RPN-55 — Extending the HP-55

What's been added in RPN-55

Selling for the same price ($395) as the HP-45 two years earlier, the HP-55 offered more registers, more conversions, and programmability. In addition, it featured the timer already implemented in the HP-45, but not made officially available to the user due to its pure accuracy and stability. The HP-55 solved this by adding a quartz-controlled oscillator.

RPN-55 SD enhances the HP-55 in many ways:



Register Arithmetic

For STO operations on primary registers only

For both STO and RCL operations on all 20 registers, including the double-register ∑+ (R.1 and R.3) working with stack registers X and Y.

Program and all registers lost on power-off

Program and all registers preserved ("continuous memory")
Range 100 hours 12, 24, 100 hours
CLx key Resets timer to 0 100 hr: Resets timer to 0
12/24 hr: Sets timer to actual time
r Not available Correlation coefficient
Not available x-estimate for given y
ENG Not available SCI mode with exponent fixed to multiples of 3
ABS Not available Make X value positive
INT Not available Integer part of X
FRC Not available Remove integer part of X
RND Not available Round X to displayed value
RAND Not available Create fractional or integer random number, as defined in Settings
P≷S Not available Swap primary and secondary registers
FUNCTIONS (with setting Replace Unit Conversions set to ON)
sinh, sinh-1 Not available Hyperbolic sine and inverse
cosh, cosh-1 Not available Hyperbolic cosine and inverse
tanh, tanh-1 Not available Hyperbolic tangent and inverse
LOGy Not available Base y logarithm of x
LN x! Not available Logarithm of factorial of x
∆% Not available Percent difference from y to x
%∑ Not available x as percentage of register ∑x
PUSH Not available Saves the entire stack on a 4-level stack
POP Not available Replaces the stack with the last PUSHed values (0 if none)
UNITS Not available Temporarily reveals the unit conversions until one of them has been executed. This also works as part of a program.
PSE Not available Pauses program execution for one second.
Capabilities 49 steps
Editing by replacing steps
99 steps
Insert and delete steps with automatic address correction
Clear Program Not available Yes, with user confirmation
Clear Program Range Not available Clear any block of steps with automatic address correction
Delete f or g key Cumbersome Yes, by entering f or g again
Prefix keys Active if pressed Active if pressed. Deactivated if pressed again. Indicators show state.
Register view Not available All registers shown on single screen including formatted display.
Run/Stop and single-step program without leaving register view.
Store and recall split times by tapping split number.
Decimal Point,
Thousands separator

Fixed to decimal point (DP).
Thousands separator not available

User-selectable, DP or comma.
Thousands separator optional
Angular mode indicator

Not available

When not DEG, mode is shown in the top-left corner of the display.
Command display

Not available

In program mode or while single-stepping, current command is shown as text below display
Current step number display

Not available

Current program step number is optionally shown below display
Program memory fill status

Not available

Red progress bar shows amount of program memory filled
Low battery

Lights all decimal points

Lights all decimal points in vintage mode.
Lights a small red indicator in extended mode.
(When iPhone charge < 10%.)

Switching to timer mode with x = 43.170992:

1. Vintage mode (100-hr only)

2. Extended mode 100-hr:

3. Extended mode 24-hr:

4. Extended mode 12-hr:

      Importing Numeric Data
      You can preset RPN55's registers with data created externally without having to type them in.
      Simply prepare the data in text format in any app that can handle text, like Notes, Mail and many others.
      The data format looks like this:
      Rn or R.n value (where n = 0…9)
To set R1 = 4.5, R6 = 6.28E-7, R.3 = 439, prepare your data like this:
      R1 4.5
R6 6.28e-7
R.3 439
      Select all of the text and paste.
Upper-/lower-case is ignored.
One or more blanks or tabs may follow the register number.
M may be used in place of R.
R.n may also be written as R1n, e.g. R.3 is the same as R13.
The order of the registers is irrelevant.
Unlisted registers are left untouched.

      Extra Functions
      Calculates the correlation coefficient for a linear regression.
Note that the method shown on page 61 of the Owner's Handbook calculates coefficient of determination, which is the square of r.
      Sets x to the positive value of x.
      Sets the fractional part of x to 0.
      Sets the integer part of x to 0.
      Rounds x to the value currently shown on the display.
      • If the RAND setting is Off:
Creates a random number between 0 (incl.) and 1 (excl.).
• If the RAND setting is On:
x ≤ 0: creates an integer random number in the range 0…232–1.
x > 0: creates an integer random number less than x, but no larger than 232–1.
      Swaps registers R0…R9 with registers R.0…R.9.
      LN x!
      Calculates logarithm of factorial.
This function allows handling factorials of very large numbers, as may occur in statistical computations. By using the logarithm of these factorials, multiplication and division are reduced to addition and subtraction.
On return:
x = LN x!
y = mantissa of x!
z = exponent of x!
(assuming x! = y·10z)
z+1 = number of digits of x!
x may be any positive number up to
4.4673259 × 1097.
      Calculates the percent difference between y and x. The stack doesn't drop.
      Calculates the percentage x is of the value in the ∑x register (R.1)
      Copies all four registers of the stack to an internal stack of stacks. Four levels are available.
      Retrieves the latest PUSHed copy of the stack. The current x value is saved in LastX. The oldest copy of the stack is copied down on the internal stack.
NOTE that CL.R clears the stack of stacks, while CL R does not. You can use P≷S  CL R  P≷S  to clear the secondary registers without affecting the stack of stacks.
Calculate the base 2 logarithm of 5 inches converted to millimeters.

g in➞mm
f LOGy      
Result: 6.99

This sequence also works as part of a program.
      Executes a one-second pause. A green indicator lights up while the pause is active.

      When you switch to timer mode, the current value of x is taken as the starting time. If the value is an invalid time, 0 will be shown. Invalid starting times are negative numbers, or times equal to or larger than 100 hours (or 24 hours), or more than 59 minutes in the minutes field, or more than 59.9999 seconds in the seconds field.

When leaving timer mode, the actual mode switching doesn't occur until the timer has been stopped by R/S.
      Press key to start and stop the timer. This also works in register view.
      100-hours mode: resets the time to zero.
12 and 24 hours: sets the time to the current real time.
      Shows or hides the hundredths digits.
      Split Times
      Pressing any digit key 0…9 while the timer is running stores the current time in the corresponding register.
Pressing any digit key 0…9 with the timer stopped retrieves the stored time from the corresponding register.

In register view, you can achieve the same by pressing any split time.
Non-zero values are highlighted in extended mode.

Split times are still available after leaving timer mode.
      • If the RAND setting is Off:
Creates a random number between 0 (incl.) and 1 (excl.).
• If the RAND setting is On:
x ≤ 0: creates an integer random number in the range 0…2^32–1.
x > 0: creates an integer random number less than x, but no larger than 2^32–1.
      Swaps registers R0…R9 with registers R.0…R.9.

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