Warning: This is intended only to be used in a lab environment for short sessions. The isolation barrier on this PCB is not strong enough to hold off high voltages for extended periods of time, or tolerate a less than clean environment.

What is this

This is a 2-port isolated USB Full-Speed hub with external power option and integrated USB CDC ACM compatible USB to serial adapter.

The barrel jack accepts 9-12V center positive power supply. The micro-USB port accepts a USB upstream connection. The two USB type-A ports provide two current-limited downstream ports. The pin headers provides a externally-referenced simple UART.

Story of Creation

Confession: I almost fried my oscilloscope when testing a piece of mains-connected equipment, not knowing that the apparent ground wire is actually connected to the mains live due to the nonpolar plug connected backwards.

For fear of creating that spark again and potentially frying my computer, I decided to somehow implement electrical isolation in my home lab. For all the isolation hardware, this is so far the most complicated one.

How does this work

At the heart of this isolation hub is Analog Devices ADuM3160/ADuM4160 USB Full Speed isolator and NN1-05S05AS isolated power supply module. Those two chips passes USB signal and electrical energy respectively from the upstream port to the USB hub chip without electrically connecting the two.

The rest of the circuit is a standard three-port externally powered USB hub, with one of the ports internally connected to a USB to serial adapter chip.

About the serial port

The integrated USB to serial chip has internal logic level shifters, so it will neither supply power to external circuitry, nor work correctly without the external circuitry supplying it a reference voltage. You need to connect the VDDIO pin to a correct power rail to use the USB to serial adapter.

If you need power to your board, use one of the downstream USB ports for that.

The UART pins are marked on the top of the board with single-letter codes: