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Converting an HP 20B into a WP 34S

The most powerful RPN Calculator you can build

Scritto da Silicon_Simon.
Domenica 30 Novembre 2025, 4:56 pm. Tag usati: calculator, retrocomputing, rpn

If you're into scientific calculators and miss, like me, the golden age of RPN machines, you're going to love this project. I'm talking about transforming a basic HP 20B business calculator into a WP 34S, arguably the most powerful RPN scientific pocket calculator ever created. 

I decided to write this post because while the conversion procedure is documented across various sources online, it's not always easy to find a straightforward, complete guide, especially when it comes to dealing with the infamous Pogo Pin cable issue. Back when HP sold these calculators, in fact, you could buy an official programming cable with a 6-pin Pogo Pin connector and erase/reset buttons. Today? Good luck finding one of those. They're practically extinct, which is a real problem because that cable is essential for flashing the new firmware.

In an age where everyone reaches for their phone or a computer when they need to calculate something, there's something deeply satisfying about having a dedicated, purpose-built tool that does one thing exceptionally well. The WP 34S embodies the best of HP's calculator legacy while pushing beyond what even HP achieved in their golden era.

This is a love letter to the calculators that got us through engineering school, that sat on engineers' desks for decades, that were passed down from mentor to student. But it's also something new: a community-driven project that proves open-source hardware and firmware hacking can create tools that rival or exceed commercial products.

What is the WP 34S Project?

The WP 34S is an open-source firmware project that breathes new life into HP's 20B and 30B business calculators. Started in 2011 by Walter Bonin, Paul Dale, and Marcus von Cube (hence "WP"), this project replaces the original business-oriented firmware with a full-featured scientific calculator that rivals (and in many ways surpasses) legendary models like the HP-42S.

The WP 34S has been stable since 2011 and runs on version 3.3 since 2014, with comprehensive documentation that's even available as a printed manual. The firmware is completely open source and actively supported by a passionate community of calculator enthusiasts on the HP Museum forums.

What makes it special? Think of it as combining an HP-42S with an HP-16C, then adding features that never existed in any calculator before. It's keystroke programmable, incredibly fast, and packed with mathematical functions that would make any engineer or scientist drool! :)

The procedure to turn the cheap HP 20b into a powerful WP34s involves:

1. Acquiring an HP 20B Business Consultant calculator,
2. Optionally modding it with two 18pF SMD capacitors and a crystal oscillator,
3. Connecting the calculator to a computer's serial port via that elusive cable,
4. Transferring the new firmware.

As you can see, there are already some challenges here for the modern hacker: first, the cable availability; second, the use of a serial port (when's the last time you saw one of those on a laptop?).

Thanks to some fantastic help from my friends Paolo and Guido—with whom I spent a very pleasant afternoon troubleshooting this—I managed to work around these issues using:

• Breadboard jumper wires (the kind you'd normally use with Arduino projects)
• An FTDI FT232 USB-to-serial adapter (we had one lying around from old ham radio projects)

This combination replaces the original Pogo Pin cable entirely and works beautifully. Here's what you'll need for this conversion.

Essential Hardware

• HP 20B or HP 30B calculator (the 30B has a better display and keyboard but costs more)
• FTDI FT232 USB-to-serial adapter (CRITICAL: Must operate at 3.3V, not 5V - the standard 5V serial voltage will permanently damage the calculator)
• Breadboard jumper wires (male-to-female and male-to-male)
• Small Phillips screwdriver
• Plastic prying tools (old credit card or phone repair spudgers work great)
• Pin or pointed tool for pressing the reset button

Optional but Recommended Components

• 2 × 18pF SMD capacitors
• 32.768 kHz crystal oscillator (the kind used in watches)
• Soldering iron and solder (if installing the crystal mod)
• Keyboard overlay (highly recommended - available from the really kind Jeff Roetman)

Software

• WP 34S firmware files (available in multiple variants)
• MySamba modified flashing software for Windows by Marcus von Cube
• Windows computer (Windows 10/11 work fine, to run with admin privileges)

The oscillator affair

Before we dive into the procedure, let's talk about the optional crystal mod. This isn't strictly necessary, but here's why you might want to do it. The crystal oscillator enables the stopwatch function in the firmware, making it significantly more accurate and reliable. If you're going through all this trouble anyway, why not go all the way and unlock every feature? The mod involves soldering two tiny 18pF SMD capacitors and a 32.768 kHz crystal oscillator to specific points on the calculator's PCB. It's delicate work, but if you're comfortable with basic SMD soldering, it's totally doable.

Firmware Variants

The WP 34S firmware comes in several flavors:

1. Full version - All standard features enabled
2. Full version with crystal support - Includes stopwatch functionality (requires the hardware mod)
3. IrDA variant - For those who want to add an infrared LED for calculator-to-calculator communication
4. WP 31S - A simpler variant with a reduced function set

My advice is to choose the version that matches your hardware modifications and needs.

Step-by-Step Conversion Process

Phase 1: Disassembly

Opening the HP 20B requires removing five screws (see Gallery below), but there's a catch:

1. Remove the two button cell batteries
2. Carefully peel back the lower rubber anti-slip strip on the back (it's attached with double-sided tape)
3. You'll find two hidden screws under this strip, remove them
4. Remove the three visible screws
5. Use a plastic prying tool to carefully separate the case halves
   • Work your way around the edges gently
   • The plastic clips aren't super fragile, but take your time
   • Don't force it

Once open, you'll have access to the mainboard with the Pogo Pin contact pads clearly visible.

