Two things came out of my pondering:
One: There are a lot of single-wire sensors available so it should not be too difficult to interface to them and ....
it turns out that back in the day, late 1980's (I had completely forgotten!), I had interfaced a Temperature Module FE33L to the TI99 through the Joystick port: A little assembly picked up the serial data and stored it as ASCII so that an EB routine could pick it up to display and print an asterix (*) on a horizontal temperature scale: the module updated at 10 second intervals to give me a real-time clock and so I was able to display/print only changes rather than continuous reading using excessive memory.
I will dig out that code if anyone is interested in adapting it.
Two: Back to the original question - I looked around the site to see if anyone had tried measuring pulse frequency on the TI and found that our ideas man had already been there:
It is a very simple matter to use opto-couplers for a wind direction vane pick-up so if I can only fiqure out a simple way to interface temperature and humidity then we have it covered for us cheap-stakes.
Edited by Meddler, Thu Feb 2, 2017 5:22 AM.