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HP 42S

The HP 42S is a programmable RPN scientific calculator that Hewlett-Packard sold from 1988 to the mid-1990s. It runs the programs of HP's earlier HP-41C and 41CV, and adds a two-line display with menus, complex numbers, matrices, a solver and numeric integration, all in a pocket-sized case.

HP 42S
The 42S as saturnus draws it, with π in the X register.

At a glance

Released31 October 1988, at $120
Discontinued1995 or 1996 (sources differ)
Code nameDavinci
ProcessorSaturn core in HP's Lewis chip, about 1 MHz
Memory8 KB RAM (over 7,200 bytes for you), 64 KB ROM
DisplayTwo lines of 22 characters, 131 × 16 pixels
Keyboard37 keys, one shift key; the top row doubles as the menu keys
Input and outputInfrared output to HP's 82240A printer, beeper
PowerThree button cells
Size148 × 80 × 15 mm
EntryRPN, a four-level stack

History

The HP-41's successor

The 42S, introduced in October 1988, was designed to run the HP-41C's programs. The Museum of HP Calculators sums it up: it lacked the HP-41C's expandability, but offered a two-line dot-matrix display with menus, more speed, a smaller size and a lower price.

The owner's manual promises that all programmable functions of the HP-41C and 41CV are built in, so their programs run on the 42S. Programs have to be keyed in by hand: there is no card reader and no way to load them from a computer. Not every extended function of the 41CX is there. A chapter of the manual explains the few differences, such as the larger display, the longer Alpha register and the greater number range, which can trip up an old program.

Wikipedia notes that, lacking the 41's expandability and input and output, the 42S was marketed as a replacement for the HP-15C rather than for the 41.

The Pioneer series

The 42S belongs to HP's Pioneer series, slim calculators built around Lewis, a custom chip with an improved Saturn processor core. The 27S, the 17B and later the 17BII share its case and board. Of these, the 42S is the RPN scientific and the one you can program.

HP sold it at $120 for its whole life. Finseth's calculator data gives March 1996 as the end of production; the Museum of HP Calculators and Wikipedia give 1995.

A bug that came back

A bug report from 1991, kept in Finseth's data: ask COMB or PERM for an impossible value and you correctly get "Invalid Data", but after swapping the arguments the error comes again. According to John W. Loux, early 42S calculators had the problem, a fix removed it, and a later update brought it back: it resurfaced in late revision B calculators and all of revision C. Any arithmetic operation clears it.

Free42 and the DM42

The 42S never had a direct successor. In 2004 Thomas Okken released Free42, a re-implementation of the 42S and its printer for phones and computers. It is a complete rewrite that uses no HP code and needs no ROM, and it runs 42S programs.

In 2017 the Swiss maker SwissMicros announced the DM42, a calculator modelled closely on the 42S whose firmware is based on Free42.

What makes it special

The bottom line of the display labels the top row of keys. The manual counts over 350 built-in functions; most are in menus rather than on the keyboard. Menus have submenus and several rows, moved through with the arrow keys, and the EXIT key goes back one level. A CUSTOM menu takes your own functions and programs, as the HP-41's User keyboard did.

Holding a key shows the name of its function. Hold it for about a second and NULL replaces the name: let go then and the function does not run. Let go before NULL and it runs.

Complex numbers, matrices and the solver

Complex numbers sit in a single stack register and appear by themselves where needed, for example as the square root of a negative number. Finseth lists the 42S as the first HP calculator to show them in polar notation as well as rectangular.

Matrices have an editor, and the matrix menu solves simultaneous linear equations. The solver finds the root of a program for any of its variables, and the integrator calculates definite integrals of a program. Both work on programs that declare their variables with MVAR.

Registers and variables

The 42S has both the HP-41's numbered registers and named variables. Registers start at 25 and grow as far as memory allows; they are really a matrix called REGS. Variables can hold complex numbers and matrices. Strings are limited to six characters on the stack, as on the HP-41, while the Alpha register holds 44.

Printing

The infrared port sends to HP's 82240A printer. The interface is one way: the manual explains that the calculator cannot tell if a printer receives the signal, so you switch printing on with PRON and off with PROFF. There is no other input or output.

Hidden things

A memory editor is built in. Jay B. Harlow described in 1990 how to reach it with a key combination, and a post of 1993 found the same editor in the 17B, 17BII and 27S.

Takayuki Hosoda showed in 2007 how to upgrade the 42S to 32 KB of memory, by replacing the 8 KB RAM chip and changing jumpers, and how to double its speed with a new crystal.

In saturnus

saturnus was tested with ROM revision C:

What it does not do: the 42S has no serial port, so Files has nothing for it, and the infrared printer output is not emulated. The memory view and the RPL editor are for the HP 48 and 49 only. The 42S has no clock, so "Set the clock" does not apply.

The command reference lists the 42S's 277 functions with the keys that run them, and examples for 89 of them, run in saturnus.

Open the HP 42S in saturnus

Sources