Tercio FD

1 min readFirmware v2 · protocol v2

The Tercio FD connects your computer to the CAN-FD bus. It appears as a standard USB serial port, forwards every frame in both directions, and reports the health of the bus four times a second.

Host: USB-C, USB 2.0 full speed, serial (CDC)
Bus: CAN-FD, 500 kbit/s + 2.5 Mbit/s
Transceiver: Microchip MCP2562FD
Termination: 120 Ω on board
Connector: JST XH, 3-pin
Board: 35 × 25 mm

What it does

Bridges framesA frame from the computer goes onto the bus as one CAN-FD frame; every frame on the bus comes back. It does not interpret commands, so anything that speaks the CAN protocol works through it.

Checks every byteOn USB, each frame is COBS-encoded and carries a CRC-16, so a glitch is dropped and counted, never misread. See Serial link.

Reports the busWhile a program holds the port open, it sends its bus health every 250 ms: error counters, bus state, traffic and dropped frames.

Updates over USBOne command restarts it into ST’s USB bootloader for a firmware update. No probe needed.

Connectors and LEDs

Power
Ground
Bus / data
Debug

Top view, to scale (35 × 25 mm). Hover or tap a connector to name its pins; select it for the full pinout.

CAN (J2)

On the board: J2 JST XH, 3-pin, 2.5 mm pitch
Mates with: JST XHP-3 housing with SXH-001T-P0.6 crimps

The bus. 120 Ω termination is built in, so the adapter always sits at one end of the bus.

Pin	Signal	Type	Function
1	CAN-H	Bus / data	CAN high.
2	CAN-L	Bus / data	CAN low.
3	GND	Ground	Bus ground reference.

USB-C (J1)

On the board: J1 USB-C receptacle, USB 2.0 full speed
Mates with: Any USB-C cable with data lines

Power for the adapter and the serial link to your computer. The bus is not powered from here.

Pin	Signal	Type	Function
—	USB	Bus / data	VBUS, D+, D−, CC1/CC2 (5.1 kΩ to GND), GND.

SWD (J3)

On the board: J3 5-pin header, 2.54 mm pitch
Mates with: ST-Link or any SWD probe

Programming and debugging. Normal updates go over USB instead.

Pin	Signal	Type	Function	MCU
1	+3.3V	Power	Target voltage sense for the probe.	—
2	SWDIO	Debug	Serial wire data.	PA13
3	SWCLK	Debug	Serial wire clock.	PA14
4	NRST	Debug	Reset, active low.	—
5	GND	Ground	Ground.	—

LEDs

PWR, greenUSB power is present.

RX, redFlickers with every frame on the bus, including the adapter’s own. No flicker at all: nothing on the bus is talking.

TX, blueFlickers when the adapter transmits a frame from your computer.

Specifications

Host side

USB: USB-C, USB 2.0 full speed, CDC serial
Drivers: None needed: Windows 10+, macOS, Linux
IDs: VID:PID 1209:0011, product tercioFD
Power: From USB. Does not power the bus or the drivers.

Bus side

Bus: CAN-FD with bit-rate switching, 500 kbit/s + 2.5 Mbit/s, 11-bit IDs
Termination: 120 Ω, always on: the adapter sits at one end of the bus
Buffers: 64 frames each way; frames not delivered within 50 ms are dropped and counted
Protection: ESD protection on USB and on the CAN lines

Hardware

Processor: STM32G431, Cortex-M4F at 170 MHz, 16 MHz crystal
Transceiver: Microchip MCP2562FD
Mounting: Four 2.5 mm holes, centres 30 × 20 mm
Board: 35 × 25 mm

Software

Works with: Tercio Control (Chrome, Edge), Python, Arduino/C++
Firmware: Open source, updatable over USB

ConnectingUSB, drivers, permissions, and the bus.Read more

Bus healthWhat the counters mean and how to read them.Read more