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Primary Electronics
Main purpose of this unit is to convert the pressure signal to an
electronic pulse-width signal. As help for added accuracy both
temperatures and static pressure of the transducer are
measured.
ASIC5
The ASIC5 components contain the basic pulse width converters
that convert the input frequency coming from the sensor in two
redundant pulse-width signals proportional to the two inductance
values L1 and L2. The two independent output time-duration
signals from ASIC 5 are applied to the secondary unit through
independent lines.
EEPROM1 Memory
This EEPROM memory is used by the "µP" described later. It
contains all the relevant information for the sensor
characterization and for the transmitter calibration.
Secondary Electronics "
This unit uses a µP and an ASIC to convert the basic measured
data into correct scaled data. Compensation for temperature
and static pressure are also performed. The output data value
is converted into a pulse-width signal that is filtered and that
activates the 4-20 mA transmitter. The bi-directional, digital
communication using the standard "HART" protocol is
implemented as part of this unit.
ASIC7
The main input to ASIC7 are the two independent pressure
signals combined on two lines from ASIC5 called TX1 and TX2.
The ASIC7 converts in two independent counter sections the
pulse widths to two sets of five 24 bits numbers (A/D conversion).
The pulse-width information are stored in two different RAM
locations and used by the µP to perform all the necessary
calculations and consistency checks and to calculate the
compensated output with correct scaling. Afterwards the µP
writes the calculation results into two 8 bit registers in ASIC7.
Watchdog1
A watchdog function is implemented in the ASIC7. It interacts
with the µP via the Reset block described later. In case of error
at first the watchdog resets the µP. After three retries it drives
the PWM output in alarm condition (UP/DOWN scale).
HART Modem
A modem circuit for demodulation is implemented in the ASIC
both for receiving and transmitting.
µ
P
The µP performs all the calculations and the diagnostic functions.
It drives also the supplementary shut down in case of errors in
the analog part.
Reset
There are four reset functions for the µP. "Power On Reset",
reset on ASIC7 request as described above and reset on
power supply too low or too high.
PWM Filter
This first order RC filter gives an average value of the pulse
width signal from ASIC 7.
T5 Time Generator
The ASIC7 generates a temperature dependent current (+0.33%
/ °C) applied to the T5 generation circuit that provides a time
duration signal (T5) used to measure the temperature in the
secondary electronics.
Watchdog2
A secondary watchdog is used to monitor the correct working
µ
P"
. . . PRINCIPLE OF OPERATION
of the main clock. In case the main clock doesn't work it gives
a signal to the supplementary shutdown logic block that provides
to force the output in safety condition.
4-20 mA reading
The internal microprocessor 8 bit A/D converter provides to
convert to a digital value the analog feedback signal from the
4÷20 mA output current loop. The obtained value is internally
compared with the digital value of the actual output current for
diagnostic purposes.
EEPROM2 Memory
The EEPROM2 memory is used by the µP to store and read
configuration data and data concerning calibration of the 4-20
mA generator.
Secondary Electronics "PS"
This unit contains the 4-20 mA transmitter, the power supply
and the basic analog part of the "HART" protocol.
Constant Current Generator 0-16 mA
This block converts the filtered DC voltage representing the
pressure into a 0-16 mA current. The block is trimmed together
with the power supply to maintain the stable 4 mA basic current
making the total current 4-20 mA.
Constant Current Generator 3.6 mA
This block generates a stable current basically close to 4 mA.
The current generator is also used by the HART protocol in the
transmit mode to generate a +/- 400 µA current. Externally this
will generate a +/- 100 mV signal across a 250 ohm minimum
resistor.
Local keys
The pressure transmitter has two screws hidden under the
nameplate. They can be used for setting ZERO and SPAN
values in the unit. The screws turn a magnet, that closes a reed
rely, that activates the µP. The ZERO gives the present sensor
value as the reference point. The SPAN gives the present
sensor value as FULL SCALE value. The screws with the
magnet can be removed to prevent unauthorized changes.
DC/DC converter
The input voltage is stabilized and regulated with a DC-DC
converter to provide the circuit power supply.
HART Rx
The HART modem receiving data are pre-filtered and buffered
in this block.
Over voltage detection
The power supply is continuously monitored. In case the
voltage exceeds a fixed dangerous value a reset command to
the microprocessor is generated.
Second shut-down
An additional current generator in parallel to the output regulator
allows having an independent shutdown of the output signal. In
case of a failure of the microprocessor due to a clock failure or
of a failure in the analog output stage the second shut-down is
activated forcing in this way the output signal to the up-scale
alarm value.
÷
4
20 mA reading
A buffer amplifier connected to the microcontroller reads a
voltage in the analog output stage proportional to the PWM
filtered voltage. It constitutes a feedback signal of the output
current.
Display
Optional. Not Safety relevant.
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