Phase 2: Crystal Oscillator Installation (Optional)

If you're doing the crystal mod:

1. Identify the three soldering points on the PCB (refer to the included images in the photo gallery)
2. Solder the two 18pF capacitors in their designated positions
3. Solder the 32.768 kHz crystal oscillator
4. Double-check all connections for cold joints or bridges

My advice? If you're doing this mod, complete it before reassembling the calculator. It's much easier to work with when everything is accessible.

Phase 3: Understanding the Pogo Pin Connections

This is where most people get stuck, so pay attention. The Pogo Pin pads on the calculator's PCB are arranged in a specific way, and you need to understand which pad does what. Thanks to Dewster's excellent hand-drawn documentation (seriously, bookmark that page - it's invaluable), we know exactly which contacts to use:

• RX (Receive, J35) - This needs to connect to TX on your FTDI adapter
• TX (Transmit, J33) - This connects to RX on your FTDI adapter
• 3.3V (power, J31) - This feed the serail connection
• Ground (GND, J34)- Connect to GND on your FTDI adapter
• Reset button (J32) - There's a small hole just above the Pogo Pins
• Erase pins (J36) - Two specific contacts that need to be shorted during the boot sequence

Phase 4: Wiring the Connection

Here's how to set up your breadboard wire connections:

1. Reassemble the calculator completely - Yes, you read that right. It's easier to work with when it's together.
2. Prepare your jumper wires:
   • One wire for RX
   • One wire for TX
   • One wire for 3.3V+
   • One wire for GND
   • One wire for creating the erase short circuit
   • One wire or pin for pressing reset
3. Connect to the FTDI adapter:
   • Calculator RX → FTDI TX
   • Calculator TX → FTDI RX
   • Calculator 3.3V+ → FTDI VCC
   • Calculator GND → FTDI GND

CRITICAL SAFETY CHECK: Verify that your FTDI adapter is set to 3.3V mode! The standard RS-232 serial protocol uses 5V, which will fry the calculator's circuitry instantly. Most FTDI boards have a jumper to switch between 3.3V and 5V—make absolutely sure it's on 3.3V.

Phase 5: The Flashing Sequence

And now, this is the tricky part. The calculator needs to be put into a special bootloader mode that opens the serial port for programming. Here's the exact sequence (and yes, order matters!):

1. Turn on the calculator - Press the ON button
2. Short the erase pin - Use a jumper wire to connect the two erase contacts (ERASE + 3.3V)
3. Keep the short in place - Don't remove the wire yet
4. Press the reset button - Use a pin or another jumper wire to press the tiny reset button in the hole above the Pogo Pins
5. Turn on the calculator again - Press ON while maintaining the short
6. Remove the short circuit wire
7. Press reset again
8. Turn on the calculator one more time - Press ON

At this point, the calculator should be in bootloader mode, ready to receive the firmware.

Phase 6: Flashing the Firmware

Now launch the MySamba software on your Windows computer:

1. Run as Administrator - This is essential. Right-click the executable and select "Run as administrator." If you don't do this, the software won't detect the COM ports.
2. Select your COM port - The FTDI adapter will create a virtual COM port. Select it from the dropdown menu.
3. Choose your firmware file - Browse to the .bin file you downloaded earlier.
4. Start the transfer - Click the send/upload button.
5. Wait for the progress bar - The entire transfer takes about 20 seconds. You'll see a progress bar in MySamba's modest interface.
6. Press reset after completion - Once the transfer finishes, press the reset button one more time.
7. Power on your new calculator - Press ON and marvel at your handwork!

Troubleshooting Notes

If the flashing doesn't work on the first try, don't panic. This procedure often requires a few attempts to get the timing right. The key is getting the calculator into bootloader mode—that erase/reset/ON dance can be finicky.

Important: Keep both button cell batteries installed during the entire process. The calculator needs stable power throughout the firmware transfer.

MySamba compatibility: The MySamba software was written quite a while ago and was designed for older Windows versions. On Windows 10 and 11, we found it works perfectly fine as long as you run it with administrator privileges. If you're having issues detecting COM ports, that's almost always the problem.

Phase 7: The Finishing Touch - Keyboard Overlay

Once you've successfully flashed the firmware, you'll notice something immediately: the key labels on the calculator no longer match their functions. At all. The WP 34S has vastly more functions than the original HP 20B, so the keyboard layout is completely different.

This is where a keyboard overlay becomes essential. I can't thank Jeff (Spiff72) enough for quickly and courteously sending me some die-cut adhesive overlays that he produces. These overlays are professionally made and transform the calculator from a confusing mess of mislabeled keys into a proper, readable scientific calculator. They're well worth the investment. Trust me, trying to memorize which business-calculator button now triggers which scientific function is an exercise in frustration.

The Result: Your New WP 34S

Fire up your newly converted calculator and you'll have in your hands:

• 400+ built-in functions including complex numbers, matrices, statistics, probability distributions, numerical integration, and solving
• Keystroke programming with alpha labels and full program editing
• RPN or algebraic entry modes (though let's be honest, if you're doing this conversion, you're here for RPN)
• 8-level stack with named registers
• Stopwatch function (if you did the crystal mod)
• Base conversions and logical operations for programmers
• Double precision floating point for maximum accuracy

All of this in a calculator that costs you maybe $30-50 plus the cost of an FTDI adapter! According to users and the project maintainers, it's the most powerful and fastest RPN scientific pocket calculator ever built. And they're not exaggerating.

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