Siemens SIMATIC ET 200iS Manual page 286

Analog Electronics Modules with HART
15.1.2 How Does HART Work?
Introduction
The HART protocol describes the physical form of the transfer:
Transmission procedures, message structure, data formats and commands.
HART Signal
The following schematic shows the analog signal with the HART signal
superimposed on it (FSK technique). The signal is composed of sine waves of
1200 Hz and 2200 Hz having an average value of zero. It can be filtered out using
an input filter so that the original analog signal is available again.
20 mA
Analog signal
C=command
4 mA
0
Figure 15-1 The HART Signal
15-2
+0.5 mA
0
-0.5 mA
1200 Hz
R
C
R=response
Time (seconds)
2200 Hz
"1" "0"
C
R
C
ET 200iS Distributed I/O Station
R
A5E00087831-02