Your thermostat is what we call a proprietary digitally communicating thermostat. It uses AV communications protocol based on RS 485 and it digitally talks to the control board. It unfortunately is not compatible with a nest. learning thermostat. In fact it’s not compatible with any thermostat other than the ones manufactured by the HVAC equipment manufacturer it is attached to.
However, most of those systems are designed to work, not only with the proprietary system, like the Carrier ION, but with traditional thermostats too. You would however have to run a new cable between the thermostat location and the equipment and that cable would need to have 6 to 8 conductors inside. I hope this helps you to understand.
Ken, The AC Cooling Wizard