【Daily Question 20090209】Smart transmitters use the HART protocol; reasons for the required loop resistance and consequences of exceeding this range
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【Daily Question 20090209】Intelligent voltage transformers and current transformers use the HART communication protocol; the required loop resistance range is 250–1100Ω. Why? What happens if it goes beyond its scope? Summary: Since the HART protocol is a digital communication protocol, it represents a method of transmitting digital signals. During transmission, a current source is used as the carrier to transmit these signals; if the current level falls below the specified range, the communication current becomes too weak, and it is not possible to transmit the ‘1’ and ‘0’ signals accurately to the terminal.
1. Smart transmitters that use the HART protocol can experience operational failures due to issues such as carrier interference, excessive capacitance in the cables, high loop resistance, etc.; when the cables are too long, the smart transmitter fails to communicate at the installation site. The reason, analyzed, is that the cable capacitance is greater than the maximum capacitance allowed by the HART communication device. The HART communication frequency is relatively high (1-2 KHz), which requires that the T=RC value of the network not exceed 0.65 μs. Therefore, the maximum capacitance of HART communication devices and networks limits the maximum communication distance ； Due to the cable, the network time constant becomes too large, resulting in severe communication distortion and loss of communication; however, this has no effect on analog signals. The problem can be resolved by using 2.5sq cross-linked cables with a Cmster value of 90PF/M. 2. The HART protocol is a digital communication protocol; it represents a method of transmitting digital signals. During transmission, the HART signal is superimposed on a 4-20MA current source, which serves as the carrier for transmitting the signals. If the signal strength falls below the required level, the “1” and “0” signals cannot be transmitted accurately to the terminal. This post was last edited by jsnjhu on 2009-2-9 13:16.]
Intelligent voltage and current transformers use the HART communication protocol; the required loop resistance ranges from 250 to 1100Ω. Why is that? What happens if it goes beyond its scope? Answer: Because the HART protocol is a digital communication protocol, it represents a method of transmitting digital signals. During transmission, a current source is used as the carrier to transmit the signals; if the current level falls below the specified range, the communication current becomes too weak, and it is not possible to transmit the ‘1’ and ‘0’ signals accurately to the terminal.
The key is that the Hart transmitter needs to be able to distinguish between 0 and 1, which is why such resistance value requirements exist. For engineers, it’s sufficient to know how to use it; there’s no need to delve too deeply into it. The main focus should be on the process itself.:victory:
Agree with what was said on floor 1; it’s very comprehensive.