4.2.8 How TH420 works
The functional principle is described below based on the function chart.
5
1
4
3
2
1 (+)
2 (-)
µC1
µC2
4 ... 20 mA U, I
out
SITRANS TH420
D
A
A
D
① LED ⑤ Galvanic isolation
② Extension port ⑥ Input
③ Test terminals A/D Analog-to-digital converter
④
Output μC1 Microcontroller, on the secon‐
dary side
μC2 Microcontroller, on the pri‐
mary side
D/A Digital-to-analog converter
U Supply voltage
I
out
Output current
Figure 4-8 Function chart for SITRANS TH420/TR420
• Input ⑥ for connecting dierent signal sources. Section: Input (Page 201).
• This signal is converted to a digital signal in an analog-to-digital converter (A/D).
• The digital signal is evaluated in a microcontroller (μC1) on the secondary side and corrected
to match the input characteristic.
• The digital signal is transferred via the galvanic isolation ⑤ to the microcontroller (μC2) on
the primary side.
• The analog output value is calculated in the microcontroller (μC2) on the primary side. The
functional status is indicated by LED ① and the communications data is prepared.
• The digital-to-analog converter (D/A) subsequently converts the signal into the output
current ④ of 4 to 20 mA.
Description
4.2 SITRANS TH320/TH420
SITRANS TH100/TH320/TH420/TR320/TR420/TF320/TF420 with 4 to 20 mA/HART
40 Operating Instructions, 05/2020, A5E41864807-AB
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