Hart Devices - Siemens Simatic RTU303xC Series Operating Instructions Manual

also need to be attached in order to guarantee a minimum differential voltage of 200 mV with
a high-resistance bus. The fail-safe resistors are usually attached at the bus master.
The HART/RS-485 extension board features fail-safe resistors that can be connected
internally. In contrast, the termination resistors need to be attached externally to the two bus
ends.
However, the fail-safe resistors also increase the power requirements and, for this reason,
these resistors are switched off outside of the update cycle of the RTU. For this reason,
communication of the remaining bus components is only possible with restrictions in sleep
mode of the RTU, for example.
Note
Connection to terminals
• The RS-485 connections are not protected against the connection of 24 V DC at the
terminals "A" and "B". The connection can lead to a defect in the expansion card or the
RTU.
• For fault-free operation of externally supplied bus devices, the potential connections of
the devices must be connected to the reference potential of the RTU. The "CTRL M"
terminal is provided on the extension board for this.
Figure 3-11
3.8.2

HART devices

Connecting HART devices
The expansion card allows the connection of up to 8 sensors (e.g. analog sensors for flow
and level measurement) via the HART Multidrop protocol to the RTU. Several HART devices
can be connected in HART Multidrop mode. Note the following:
● The HART addresses of the connected devices must be unique.
● The current must be set to 4 mA.
RTU303xC
Operating Instructions, 06/2019, C79000-G8976-C382-06
Connection of Modbus devices to extension board
Connecting up, installation, commissioning
3.8 Connecting devices to the extension board
81