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The two-wire, three-wire, and four-wire systems that Yikongjun is discussing with everyone today refer to the differences in working principle and structure among various transmitters that output analog DC current signals; they do not merely refer to the wiring methods of these transmitters. First, let’s take a look at their definitions. Two-wire system: It involves two wires, which are used both for transmitting power and signals; in other words, the load connected to the sensor and the power supply are connected in series. The power supply is supplied from an external source and, together with the load, drives it. Three-wire system: In a three-wire sensor, the positive terminal of the power supply is separated from the positive terminal of the signal output, but they share a common COM terminal. 4-wire system: two wires for power and two wires for signals. The power supply and the signal operate separately. The terms for multiple-wire systems came into use after the advent of two-wire transmitters. This is the result of the widespread use of electronic amplifiers in instruments; the essence of amplification is a process of energy conversion, which relies on power supply. Therefore, the first to appear were four-wire transmitters ； That is, two wires are responsible for supplying power, while the other two wires are responsible for delivering the amplified signal that has been converted (such as voltage, current, etc.). However, at present, many transmitters use a two-wire system. Next, let’s take a closer look at the differences between transmitters of different wiring types Differences between transmitters with different wiring systems: 1. Two-wire system: To use a two-wire transmitter, the following conditions must be met simultaneously: 1. V ≤ Emin – Imax·RLmax. The output voltage V of the transmitter is equal to the specified minimum supply voltage minus the voltage drop caused by the current across the load resistance and the transmission wire resistance. 2. I≤Imin: The normal operating current I of the transmitter must be less than or equal to the transmitter’s output current. 3. P＜Imin(Emin-IminRLmax): The minimum power consumption P of the transmitter must not exceed the value given by this formula; it is usually less than 90 mW. In the formula: Emin = minimum supply voltage; for most instruments, Emin = 24(1–5%) = 22.8 V, with 5% representing the allowable negative variation for a 24 V supply ； Imax=20mA ； Imin=4mA ； RLmax=250Ω+resistance of the transmission wire. If the transmitter meets the above three conditions in its design, two-wire transmission can be achieved. The so-called two-wire system means that the power supply and the load are connected in series, sharing a common point; the signal communication as well as the power supply between the field transmitter and the instruments in the control room are accomplished using just two wires, which serve both as power lines and signal lines. In two-wire transmitters, the current at the signal start point is 4mA DC, which provides a static operating current for the transmitter. Meanwhile, the electrical zero point of the instrument is also 4mA DC, and it does not coincide with the mechanical zero point. This \"living zero point\" facilitates the detection of faults such as power loss and wire breaks. Moreover, the two-wire system facilitates the use of safety barriers, contributing to safety and explosion prevention. Figure 1: Schematic diagram of the wiring for a two-wire transmitter. As shown in Figure 1, a two-wire transmitter is powered by 24V DC; its output signal is 4-20mA DC, with a load resistance of 250Ω. The negative terminal of the 24V power supply has the lowest potential and serves as the common signal line. For intelligent transmitters, an FSK modulated signal based on the HART protocol can also be added to the 4-20mA DC signal. II. Three-wire system: Some instrument manufacturers, in order to reduce the size and weight of transmitters, improve their resistance to interference, and simplify wiring, switch the power supply for these transmitters from 220V AC to low-voltage DC power. If the power is supplied by a 24V DC power supply unit, the low-voltage power supply facilitates the use of a common negative wire, which leads to the development of three-wire transmitter products. Figure 2: Schematic diagram of the wiring for a three-wire transmitter. As shown in Figure 2, a three-wire transmitter uses one wire for the positive power supply terminal, another wire for the positive signal output terminal, and one shared wire for both the negative power supply terminal and the negative signal terminal. Its power supply is typically 24V DC; the output signal can be 4-20mA DC with a load resistance of 250Ω, or it can be 0-10mA DC with a load resistance of 0-1.5KΩ ； Some also have mA and mV signals, but the load resistance or input resistance varies depending on the type of output circuit. III. Four-wire system: Due to the widespread use of the 4-20mA DC (1-5V DC) signal standard, uniformity in signal formats is required in control systems to facilitate connections. As a result, certain instruments that are not electrically driven, such as those used for on-line analysis, measurement of mechanical or electrical parameters, etc., need to be able to output signals in the 4-20mA DC format. However, due to complex conversion circuits and high power consumption, it is difficult for these instruments to meet all three requirements for two-wire transmitter design; hence, they cannot operate in a two-wire system. Instead, an external power supply must be used to create four-wire transmitters that can output 4-20mA DC signals. Figure 3: Schematic diagram of the wiring for a four-wire transmitter. As shown in Figure 3, four-wire transmitters are typically powered by 220V AC, although some are also powered by 24V DC. The output signal can be 4-20mA DC with a load resistance of 250Ω, or 0-10mA DC with a load resistance of 0-1.5KΩ ； Some also have mA and mV signals, but the load resistance or input resistance varies depending on the type of output circuit. In the above three diagrams, the signal input to the measuring instrument is a current signal; if a resistor RL is connected in parallel, then a voltage signal will be received. As can be seen from the above description, due to the different working principles and structures of various transmitters, different products have been developed, which in turn determines the two-wire, three-wire, and four-wire wiring configurations for transmitters. For users, when making a selection, they should take into comprehensive consideration the actual conditions of their own organization, such as the standardization of signaling systems, explosion-proof requirements, specifications for receiving equipment, and investment factors. It should be noted that the 4-20mA DC signals output by three-wire and four-wire transmitters, due to the differences in their output circuit principles and structure compared to two-wire transmitters, raise the question of whether their negative output terminal can be connected to the negative wire of a 24V power supply in practical applications Can we share the same space? This is something to keep in mind; if necessary, isolation measures can be taken, such as using distributors and safety barriers, in order to share power and ground with other instruments and to avoid additional interference. Converting from two-wire to four-wire configuration: If a four-wire transmitter that transmits signals in the form of 0-10mA DC is to be converted to a two-wire configuration, the first problem that arises is that its starting current is zero. With no current flowing, the electronic amplifier of the transmitter cannot establish a operating point, which makes it difficult for the transmitter to function properly. If a DC power supply is used and the instrument’s original constant current characteristic is to be maintained, when the load resistance of the transmitter is between 0–1.5 KΩ and the feedback coil resistance connected in series is around 2 KΩ, the voltage drop across these two components will exceed 24 V when the output current is 10 mA. In other words, it is impossible to maintain a constant current characteristic when using a 24 V DC supply with a load resistance of 0–1.5 KΩ; therefore, two-wire transmission is not feasible in such a scenario. In the 1970s, some instrument manufacturers worked on converting four-wire transmitters that operated with 0–10mA DC signals into two-wire transmitters. The approach involved modifying the original transmitter circuit and raising the supply voltage to 48VDC; however, the starting current of the transmitter still could not be zero. To address this issue, a negative current was used to counteract the 4mA output current across the load resistor. But such products also failed to gain promotion and application. There is no need to change from a two-wire system to a four-wire system; moreover, it represents a technical regression.