Rheonics SMET provides clients HART interface to get digital data over the traditional 4-20 mA channel. To enable HART data communication, the Rheonics SMET uses an external HART communication module. This document provides guidance for field connection by an end user.
Figure 1: Schematic of HART connection to Sensor Electronics (SMET)
Figure 1 shows connection of the SMET to the external system at a customer location. These are the external 24V DC power supply, PLC with 4-20mA input with HART communication and connection for the HART communicator which is typically used by customers in field to check and configure the HART device.
The SMET is delivered with 4 connection blocks labeled GND, 24V, GND-S and 4-20mA.
GND and GND-S are for grounding to the power supply and PLC respectively. It is important to note that the GND & GND-S are connected in the SMET so care must be exercised to ensure there is no ground loop due to grounding mismatch between the PLC and external power supply. It is advised that external power supply and PLC share the same ground.
Live wire (+) from Power supply is connected to the available terminal on the connection block labeled 24V. Ground (-) from power supply should be connected to the available terminal on the connection block labeled GND.
For 4-20mA and HART connection, the 2nd channel on the 4-20mA on SMET is used. Connect the ground (-) from the PLC 4-20mA input to the available terminal on the GND-S. Connect the positive (+) wire from the PLC 4-20mA channel to the 4-20mA labeled connection block. Use the available terminal on that connection block.
This completes the connection of the SMET to power and PLC for the 4-20mA channel and HART.
Connect the remaining two 4-20mA channels to the PLC if required. Refer to Connecting 4-20 mA outputs for instructions on connecting standard 4-20mA outputs from the SMET to your PLC.
The HART communicator or the HART modem in connected in parallel to the 250 Ω resistor in the 4…20 mA line.
Note: SMET 4-20mA outputs are powered from SMET so no external power is required to power the 4-20mA channels on the SMET.
Device variables are set into the HART module with units.
Primary Variable (PV)
Secondary Variable (SV)
cP (mPa. S)
Third Variable (TV)
Forth Variable (FV)
Table 1: The HART variables and 4-20mA measurements.
Figure 3 and Table 1 shows the HART variables that are transmitted digitally over the second 4-20mA channel of the SMET.
Figure 2 shows the 4-20mA channels 1-3 which carries the analog signals. Channel 2 in addition also carries HART signal.
Figure 2: SMET showing the 4-20 mA output channels..
Figure 3: The HART variables and 4-20mA measurements.
HART communicator connection and Resistor
For the connection of a HART modem or HART communicator with the SMET, a resistor of the value ~250Ω is needed.
Certain settings on the SMET are fixed and should not be changed to avoid compromising HART communication. These are:
- Modbus slave address in the SMET is set and should not be changed.
- 4-20mA channel 2 is set to density in SMET with 4-20mA range set to 0 – 1-5g/cc – these limits should not be changed as it sets the primary variable range as well.
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