Regarding HART protocol communication
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Why do transmitters that support the HART protocol fail to communicate with a handheld device (that supports HART) if the transmitter is connected directly to 24V power? The transmitter, when placed on-site, can communicate with the handheld operator (which supports HART).
The resistance of the power supply circuit is low; connect a resistor of around 250 ohms in series.
The answer above is correct; a termination resistor is needed
For HART communication, the loop resistance must be greater than 230 ohms. Actually, by connecting it directly to 24V and placing a 250-ohm resistor in series with the power supply, communication is also possible. Communication is possible at the site because the resistance of the entire measurement circuit is greater than 250 ohms. The resistance of this circuit comes from the safety barrier, the sampling resistors of the cards, and so on……
On-site, we generally use a HART-enabled handheld device to adjust the meters. Are there any other widespread applications for the HART protocol?
A resistor of around 250 ohms should be added to the circuit; this may be necessary because the resistance already present in the circuit is greater than the required value.
Because the Hart protocol converts analog signals into digital ones, it must be used in conjunction with 4-20mA signals to function. Profibus and Fieldbus can be omitted, as they are purely digital.
A 250Ω resistor is required, but since the transmitter is connected to the DCSAI module and there is already wire resistance involved, communication is possible without using a 250Ω resistor.
Everyone has said that there needs to be a resistor in the circuit with a value of at least 250; connecting 24V directly means there is no resistor, so adding one will do. Hart – many of the devices used on site are from Hart. The Rosemount 475 can be used to calibrate not only instruments made by Hart itself, but also instruments of other brands, as well as valves and positioners; they’re all from Hart.
I agree with the opinion from the 4th floor: the transmitter needs to be removed for calibration; if there is no 250Ω resistance in the circuit, communication is not possible. In on-site circuits, communication can occur if the resistance of the safety barrier and the display circuit is 250 ohms.