I just posted a how-to article describing adding additional analog inputs to a Kantronics TNC. Out ham radio club was installing a new APRS repeater, and had asked if there was any way of monitoring multiple analog values (specifically, two battery banks, the presence or absence of AC mains, and a temperature reading). I spent some time studying the schematics and doing some research on the web. I was unable to find any specific how-to articles other than vague references to using additional CPU pins. After building and testing the circuit, I decided to post an article describing the process for anyone interested in doing the same thing.
Adding Additional Analog Inputs on a Kantronics TNC