Siemens SITRANS TK Bedienungsanleitung Seite 36

7NG3120, 7NG3121, 7NG3122
1.3 Operating principle
The measured signal supplied by a resistance-based sensor (2, 3 or 4-wire
connection) or by a thermocouple is amplified in the input stage. The voltage,
which is proportional to the input quantity, is then converted to digital signals in an
analog/digital converter ¡. Via electrical isolation ©, these signals are forwarded
to the microprocessor. In the microprocessor ¢ they are converted in accordance
with the sensor characteristic and further parameters (measuring range,
attenuation and ambient temperature etc.).
The signal prepared in this way is converted in a digital/analog converter £ to an
impressed direct current of 4 to 20 mA. The auxiliary power source is located in the
output signal circuit.
Sensor
TC
¡
RTD
I
Analog/digital converter
¡
Electrical isolation
©
Microcontroller
¢
Digital/analog converter
£
Auxiliary power source
¤
PC/Laptop
¥
Interface (HART modem) for SIMATIC PDM
¦
Figure 1 Block diagram: operating principle of the SITRANS TK-H
The transmitter is parameterised and operated with a PC ¥ that is connected to
the two-wire line via the interface ¦. A hand-held communicator can also be used
for this purpose. The signals needed for communication in conformity with the
â
HART protocol V 5.7 are superimposed on the output current in accordance with
the frequency shift keying (FSK) method.
The data specific to the transmitter and the parameterisation data is stored in a
non-volatile memory (EEPROM).
36
SITRANS TK-H
¢
©
#
Auxiliary power
¤
£
#
I
¥
Instruction Manual
Load
¦
HART
modem
SITRANS TK/TK-H
C79000-B7174-C12-07