Problems related to connecting a hart communicator to field device?

When we connect the hart communicator to field device it shows that the device is not found and what is the reason for this?

It could be due to several reasons such as

• It could be because the device description file missing, so add the latest DD file version of the instrument.

• Check the continuity in the hart cable

• You must check the transmitter configurations

• It could also be because of the faults in the transmitter or in other devices

• if you are getting power from the control room plz check the polarity and lose connection

• Check the continuity on the hart communication cable Ensure the transmitter is Hart enabled If bench setup uses 250 ohms in series for impedance matching. Check security switches in the transmitter which can also block communication Incase checking in JB. Ensure it is connected to the correct terminal In hart maximum Generic mode DD of each old model will support a new one. Else update dd in Hart.

For HART communication to be successful.

  • the transmitter needs to be powered
  • the transmitter needs to be HART. Some transmitters use other protocols and do not talk HART.
  • the loop needs 250 ohms of resistance. There are receivers whose analog inputs do not have 250 ohm dropping resistor. Those loops need to have a resistor added in series in the loop for the HART signal to be ‘seen’ by the HART communicator. This always applies to bench testing a HART transmitter.

Other considerations

  • HART ID number Every HART transmitter has a HART ID address number. Both devices need to use the same HART address ID number. Factories ship HART transmitters configured for address 0, but that ID address number can be changed.

Point-to-point HART communication using a communicator directly connected to a transmitter’s field terminals, uses the HART address 0 (zero). (Both the transmitter and the communicator are configured to use HART ID 0).

Numbers greater than zero are used for HART multidrop applications (the exception, not the rule). A field transmitter with an address other than 0 will output only 4mA, regardless of the PV value, because that is operational mode for multidrop.

If either device uses a different HART ID number (the communicator can be configured to talk to some ID number other than 0, which is the ‘normal’ ID number), HART comm will fail.

  • Some devices allow HART to be disabled. HART needs to be enabled to work.
  • the Test terminals on a DP pressure transmitter do not carry the HART signal. Connections need to be across the transmitter (+)/(-) terminals, not the Test terminals.
  • There is HART spec for power supplies. It is not common, but a power supply can filter out the HART signal.
  • A minimum voltage is needed to drive the HART signal. A transmitter will power up at its lift-off voltage, but there might not be sufficient voltage to drive a HART signal, if the loop resistance is too high or conversely the power supply voltage is too low. A wireless THUM adapter requires a higher operating voltage than wired HART.
  • the HART communicator’s probes can have corrosion on its probe clip or an open circuit in the wiring and cause a lack-of-connection
  • Another device in the loop like a repeater/isolator or a loop powered indicator, can ‘strip’ the HART signal off the 4-20mA signal. Any device in a 4-20mA loop using HART needs to be “HART enabled” so the HART signal is not stripped off.
  • electrical noise on the signal lines can disrupt the HART signal
  • a communicator is a HART master (field device is a slaver) and the HART protocol allows for two HART masters, but there can be conflicts if both attempt to operate simultaneously.
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