It is the communication module which looks for messages on the input wire and, if they are present and valid, responds with the correct action. The response is given by overriding the normal 4-20 mA signal with 16 mA digitally coded pulses.
PROM : Stores meter body data (Input / output characteristic, temperature characteristic Static Pressure characteristic, model data, usable range etc.)
EEPROM : Stores data on transmitter parameters (Tag number, measuring range, linear or sq. root output selection, damping time selection, zero and span calibration) set with SFC.
RAM : When instrument power is turned ON, back-up data stored in EEPROM is automatically transferred to RAM.