4. Connecting the adapter

Pololu Isolated USB-to-I²C Adapter pinout and LEDs.

Pololu Isolated USB-to-I²C Adapter with Isolated Power pinout and LEDs.

The Pololu Isolated USB-to-I²C Adapter connects to a computer’s USB port via a USB C cable (not included). You can connect I²C target devices to the adapter through either a JST SH-style 4-pin connector (compatible with Qwiic and STEMMA QT) or four 0.1″-pitch through-holes that are compatible with standard 0.1″ male headers and female headers.

Pin	Cable color	Name	Function (Isolated USB-to-I²C Adapter)	Function (Isolated USB-to-I²C Adapter with Isolated Power)
1	Black	GND	I²C ground; this pin is NOT connected to USB GND
2	Red	VCC	I²C bus voltage input (2.25 V to 5.5 V)	I²C bus voltage output (3.3 V or 5 V, set by slide switch at start-up)
3	Blue	SDA	I²C data line
4	Yellow	SCL	I²C clock line

The adapter’s GND pin should be connected to the ground of your I²C target device(s). Because the adapter provides full galvanic isolation, the I²C side does not share a common ground with the USB connection.

The adapter’s VCC pin is either an input or an output, depending on the model of adapter you have:

• The Pololu Isolated USB-to-I²C Adapter (green board) has a VCC (IN) pin, and you must supply 2.25 V to 5.5 V across this pin and GND. This should match the logic level voltage of your I²C target(s).
• The Pololu Isolated USB-to-I²C Adapter with Isolated Power (blue board) has a VCC (OUT) pin. When the adapter’s VCC output is enabled (controllable by the Enable VCC Out command), this pin supplies 3.3 V or 5 V power (up to 200 mA) through the adapter’s isolated DC-DC power module. The voltage level is set by the position of the slide switch at start-up.

The adapter pulls the SCL and SDA lines of the I²C bus up to VCC when they are not being driven low. Depending on your setup, you might need to add pull-up resistors from SCL and SDA to VCC to ensure that the signals rise fast enough. The adapter has 10 kΩ on-board pull-up resistors, and many I²C target boards will have pull-ups too, but that might not be enough, especially if you want to use speeds faster than 400 kHz or have long wires. The I²C-bus specification and user manual (1MB pdf) has some information about picking pull-up resistors in the “Pull-up resistor sizing” section.

Wiring diagram for connecting I²C targets to the Pololu USB-to-I²C Adapter.

Wiring diagram for connecting I²C targets to the Pololu USB-to-I²C Adapter with Isolated Power.