Communication Format - Siemens 7MF4333 User Manual

UMSITRAPDS3-1
1 Measuring cell
2 Pressure connection
3 Seal diaphragm
4 Oil fill
5 Absolute pressure
p sensor
e
Input variable pressure
FIGURE 8-6 Absolute Pressure Measuring Cell, Gauge Construction

8.7 COMMUNICATION FORMAT

The transmitter communicates, via the HART protocol, with a HART Communicator and any HART
capable Primary Master controller connected to the network.
HART communication uses phase-continuous frequency-shift-keying (FSK) at 1200 bits/sec and
frequencies of 1200 Hz (logic 1) and 2200 Hz (logic 0). HART communication is superimposed (AC
coupled) on the analog 4-20 mA signal. Because the digital signaling is high frequency AC, its DC
average is zero and does not interfere with analog signaling.
An active filter connected to the loop input receives HART transmissions. The filter effectively rejects
low frequency analog signaling and other out-of-band interference, preventing a compromise of the
digital reception. The filtered signal is applied to a zero crossing detector, which converts the filtered
information into clean pulses of uniform amplitude before introduction to the Bell 202 modem.
The modem receives and processes (e.g., demodulates) the serial FSK signal and outputs a signal to the
microcontroller where serial to parallel conversion is performed.
In response to the received signal, the microcontroller outputs a signal to the modem where it is
modulated and fed into the feedback circuit of a voltage-to-current converter for transmission over the
loop.
November 2010
Circuit Description
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8-5