Page 18 says that the Main Set Point is Modbus address 138.
What exact parameter do you want to read as the "PV" ? The AC20 Modbus user manual says on page 37 that it can run with the Carbon Potential, Dew Point, or Sensor Output (millivolts) as the primary control variable.
The more detail you put into your request the better data you can get from the Forum.