
Random number generator  Yes 
Linear regression, estimate  Yes 
Curve fitting   
Correlation coefficient  Yes 
Normal distribution  Yes 
Factorial  Yes 
Gamma  Yes 
Combinations and permutations  Yes 
Scientific and Time  
Polar / rectangular conversions  Yes 
Base conversions (2, 8, 10, 16)  Yes 
Arithmetic in bases 2, 8 and 16  Yes 
Bit operations (AND, OR, XOR, ...)  Yes 
Selectable word size (164 bits)  Yes 
Selectable complement mode (1's, 2's, unsigned)   
Decimal hrs / hrs.min.sec. conversions  Yes 
Hrs.min.sec arithmetic  Yes 
Time of day clock   
Timer   
Dynamic calculations (timer × rate)   
Alarms   
Unit Conversions  
Centimeters / inches  Yes 
Millimeters / inches  Yes 
Meters / feet  Yes 
Kilograms / pounds (mass)  Yes 
Newtons / pounds (force)  Yes 
British thermal units / joules  Yes 
Liters / gallons  Yes 
° Celsius / ° Fahrenheit  Yes 
Other  Yes 
Userdefined units  Yes 
Time Value of Money  HP28S 
Number of payments (n)   
Interest rate (i)   
Present Value (PV)   
Payment (PMT)   
Future Value (FV)   
Cash Flow Analysis  HP28S 
Net present value (NPV)   
Internal rate of return (IRR)   
Amortization  HP28S 
Accumulated Interest (ACC INT)   
Balance (BAL)   
Depreciation  HP28S 
Straight line (SL)   
Sum of the years digits (SOYD)   
Declining balance (DB)   
Bonds and calendar functions  HP28S 
Bond price   
Bond yield   
Date arithmetic / calendar functions   
Memory and Programming  HP28S 
Programming language(s)  RPL 
RAM / Max # of variables or addressable registers  32K Bytes 
Max # of program steps  Unlim 
RAM / Max vars or regs with full optional RAM   
Max # of prog steps with full optional RAM   
Merged program steps  Yes 
Manual register / program memory allocation   
Automatic register / program memory allocation  Yes 
Overlapped register and program memory   
Continuous memory  Yes 
Flags  64 
Conditional tests  Many 
Subroutine levels  Unlim 
Indexed looping (increment by +/ 1, end at 0)  Yes 
Indexed looping (choice of increment and end)  Yes 
Multiple index / loop registers  Yes 
Line number addressing   
Branching by relative line number   
Global program label addressing  Yes 
Local program label addressing  Yes 
Indirect / computed addressing and control  Yes 
Multiple indirection registers  Yes 
Pause  Yes 
Alpha messages and prompts in programs  Yes 
Alpha program listings  Option 
Alpha string manipulation  Yes 
Program step insert / delete  Yes 
I/O and Expansion Features  HP28S 
ROM modules   
RAM modules or expansion option   
General expansion slots / connector   
Magnetic card reader/writer   
Tape drive   
Disk drive   
Upgradeable firmware   
Optical wand   
Serial port   
Infrared input   
Infrared output  Yes 
Printer  Option 
Networking   
Other   
General  HP28S 
Sounds  Yes 
Self test  Yes 
Hard case (in addition to soft case)   
Additional Features   
Batteries  3 N cells 
Length (inches)  6.2" 
Width  3.75" 
Height  0.7" 
Weight (oz or lbs)  8oz 
Year of introduction  1987 
Features in optional ROMs are listed as Option. 
HP28S notes:

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