The USB Null Modem Cable Is Now A Thing

August 25, 2020 by Lewin Day 87 Comments

The classic serial null-modem cable was, among other things, used to connect two computers together for communications and file transfer. Largely eliminated in daily use by the advent of home networking, there are still fringe applications where such a thing can come in handy. [Nick Sayer] needed just such a tool, but one that would work in a modern USB environment. Enter the isolated USB null-modem.

The device consists of two USB Communication Device Class, or CDC chips, creating a USB serial port for each attached computer. The TX and RX lines are cross-connected to allow communication between the two sides. Rather than directly connect the lines, however, they pass through an opto-isolator. This is important, as it allows two computers at different ground potentials to be safely connected to each other without damage.

[Nick] originally created the device to solve a specific problem at his day job, but community response was large enough that he was kind enough to share the project online. Expect to see devices available on Tindie in future for those that need a hookup. While it’s not something everyone will need, for those that do, it should come in handy. If you’re looking for other useful applications for USB-serial devices, there’s plenty – you can even try your hand at software-defined radio!

Loading Programs Onto A TRS-80 Model 100

March 5, 2012 by Mike Szczys 41 Comments

We’d guess that you don’t have a TRS-80 Model 100 computer sitting around. But we’ve heard that the decades-old hardware is built like a tank so if you search around you can probably get your hands on a working unit. The Model 100 boasted some nice features, one of which was a 300 baud modem allowing you to transfer data onto the device. [MS3FGX] wanted to give it a try but had to do some work to get the Model 100 to communicate with modern hardware.

This could have been a much more involved process, but since the Model 100’s modem uses common communications standards it’s really just a matter of hooking it up and choosing the right COM port settings on a computer. In this example a Linux box is used with the program Minicom. It is configured to communicate at 300 baud 8N1 (8 data bits, no parity bit, and one stop bit).

With software in place you’ll need to make your own cable. [MS3FGX] does this using a DB-25 connector for the Model 100 side, and a DB-9 connector for the serial port on the Linux box. He’s got a pin-out for the cable on the second page of his guide. It sounds like it should be no problem to use a USB-serial converter if you don’t have a serial port.

Once everything is in place you’ll be able to transfer BASIC programs from your computer to the Model 100.