I've got a ton of emails about the adapter for the Kenwood TM-D710A. The microphone on this unit uses a digital pulse counter based on 4017 logic IC's to determine, over a single wire, which key is pressed. This same principle can be use in other projects to read a keypad, control LED's etc with on a pin or two from a microcontroller. For reference below, are the articles on the TM-710A Adapter and associated projects.
http://www.shaneburrell.com/?p=688 - Keyboard Adapter itself - This board is line powered from the microphone cable. It effectively sits between the Kenwood radio unit and Microphone. It allows you to place a keyboard in between the two to allow functionality that Kenwood never intended per say.
The theory of operation: The microprocessor in the radio head unit sends pulses to the microphone. On each pulse the line is pulled low which the microcontroller spies on via another pin attached to the pulse pin. On a scope, it's very easy to see the line being pulled low depending on the keypress. In hacking the pulses, its just a matter of seeing what key generates what pattern. In the TM-D710A adapter I used a AVR to talk to the microphone and read a standard keyboard. The One Signal Wire board design above was used to Emulate the mic controlled by the AVR.
Below is a video some Arduino code I initially developed to scan the microphone patterns driving the Kenwood Mic from the AVR. If you look at the scope this should give you a good idea how the pulse counter is working.
The TM-D710A keyboard hack was a really fun project and hit multiple stages of hack/design. The 4017 counter is a pretty neat way of reading/controlling things using on 2 pins from a micro-controller.