“Blooper” Is a Null Modem Cable for the Modern USB Era

What do you do when you want two computers to talk to each other directly, without any networking hardware in-between? In the past, you might have used a null modem cable or an Ethernet crossover cable — both just swap the receive and transmit signals to allow direct communication. But your computer today might not even have an Ethernet port and it definitely doesn’t have a serial port. That’s why Clem Mayer invented this USB Looper, aka “Blooper,” for the modern era.

Blooper serves a purpose similar to those old null modem cables. It lets two computers talk to each other without involving a network. That’s handy for headless control, transferring files, and more. It can also log serial output and even let a computer talk to itself by looping back serial data to another USB port (hence the name), which can be useful when debugging or testing functionality.

As you may have guessed, that is possible because Blooper isn’t just a “dumb” set of wires, like a null modem cable or an Ethernet crossover cable. It is actually a very compact device with a USB-C port on each end. Plug in cables and connect them to two computers (or a single computer), then start communicating. The computer sees a USB serial device, so standard serial utilities will work just fine.

All of that is possible because the small Blooper PCB, custom-designed by Mayer, contains two FTDI FT230XS chips. The rest of the hardware is just resistors, LEDs, the USB-C ports, and diodes to prevent reverse-powering — you don’t want one computer’s USB port trying to provide power to the other computer’s USB port.

You should be able to use Blooper in many ways without any special software. But Mayer does provide Python scripts to help. One example is for datalogging, another is for full-duplex chat, and another is for transferring files.

Put it all together and Blooper is a genuinely useful tool to have at your disposal, which you can build with about $10 worth of components.