The uSDR is a simple SSB and CW SDR built around an ATMEGA328P. The hardware and firmware are open so there are many design variations and kits available. Shortly after becoming licensed, I decided to build a kit based on WB2CBA's design.
Below left is the completed kit before any modifications were made. Below right shows the uSDX opened, the display removed, and my "controller" hardware plugged into the connector previously occupied by the display. From the uSDX's point ov view, my controller hardware appears as a regular LCD display so no firmware changes were required and it's easy to switch back and forth between my controller and the original LCD.
The controller hardware connects to a computer over USB and the screenshot below shows the controller app running on that computer. The simple layout of the app directly mirrors the user interface of the uSDX itself.
A couple videos on YouTube show the controller in action, with the second demonstrating that it was even possible to implement CAT over this simple bridge.
If you're interested in details, see the following Github repos: