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VP-Digi: A Powerful, Affordable, and Open-Source APRS Digipeater Controller

VP-Digipeater APRS

If you’re into APRS and looking for a cheap, functional, and open-source digipeater solution, VP-Digi might just be what you’re after. Designed with accessibility and performance in mind, VP-Digi turns the inexpensive STM32 “Blue Pill” board into a full-featured APRS digipeater controller and KISS modem.

Let’s take a closer look at what makes VP-Digi such a practical choice for amateur radio operators.

💡 What Is VP-Digi?

At its core, VP-Digi is a versatile APRS digipeater and TNC that can also operate as a packet monitor. It’s built around the STM32F103C8T6 microcontroller, commonly found on the “Blue Pill” board — a low-cost development board available for around $4 USD on sites like eBay or Aliexpress.

Despite the minimal cost and simple components, VP-Digi packs a punch, offering capabilities that are often found in much more expensive gear.

Developed by SQ8VPS, the project is fully open source and well-documented. You can find the official project page here:
🔗 https://sq8l.pzk.pl/index.php/vp-digi-cheap-and-functional-aprs-digipeater-controller-with-kiss-modem/

🛠️ Key Features

  • APRS Digipeater:
    Handles up to 8 configurable beacons and supports both type n-N aliases (like WIDEn-N, NYn-N) and simple aliases (like CITY, AREA).
    The digipeater also includes:
    • Viscous delay (borrowed from aprx) to reduce packet collisions
    • Callsign filtering (whitelist or blacklist)
    • Direct-only and fill-in digi modes
  • KISS and Monitor Modes:
    Each of the available interfaces (USB and 2 UARTs) can be used for:
    • KISS TNC operation
    • Raw frame monitoring
    • Serial configuration
  • Simple Configuration:
    Just connect to the device using any serial terminal (e.g., PuTTY, minicom) to issue commands. All settings are stored in flash memory, so they persist after reboot.

🔍 Modem Support (Since v2.0.0)

One of the biggest updates is the inclusion of four selectable modem types, covering most use cases:

  • 1200 Bd Bell 202 (VHF)
  • 9600 Bd G3RUH (UHF)
  • 300 Bd Bell 103 (HF)
  • 1200 Bd V.23 (alternative for VHF)

Additionally, VP-Digi now supports FX.25, a forward error correction protocol that wraps AX.25 frames with redundancy, improving reliability over noisy links — and it’s backward-compatible!

🎧 Better Carrier Detection

VP-Digi uses a smart digital carrier detect (DCD) algorithm. Instead of relying on demodulated audio or squelch, it directly analyzes the incoming waveform to identify valid modulated signals. This approach provides:

  • Greater sensitivity
  • Better noise immunity
  • Reliable performance even with open squelch radios

📦 Open Source and Easy to Build

Everything about VP-Digi is open-source:

  • 🧰 GitHub Repository: https://github.com/sq8vps/vp-digi
  • 📄 Documentation: Included in the GitHub repo and project site
  • 📥 Precompiled firmware (HEX): Available for easy flashing
  • 🛠️ DIY-friendly: Just a few basic components needed

So if you’re comfortable soldering and flashing firmware, you can build one yourself at very low cost. All you need is the STM32 “Blue Pill”, a few passive components, and a USB-to-serial adapter.

🛠️ Changelog Highlights

Here are some key milestones from the VP-Digi development:

  • v2.0.0 – FX.25 support, four modem types, enhanced DCD
  • v1.2.0 – Per-alias viscous delay, direct-only mode, improved audio compatibility
  • v1.1.2 – Hardware fix: RX diode moved from PB3 to PB5
  • v1.1.0 – Support for TX/RX LEDs, improved internal modem
  • v1.0.0 – Initial public release

For the full changelog, visit the Changelog page.

📚 Resources

🎤 Final Thoughts

VP-Digi is a fantastic project for any ham looking to experiment with APRS, build a personal digipeater, or set up a lightweight KISS modem. With broad modem support, reliable performance, and a budget-friendly build, it’s ideal for both beginners and tinkerers alike.

Whether you’re planning to deploy a fill-in digi in your area, monitor APRS traffic, or integrate a KISS TNC into your SDR setup — VP-Digi has you covered.

Thanks to the efforts of SQ8VPS, this project is freely available to the amateur radio community. Let’s support it, build it, and spread the word. 📡

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