Thank you for the link. I followed what that person did to get the Nest thermostat to work.
He copied the digital signals and used a nano rdunio which is a simple computer. Ardunio are often used for robotics. The high low state is used to trigger relay switches, That translates into digital signals.
All he accomplished was getting a thermostat with WiFi access. The controller at the a/c is still doing the same thing it was with the other thermostat.
Making a 12 vdc to 28 vac is not hard to do. Replacing the a/c controller like the Ardunio would be required to use a home thermostat.