Connect a Pair of USB Hosts via Serial Connection with the Isolated USB Null Modem
The device features a pair of USB CDC chips, creating a USB serial port for each attached computer.
Null modems were an excellent way for two people to play PC games against each other back in the early '90s. Sure, you could use the same RS-232 cable to connect to a printer or terminal, but rolling a limited LAN-type party seemed like the pinnacle of what null modem technology was capable of producing. Networking and wireless communication platforms all but killed the null modem for home use, but the technology is still alive and well for a myriad of different communication applications.
To that end, Nick Sayer needed a way to connect two USB hosts via CDC devices, and found himself in need of a null modem, only one that could work in a USB environment and offer total galvanic isolation. Since none were available, he decided to design his own, which features a pair of CDC (Communication Device Class) chips, which create a serial USB port for each connected computer. Instead of using directly connected lines, Sayer's Isolated USB Null Modem's TX and RX lines are cross-connected to allow communication between the two sides. However, they do pass through an optoisolator.
Channeling through that optoisolator enables two PCs with different ground potentials to connect to each other without causing damage. Sayer explains, "The primary side of each isolator is just an LED, but in this case, the forward current of the LED is quite low - the absolute maximum is 8 mA. By using a 1 kΩ series resistor, we wind up with something like 3.5 mA, which is both comfortably above the minimum activation threshold and below the maximum. The output is a simple logic output that's compatible with the inputs on the UART chip. The tricky part is in understanding that the secondary side shares the 5 volt and ground of the host it talks to, but the matching primary side shares its ground with the other host."
As mentioned earlier, Sayer stated that he needed a device that could function as a null modem, but provide ground loop isolation as well. Since the community response on his project page was large enough, he now offers it on his Tindie store for those interested.