a. Remove the old charger circuit.
b. Add a connector for I2C communication with the Charger Board.
c. Add a separated regulator for 3.3V for the Head to be able to reset/turn off the head from IOBoard.
d. Add switching buck-boost regulator for the modem(Battery to 3.3V).
e. Remove U21 and connect Atmel D4 to enable pin of U1(+5V booster).
f. get rid of U22(modem voltage switch) and connect D5 to the modem regulator enable pin.
Some minor changes:
a. Change the 3.3V regulator to a different one but pin compatible(BOM change).
b. Remove the FLASH circuit which is not populated on the IOBoard.
c. Change footprint for the level shifter for resetting the PCDuino.
d. Fixed modem mounting holes.
e. Replace MODBus pin connections with 2 or 3 Atmel pins for debugging. Add Atmel reset to the old MODBus connector.
f. Remove two I2C current sensors for battery and the charger.
g. Remove the ladder resistors (R2R-1 and R2R-2 and R202 ...) and free up A6 on the Atmel. But use it to monitor 5V.