SMAR HI302, Руководство пользователя

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Installation
2.4
NOTE
The 250 Ω resistor (see the previous picture) in series with each equipment is the analog input
impedance, for example a PLC’s input, that reads the 4-20 mA signal. If the analog input
impedance is less than 250
Ω , connect a lower resistor, so that the association of both
impedances is at least 250 Ω .
For instance, suppose that a TT301 is connected to a PLC’s input with an impedance equal to
50
Ω . So, connect a 200 Ω resistor in series with the device’s feeding in order to enable the HART
communication. The user can also use a 250 Ω instead of a 200 Ω resistor.
HART Device Installation
Now we will describe the main communication features regarding the device installation.
For more detailed information about the devices, please read the specific device manual.
Concerning the HART Communication, take into consideration that the superimposition of
a modulated signal on an analog current signal can deteriorate, if some precautions are
not taken. It is important to mention that the HART communication does not affect the 4-
20 mA analog signal, since the average value of a FSK modulated signal is zero. Thus, if
the HART device is already installed, make sure that the minimum impedance (25
Ω ) is
used and connect the HI302 channel in parallel to the device.
Device Physical Types
Low Impedance Devices
Low impedance devices are typically signaling elements intended to receive current
analog signals or serve as master for a multi-drop network. As an example of a low
impedance device we can mention the FY301.
High Impedance Devices
High impedance devices control current, either as a mean of analog signaling or at a fixed
level in a multi-drop environment. As an example of a high impedance device we can
mention the LD301 and the TT301.