SMAR LD303, Operation And Maintenance Instruction Manual, Page: 24 / 110

LD303 - Operation and Maintenance Instruction Manual
2.2
As the distance (d) between the fixed plates CH and CL is constant. It is possible to conclude that
the expression (CL - CH)/(CL + CH) is proportional to
∆
d and, therefore, to the differential pressure
to be measured.
Thus it is possible to conclude that the capacitive cell is a pressure sensor formed by two capacitors
whose capacitances vary according to the applied differential pressure.
Functional Description – Electronics
Refer to the block diagram Figure 2.2. The function of each block is described below.
Figure 2.2 - LD303 Block Diagram Hardware
Oscillator
This oscillator generates a frequency as a function of sensor capacitance.
Signal Isolator
The control signals from the CPU and the signal from the oscillator are isolated to avoid ground
loops.
Central Processing Unit (CPU), RAM, FLASH and EEPROM
The CPU is the intelligent portion of the transmitter, being responsible for the management and
operation of measurement, block execution, self-diagnostics and communication. The program is
stored in a FLASH memory for easy upgrade and saving data on power-down event occurrence. For
temporary storage of data there is a RAM. The data in the RAM is lost if the power is switched off,
however the main board has a nonvolatile EEPROM memory where the static data configured that
must be retained is stored. Examples of such data are the following: calibration, links and
identification data.
Sensor EEPROM
Another EEPROM is located within the sensor assembly. It contains data pertaining to the sensor's
characteristics at different pressures and temperatures. This characterization is done for each
sensor at the factory. It also contains the factory settings; they are useful in case of main board
replacement, when its does an automatic upload of data from the sensor board to main board.
Profibus Modem
Monitors line activity, modulate and demodulate communication signals, inserts and deletes start
and end delimiters, and checks integrity of frame received.