II - DCE FEL ČVUT v Praze

SIMATIC S7 HART Analog Modules
4.2.1 How Does HART Function?
Introduction
HART signal
4-4
20 mA
Analog signal
4 mA
Fig. 4-2 The HART signal
HART commands
and parameters
0
The HART protocol describes the physical characteristics of transmission:
data transfer procedures, message structure, data formats, and commands.
Figure 4-2 shows the analog signal with the HART signal (FSK procedure).
The HART signal is composed of sine waves at 1200 Hz and 2200 Hz and
has a mean value of zero. It can be filtered out with an input filter, leaving
the original analog signal unaffected.
C
R = Response
C = Command
+0.5 mA
0
–0.5 mA
1200 Hz
“1”
Time (seconds)
2200 Hz
“0”
R C R C
The adjustable properties of the HART field devices (HART parameters) can
be set with HART commands and read using HART responses. The HART
commands and their parameters are defined in three groups with the
following properties:
Universal
Common-practice
Device-specific
Universal commands and their parameters must be supported by all
manufacturers of HART field devices; common-practice commands should
also be supported. There are also device-specific commands that apply to a
particular field device.
I/O Modules with Intrinsically-Safe Signals
C79000-G7076-C152-04
R