Siemens SIMATIC S7-1500 Manual page 20

Wiring
3.1 Wiring and block diagrams
Connection: 3- and 4-wire connection of resistance sensors or thermal resistors (RTD)
The example in the figure below shows the pin assignment for 3- and 4-wire connection of
resistance-based sensors or thermal resistors.
①
4-wire connection
②
3-wire connection
③
Analog-to-digital converter (ADC)
④
Backplane bus interface
⑤
Supply voltage via power supply element
⑥
Equipotential bonding cable (optional)
Figure 3-6 Block diagram and pin assignment for 3- and 4-wire connection
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CHx Channel or 4 x channel status (green/red)
RUN Status display LED (green)
ERROR Error display LED (red)
PWR LED for power supply (green)
Analog Input Module AI 4xU/I/RTD/TC ST (6ES7531-7QD00-0AB0)
Manual, 09/2016, A5E32366209-AC