What is it?

This is a pair of USB CDC (serial port) devices whose communications lines are cross-connected with a galvanically isolated null modem.

Each side has a USB C connector intended to connect to a host. The two hosts will each see a single CDC device. What one host transmits, the other host receives and vice-versa. Additionally, the RTS/CTS pairs as well as DSR/DTR and DCD are likewise cross-connected in the style of a null modem (RTS<->CTS, DTR<->DCD+DSR).

The two hosts are completely separate and galvanically isolated from each other. Each only electrically sees its own USB CDC device. The signals are transmitted via isolators rated for 1 kV of standoff isolation.

The TX lines of each side have an LED connected so you can see data flow. One host is red, the other green so you can see from afar which side is transmitting.

Why did you make it?

You can purchase USB CDC pigtails and connect the RS-232 interfaces with standard null modems, but this device is much smaller and tidier. And in situations where the two hosts might not share a common ground potential, the galvanic isolation is an important precaution against damage.