Pocket Pico TNC

A complete packet radio TNC on a Raspberry Pi Pico, recreating the Heathkit HK-21 Pocket Packet

Then and now

Black Heath Pocket Packet TNC sitting on a Tandy computer keyboard — The original Heathkit HK-21 Pocket Packet from the 1980s

Gray 3D-printed Pocket Pico case with TTL and RIG jacks and PTT, DCD, CON and STA light holes — The Pocket Pico: TTL and RIG jacks, plus the same PTT, DCD, CON and STA lights

The Pocket Pico is a terminal node controller (TNC) for amateur packet radio. It recreates the Heathkit HK-21 Pocket Packet, a pocket-sized TNC from the 1980s, by running the original HK-21 firmware on a Z80 processor emulator inside a Raspberry Pi Pico. The Pico also does the radio modem work in software, sending and receiving 1200-baud Bell 202 audio tones without a dedicated modem chip.

Unlike the many modern TNCs that only handle APRS, the Pocket Pico is a full TNC. It supports the complete AX.25 Level 2 protocol for connected-mode packet, so you can connect to other stations, nodes and bulletin boards just as with a classic 1980s TNC. It includes its own Personal Bulletin Board System (PBBS) where other stations can connect and leave messages, and it also works in KISS mode for today's packet software.

Paired with a custom-designed circuit board, the Pico becomes a complete, self-contained unit: a PBBS and KISS TNC with a radio connection and USB and TTL serial ports, in a 3D-printed case.

Highlights

Emulates the Heathkit HK-21 Pocket Packet, commands included, so it works with older packet software
Full AX.25 Level 2 connected-mode packet, not just APRS
Built-in Personal Bulletin Board System (PBBS)
DIP switches put the console (PBBS) and the KISS TNC on the USB or TTL serial port, or run one of them on both
Adjustable transmit audio level
Software Bell 202 AFSK modem, with no modem chip needed
USB serial plus a 3.3 V TTL serial port
Status LEDs for PTT, DCD, CON and STA, like the original
Works with the radio's squelch open
Open source (BSD 3-Clause), with the circuit board and 3D-printed case designs in the repository

DIP switch settings

Switches 1–3 choose which port gets the TNC console (with the PBBS) and which gets KISS. Switch 4 turns the status LEDs on or off, which saves power on battery. Changes take effect on their own; the Pocket Pico resets itself into the new mode.

1	2	3	USB port	TTL serial
Off	Off	Off	Console	KISS
On	Off	Off	KISS	Console
Off	On	Off	Console	Console (mirrored)
On	On	Off	KISS	KISS
On	On	On	Resets into the bootloader, for firmware updates

KISS on the TTL port runs at 115,200 baud, 8 data bits, no parity, 1 stop bit. From the Pocket Pico manual, page 5.

Video

The build

Purple Rev 0.70 circuit board with a Raspberry Pi Pico H mounted on headers and a blue DIP switch — Rev 0.70 board with the Raspberry Pi Pico and DIP switch

Other side of the purple board showing two audio jacks, four clear LEDs and a blue trimmer — Radio and TTL jacks, status LEDs and RV1, the transmit audio level trimmer

The assembled board screwed into the open gray 3D-printed case — Board fitted in the 3D-printed case

Closed case showing the micro-USB opening and an access hole for the blue trimmer — USB port and access to the transmit audio trimmer

Closed case with a window showing the four-position DIP switch — DIP switches: choose the console and KISS ports, and turn the LEDs on or off

Links

Pocket Pico TNC on GitHub User manual (PDF) Watch on YouTube

Credits

The Pocket Pico firmware and board are by pfiliberti, who built it on the original PICO TNC by amedes, keeping its software modem and adding a Z80 emulator that runs the HK-21 firmware.