Electrical connections
34 265Gx, 265Ax IM/265Gx/Ax-EN-07
5.7 Communication setup
(4 … 20 mA; HART protocol)
The transmitter can be operated via a modem, using a PC or laptop. The modem can be
connected to the transmitter in parallel at any point in the signal circuit. Communication between
the transmitter and modem occurs via AC signals that are superimposed on the analog
4 ... 20 mA output signal. This modulation occurs without averaging and does not, therefore,
affect the measuring signal.
Communication between the transmitter and PC or laptop is only possible if the signal circuit is
set up as shown in Fig. 12. The resistance between the connection point for the FSK modem
and the supply unit must be at least 250 Ω (including internal resistance of the supply unit). If
this value is not achieved within the context of normal installation, an additional resistor must be
used.
In the case of supply modules with HART communication, the additional resistor is often
installed permanently by the manufacturer. Some of these modules offer the option of
communicating directly over the supply module in the operating mode "FSK bus".
Power can be supplied by supply units, batteries, or power supplies; these must be designed to
ensure that the operating voltage U
B
of the transmitter always remains between 10.5 V DC and
45 V DC (for LCD displays with backlighting: 14 … 45 V DC).
In addition, the maximum current of 20 … 22.5 mA resulting from overranging must be
considered, depending on the corresponding parameterization. This yields the minimum value
for U
S
. If additional signal receivers (e.g., displays) are looped into the signal circuit, their
resistance must also be considered.
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