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VP-Digi

VP-Digi is a standalone AX.25 digipeater controller and KISS TNC modem intended, among other uses, for APRS.

The project was designed as a small, inexpensive and energy-efficient STM32-based device. It combines the functions of a hardware TNC, Packet Radio modem and advanced APRS digipeater.

Official project repository:

https://github.com/sq8vps/vp-digi

Documentation:

https://github.com/sq8vps/vp-digi/blob/main/doc/manual_pl.md

VP-Digi is an open-source project and continues to be developed. New versions bring fixes, modem improvements, FX.25 development and refinements to AX.25 and KISS handling.

Why VP-Digi?

VP-Digi is an interesting alternative to traditional TNCs and larger computers used for APRS stations.

A typical installation can look like this:

Radio
  |
audio + PTT
  |
VP-Digi
  |
KISS / USB / UART
  |
APRSBox / computer / other application

It can also operate completely independently as a digipeater:

Radio
  |
audio + PTT
  |
VP-Digi

In this role it does not require a Raspberry Pi, PC or operating system.

Modern architecture

VP-Digi is built around an STM32F103 microcontroller.

This architecture allows it to handle digitally:

  • modulation and demodulation,
  • channel busy detection,
  • AX.25 encoding and decoding,
  • FX.25,
  • duplicate filtering,
  • digipeater functions,
  • KISS TNC,
  • beacon generation.

It is therefore not just a tracker or packet generator, but a complete radio-layer controller for Packet Radio and APRS.

AX.25 and APRS compatibility

One of the most important VP-Digi features is proper AX.25 support, the protocol underlying classic Packet Radio and APRS.

The device can operate as:

  • AX.25 modem,
  • KISS TNC,
  • APRS digipeater,
  • beacon generator,
  • Packet Radio modem,
  • radio interface for external software.

Thanks to KISS, it can work with many applications without relying on a proprietary vendor protocol.

Example:

VP-Digi
   |
KISS
   |
APRSBox

or:

VP-Digi
   |
KISS
   |
Packet Radio software

Supported modems

VP-Digi supports several modem types.

1200 Bd Bell 202

The classic VHF APRS modem:

AFSK 1200 Bd
1200 / 2200 Hz

This is the standard mode used by classic APRS on 2 m.

300 Bd Bell 103

A mode mainly intended for HF:

AFSK 300 Bd
1600 / 1800 Hz

9600 Bd G3RUH

VP-Digi also supports:

GFSK 9600 Bd
G3RUH

This mode requires a suitable radio path and usually flat audio access.

Not every radio can handle 9600 Bd through a normal microphone input and speaker output.

1200 Bd V.23

An alternative modem is also available:

AFSK 1200 Bd
1300 / 2100 Hz

compatible with V.23.

FX.25

VP-Digi also supports FX.25.

FX.25 extends AX.25 with Reed-Solomon forward error correction. It can improve the probability of successful packet reception in difficult RF conditions while remaining compatible with classic AX.25.

VP-Digi can:

  • receive AX.25,
  • receive FX.25,
  • transmit AX.25,
  • transmit FX.25.

FX.25 can be configured independently for receive and transmit.

Channel busy detection

VP-Digi does not base DCD only on successfully decoded data.

Channel activity is detected from the presence of a valid modulating signal.

This allows the device to recognize a busy channel earlier and helps reduce collisions.

In practice, the connected radio should normally operate with its squelch open.

KISS TNC

One of the key VP-Digi features is KISS mode.

In this setup VP-Digi handles the radio layer:

AX.25
modulation
demodulation
PTT
DCD

while an external application handles higher-level logic.

This can be:

  • APRSBox,
  • a Packet Radio client,
  • Winlink software,
  • custom software,
  • diagnostic tools.

This means VP-Digi is not limited to digipeater operation.

USB and two UART ports

The device provides:

  • USB,
  • UART1,
  • UART2.

Each port can operate independently as:

  • KISS,
  • frame monitor,
  • configuration terminal.

This allows flexible installations, for example using one port for KISS and another for monitoring and diagnostics.

APRS digipeater

VP-Digi includes an advanced digipeater engine.

It can configure, among other things:

  • 4 New-N aliases such as WIDEn-N,
  • 4 simple aliases,
  • maximum hop count,
  • trace mode,
  • direct-only mode,
  • viscous delay,
  • packet filtering,
  • duplicate buffer.

This gives much more control than a simple packet repeater.

Viscous delay

VP-Digi supports viscous delay.

Instead of repeating a packet immediately, the digipeater can briefly wait and check whether another station has already repeated it.

If so, its own retransmission can be suppressed.

This helps reduce:

  • unnecessary duplicates,
  • channel occupancy,
  • retransmissions.

It is especially useful where several digipeaters have overlapping coverage.

Direct-only

Individual aliases can also use direct-only mode.

In this mode the digipeater repeats only packets heard directly from the source station.

This helps prevent unnecessary re-digipeating of packets that have already passed through other digipeaters.

Station filtering

VP-Digi can filter packets by callsign.

Two modes are available:

  • blacklist,
  • whitelist.

This allows selected stations to be blocked or only selected callsigns to be accepted.

Duplicate protection

The digipeater includes a buffer for detecting repeated packets.

This prevents the same frame from being retransmitted multiple times when received over different paths.

The buffer time is configurable.

Beacons

VP-Digi supports up to 8 independent beacons.

For each beacon you can configure:

  • content,
  • interval,
  • delay,
  • APRS path,
  • enable or disable state.

Example:

!5002.63N/02157.91E#VP-Digi

Frame monitor

VP-Digi can also operate as an AX.25 traffic monitor.

Received packets can be viewed on a serial port, which is useful for:

  • station commissioning,
  • configuration checks,
  • audio path diagnostics,
  • Packet Radio and APRS traffic analysis.

Low power consumption

One of the major VP-Digi advantages is its very low current consumption.

According to the project documentation, the device draws approximately:

40-50 mA

This is especially useful in stations:

  • powered from batteries,
  • powered from solar systems,
  • installed where mains power is unavailable,
  • operating as autonomous digipeaters,
  • running continuously.

In such installations the radio itself usually consumes much more energy than the controller.

Open and actively developed

VP-Digi is an open-source project.

The repository provides:

  • source code,
  • schematic,
  • documentation,
  • ready-made firmware,
  • change history.

The project continues to be developed, with new releases adding fixes and features.

For infrastructure intended to operate for a long time, that is an important advantage.

Who is VP-Digi for?

VP-Digi is a good fit when you need:

  • an autonomous APRS digipeater,
  • an energy-efficient 24/7 node,
  • a hardware KISS modem,
  • a Packet Radio modem,
  • a radio interface for APRSBox or other software,
  • a platform for AX.25 and FX.25 experiments,
  • a small field or solar-powered station.

VP-Digi vs a software modem

VP-Digi and software solutions such as Dire Wolf represent two different approaches.

VP-Digi

microcontroller
low power consumption
no operating system
standalone digipeater
KISS

Software modem

computer
sound card
more processing power
greater DSP flexibility
easier integration with network services

For an autonomous installation VP-Digi can be simpler and more energy-efficient.

In a larger station it can work as a KISS modem for external software.

Summary

VP-Digi is a good example of modern APRS hardware.

It combines:

  • AX.25 compatibility,
  • FX.25 support,
  • several modem types,
  • an advanced digipeater,
  • KISS,
  • USB and UART,
  • very low power consumption,
  • open-source firmware,
  • active development.

It can therefore operate both as a standalone digipeater and as a modem within a larger APRS system.

It is particularly interesting where protocol compliance, reliability, low power consumption and operation without a full computer matter.

Documentation

Main sources: