VP-Digi: cheap and functional open-source APRS digipeater controller with KISS modem

VP-Digi is an open-source, very cheap, functional, easy to assemble and run, APRS digipeater controller, which can work also as a KISS modem or a frame monitor.

The device is based on a popular „Blue Pill” board with STM32F103C8T6 microcontroller. This board can easily be found on websites like Ebay, Aliexpress etc. at the price of about 4$. VP-Digi requires only a few typical external components to operate. Thanks to the use of STM32 processor, the VP-Digi can offer many more capabilites than similar devices based on Arduino, at a very similar costs.

VP-Digi offers 8 configurable beacons and an APRS digipeater, which is capable of handling 4 type n-N aliases (e. g. WIDEn-N, NYn-N) and 4 simple aliases (e. g. CITY, AREA). Moreover, the digipeater incorporates the viscous delay function know from aprx, which can reduce unnecessary traffic. There is also a posibility to filter packets by sender callsign, either in blacklist or whitelist mode. The digipeater can be freely configured, e. g. to work as regional (Wn) or fill-in (W1) digi.

The device is equipped with an USB port and two UART ports. All of them can work independently in KISS TNC, frame monitor or configuration mode. The configuration is done by simple commands using any terminal program and is stored in the embedded flash memory.

Monitor mode

Since version 2.0.0, VP-Digi comes with four selectable modems: 1200 Bd Bell 202 (VHF standard), 9600 Bd G3RUH (UHF standard), 300 Bd Bell 103 (HF standard) and 1200 Bd V.23 (alternative VHF standard).

Since version 2.0.0, the FX.25 protocol support is included, which enables error correction for better reliability. The FX.25 protocol is fully compatible with existing AX.25 protocol.

Additionally, a digital carrier detection (DCD) is implemented. It uses an approach of looking for correct modulated signal rather than checking demodulated data. This algorithm provides much better results, is more sensitive, but at the same time is more immune to noise. That’s way VP-Digi works very well with devices with open squelch.

Full documentation – assembly, configuration, etc. – is available here.

The latest compiled firmware (HEX) is available here.

The repository with source code and documentation is available here.

Changelog is available here and will be updated only there.
v. 1.2.0 – 10 September 2021 – source code is available. Code partially rewritten. Viscous delay and direct-only modes enabled for each alias separately. Special mode for radios with flat audio output. Erase configuration command.
v. 1.1.2 – 27 October 2020 – schematic fix, RX diode output moved from PB3 to PB5 (hardware and firmware changes) – thank Alfredo, IZ7BOJ for spotting these errors
v. 1.1.1 – 10 September 2020 – bug fix (4-7 digi aliases configurations were displayed incorrectly)
v. 1.1.0 – 02 August 2020 – external TX and RX LEDs, small internal modem configuration changes
v. 1.0.0 – 01 August 2020 – inital release