[SiriusHardware]

These units only have a 2-terminal plug, the data is superimposed on the supply voltage so you should see that the normally steady supply coming down the cable inner core flickers between two different voltages if you put a scope on it and press buttons.

The chip is actually one which was specifically made to go in remote controls. I have a homebrew version of one of these based on a PIC chip. It took me a while to get it to work because I did not realise the original R/C chip sends each 'packet' several times over for any one keypress, and the firmware in the HF150 actually looks for this repetition. My initial attempt only sent the data packet once and was ignored.

The most likely physical failure mode is failure of the cable where it exits the keypad or right next to the strain relief on the jack plug, but I assume you have already metered that out.

Edit: Just fired mine up again. As you type digits the number you are entering should build up on the display in right-entry mode like on a calculator. If you are entering fewer than 5 digits you may have to hit '#' to confirm. If you enter 5 digits then the frequency entered is selected on entry of the 5th digit.