I made this as a wired alternative to my Bluetooth-based BTcon because I've had compatibility issues with the BT protocol on some OS's and there are times during development where the near-field EMI prevents reliable communication. I still need isolation from the DUT and Silicon Lab's Si8621AB-B-IS digital isolator serves this function.
The MCP2200 makes the design quite simple with just a handful of parts; actually it is a derivative of Microchip's USB-UART demo board (ADM00276). I would have used this but I wanted a thumb-drive form factor without the classic 6-pin ICSP connector. I've also improved the decoupling to ensure reliable operation in noisy environments. I've tested it at 115.2kbps with no data loss on a variety of devices.
The Si8621AB provides 2.5kV of isolation and is a UL 1577 recognized component. That said, the board is not designed to meet any specific UL creepage or clearance requirements and I have not performed high voltage testing.
Because of the isolator, the DUT must provide 5V to power its half of the SI chip. Included in the BOM are the USB micro-AB plug components I've used to make a straight-thru cable for connections. Four wires are required (+5, GND, Rx, Tx) but I recommend the use of shielded cable for improved noise immunity.
The MCP2200 must be programmed with Microchip's configuration utility in order for the Rx/Tx LEDs to work. The unused GPIO pins should also be configured as logic low outputs for noise immunity.