A very good article that provides basic knowledge about instruments (reposted from Industrial Control Network: kndt2007)

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8 Questions and Answers about Transmitters: What are the two-wire and four-wire signal transmission methods for transmitters? ......What is...... In the two-wire transmission method, the power supply, the load resistor, and the transmitter are connected in series; that is, two wires are used to transmit both the power required by the transmitter and the output current signal. Currently, most transmitters are of the two-wire type ； In the four-wire system, the power supply and the load resistor are connected to the transmitter separately; that is, the power supply and the transmitter’s output signal are transmitted using two wires each. ......Please see the eight questions and answers about transmitters. I. What is a two-wire current transmitter? What does two-wire mean? What are the advantages of the two-wire system? The two-wire system refers to a situation in which the field transmitter is connected to the instruments in the control room using only two wires; these two wires serve both as power lines and as signal lines. Compared to three-wire systems (one positive power line, two signal lines, one of which is common ground) and four-wire systems (two positive and negative power lines, two signal lines, one of which is ground), the advantages of the two-wire system are as follows: 1. It is less affected by parasitic thermocouples, resistance drops along the wires, and temperature drift, allowing the use of much cheaper and thinner wires ； It can save a large amount of cable and installation costs ； 2. When the output resistance of the current source is high enough, the voltage induced in the wire loop through magnetic field coupling has no significant effect, as the current caused by interference sources is very small; generally, using twisted pairs can help reduce such interference ； For two-wire and three-wire systems, shielded wires must be used, and the shielding layer of these wires should be properly grounded. 3. Capacitive interference can cause errors related to the receiver resistance. In a 4–20mA two-wire loop, the receiver resistance is typically 250Ω (with a sampled Uout of 1–5V). This resistance is low enough not to cause significant errors; as a result, the allowable length of wires is greater than that in voltage telemetry systems ； 4. Each individual reading or recording device can be connected to different channels with wires of varying lengths, without any difference in accuracy resulting from these length differences; this enables decentralized data collection. The advantage of decentralized collection is that data is collected in a dispersed manner while control is centralized.... 5. Using 4mA as the zero level makes it very convenient to determine whether there is an open circuit, a short circuit, or if the sensor is damaged (0mA condition). 6. It is very easy to add one or two lightning and surge protection devices at the dual-output ports, which helps ensure safety against lightning and explosions. Neither three-wire nor four-wire transmitters possess the aforementioned advantages, and they are set to be replaced by two-wire transmitters. This can be inferred from industry trends abroad as well as the supply and demand for transmitter chips. Current transmitters need to be installed on the power lines of the equipment at the site of use, while monitoring systems based on microcontrollers are located in control rooms that are far away from the equipment, usually several dozen to several hundred meters away, or even further. The environment at the device site is quite harsh; strong electrical signals can cause various electromagnetic interferences, and lightning induction can generate strong surge pulses. In such conditions, a challenging issue in microcontroller-based systems is how to transmit weak signals over long distances reliably in such adverse environments. The emergence of two-wire transmitters has helped to solve this problem effectively. We designed a small, inexpensive piercing-type two-wire current transmitter based on the DH4-20 transceiver module as its core. It features a low offset voltage (<30μV), low voltage drift (<0.7μV/C°), and ultra-low non-linearity (<0.01%). It isolates the current in the power lines of the field devices and converts it into a standard current signal ranging from 4 to 20 mA, with a linear variation in value; this signal is then sent to the input interface of the monitoring system via a pair of twisted pairs. The same twisted pairs are also used to deliver the 24V operating voltage from the monitoring system to the current transmitter. The measurement signal and power supply are transmitted simultaneously over twisted pair cables, which eliminates the need for expensive transmission cables; moreover, since the signal is transmitted in the form of electricity, its resistance to interference is greatly enhanced. II. How is the 4-20mA output of a current transmitter converted? The output of a two-wire current transmitter is 4–20 mA, and this value is converted into an analog voltage signal of 1–5V or 2–10V through a precision resistor of 250 Ω. There are various methods for converting this to a digital signal; since the system is intended for long-term use in harsh industrial environments, it is necessary to consider the safety and reliability of the hardware system. The input module of the system uses a frequency conversion device, the LM231, to convert analog voltage signals into frequency signals, and an optocoupler, the TL117, to isolate analog signals from digital signals. At the same time, the analog signal processing circuit and the digital signal processing circuit each use two separate sets of power supplies, with the analog ground and the digital ground being separated from each other; this helps to improve the safety of the system’s operation. Using the voltage-to-frequency conversion device LM231 also provides a certain degree of protection against high-frequency interference. III. What are the advantages and disadvantages of current-output and voltage-output types? In many applications controlled by microcontrollers, transmitters are used to convert signals that cannot be measured directly by the microcontroller into electrical analog signals that can be processed by it, such as current transmitters, pressure transmitters, temperature transmitters, flow rate transmitters, etc. Most early transmitters were of the voltage-output type, that is, they converted the measured signal into a 0-5V voltage output; this was a direct output from the operational amplifier, with a certain signal power level