【Daily Question 20090419】Use of the hand controller

The resistor is connected in parallel with the output wire of the hand controller; it can convert current signals into voltage signals

A resistor must be added during offline calibration of the transmitter to provide the load resistance required for communication between the circuit and the communicator; this value is usually 250 ohms, and the resistor is connected in series within the circuit.

The transmitter has a resistance R (250 ohms–600 ohms) in series with the power supply. If this resistance is too low, communication cannot take place; if it’s too high, the transmitter cannot function.

O(∩_∩)O~ I don’t know; I was just demoted to a security guard today. Scuba diving*, O(∩_∩)O~

A resistor must be used if the line resistance is less than 250; otherwise, communication is not possible.

I’ve seen that the required loop resistance for the HART protocol is 250–1100Ω; I’m not sure at what value it becomes unusable, as I haven’t tried it yet. Anyway, when using 375 for tuning, a 250-ohm resistor is usually connected in series with it. As for why resistors are used, it is because the HART communication protocol is a digital communication protocol – 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.

HART (Highway Addressable Remote Transducer) is an open communication protocol for addressable remote sensors; it was developed by the American company Rosement in 1985 as a communication standard used between field smart instruments and equipment in control rooms. HART devices provide communication with a relatively low bandwidth and moderate response time. After more than 10 years of development, HART technology has become highly mature abroad and has emerged as the industry standard for smart instruments worldwide. The HART protocol uses FSK frequency-shift keying signals based on the Bell202 standard; it overlays an audio digital signal with an amplitude of 0.5 mA on the low-frequency 4-20 mA analog signal to enable two-way digital communication, with a data transfer rate of 1.2 Mbps. Since the average value of an FSK signal is 0, it does not affect the magnitude of the analog signal transmitted to the control system, ensuring compatibility with existing analog systems. In HART protocol communication, the main variables and control information are transmitted via 4-20mA; when necessary, additional measurement data, process parameters, device configuration, calibration, and diagnostic information are accessed through the HART protocol. HART communication uses a half-duplex mode of transmission; its feature is that it enables digital signal communication over existing analog signal transmission lines. It represents a transitional solution in the shift from analog systems to digital systems, and therefore possesses strong market competitiveness during this period of transition, which has led to its rapid development. HART specifies a series of commands that operate in a command-based manner. It has three types of commands. The first type is called general commands, which are commands that all devices understand and execute ； The second category is known as general behavior commands; the functions they provide can be implemented in many field devices (though not all of them). These commands include a library of functions commonly used by field devices ； The third category is known as special device commands, which are designed to enable special functions in certain devices. Such commands can be made available for use by the community, or they can be exclusive to the company that developed them. In a field device, these three types of commands are usually found to exist simultaneously. HART uses a unified device description language, DDL. Field device manufacturers use this standard language to describe the characteristics of devices. The HART Foundation is responsible for registering and managing these device descriptions, compiling them into a device description dictionary. Master devices utilize DDL technology to understand the characteristic parameters of these devices, thereby avoiding the need to develop dedicated interfaces for them. However, due to this analog-digital mixed-signal format, it is difficult to develop a communication interface chip that can meet the requirements of various companies. HART can utilize bus power, meets intrinsic safety explosion-proof requirements, and can form a dual-master system with a handheld programmer and a management system host as the main devices. The HART communication protocol is a digital communication protocol; it represents a method for transmitting digital signals. An industrial standard for a communication protocol between intelligent field devices and control systems, initiated by ROSEMOUNT; it is the most widely used communication protocol in the process industry. During transmission, the current source acts as a carrier for the display signals, transmitting them; if it exceeds the specified range, the communication current becomes too weak to accurately transmit the “1” and “0” signals to the terminal. HART instruments are smart instruments with HART functionality.

1. If the impedance in the circuit is less than 250 ohms, it will affect the communication with the handheld controller； 2. If the impedance in the HART circuit is less than 250 ohms, an additional resistor is required ； 3. When installing a temporary optional 250-ohm load impedance, it can be done as follows: (1) Insert the load impedance into the lead socket ； (2) Open the circuit to connect the impedance in series with the circuit ； (3) Use lead terminals to close the circuit.

When calibrating the transmitter offline using BT200 and 375, we always add a 250-ohm load resistor.

When communicating using a HART handheld device, a sufficient resistor should be connected in series in the circuit, with the ends of the communicator connected to the ends of this resistor. As long as the resistance is within the specified range, the communicator can detect the digital signal and establish communication. If the loop resistance does not meet the requirements, it needs to be increased; if the resistance is too high, it will affect the voltage across the transmitter and also cause communication failures.

Summary of answers: 1. The required loop resistance for the HART protocol is 250–1100Ω; if the impedance in the loop is less than 250 ohms, it will affect communication with the handheld device； 2. When the impedance in the circuit is less than 250 ohms, an additional resistor is required; this resistor can be connected in series at various points in the circuit, but the hand controller must be connected in parallel with the resistor ；