SMAR HI302, User Manual

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Installation
2.6
Notice that it is very important to keep the minimum impedance (25 Ω ) to allow HART
communication. Sometimes the voltage supply must be increased to ensure that the
system is in the operation area.
HI302-N (without Analog Conversion)
This HI302 option has only HART communication and no circuit for analog conversion.
The HI302-N does not have an internal resistor, so use an external resistor or an active
impedance (PSI301) if many devices are used.
•
Typical Multidrop
There are two ways to perform this connection. The resistor can be installed in series with
the power supply or in parallel to the HART channel. The first way is shown below:
•
Multidrop with 4-20 mA enabled
Be careful with this topology because some types of I/O devices do not accept the
connections presented below, for example, the SDCS’s that feed the devices via internal
power supply. The I/O device must receive external feeding, like PLC cards or field
devices. Besides its complexity, this connection allows an improvement in the use of the
HI302 channels and, as the current that flows in the loop is tenths of mA, use an active
impedance instead of a simple resistor. See the next picture: