Hello Everyone,
Why i can not communicate by HART communicator on a HART supported instrument that is connected to PLC? But when i am connecting same instrument to DCS, then i can communicate with HART communicator.

 

1. The most common issue is loop resistrance. A HART loop requires a minimum of about 250 Ohms in order to develop sufficient signal strength. That resistance is usually supplied by the resistor across the (+) and (-) terminals on the analog input card. Many brands and models of AI cards use input resistance values less than 250 Ohms. To add a resistor to the loop, add a resistor in series in the loop, but not across the (+)/(-) terminals. The total resistance can be more than 250 Ohms, but not less than 250 Ohms.

2. If there is a loop powered panel meter or signal isolator in the HART loop, the unit can filter or strip the HART signal off the 4-20mA signal.

3. Are you sure the HART polling address on the transmitter is HART address 0? Address 0 is the default address for point-to-point communications with a HART communicator.

4. It is not a common problem, but the DC power supply can generate too much noise and ripple that interferes with the HART signal. A warning in a Rosemount manual states, "the power supply used, and circuitry common to the transmitters, should not have more than 20 ohms of impedance at 1200 Hz".