What is a two-wire system? Two-wire advantages

What is a two-wire system? What are the advantages of the two-wire system? The two-wire system means that only two wires are used for the connection between the field transmitter and the control room instrument. These two wires are both power lines and signal lines. Compared with the three-wire system (one positive power line, two signal lines, one of which is GND) and the four-wire system (two positive and negative power lines, two signal lines, one of which is GND), two The advantages of the wire system are: 1. It is not easily affected by parasitic thermocouples and resistance voltage drop and temperature drift along the wire, and very cheap and thinner wires can be used; a lot of cables and installation costs can be saved; 2. Output resistance in the current source When it is large enough, the voltage induced in the wire loop by the magnetic field coupling will not have a significant impact, because the current caused by the interference source is extremely small, generally using twisted pairs can reduce the interference; two-wire and three-wire systems must be shielded The shielding layer of the shielded wire should be properly grounded. 3. Capacitive interference will cause errors related to the receiver resistance. For a 4-20mA two-wire loop, the receiver resistance is usually 250Ω (sampling Uout=1-5V). This resistance is too small to cause significant errors. Therefore, it can be The allowable wire length is longer and farther than the voltage telemetry system; 4. Each single reading device or recording device can be switched between different channels with different wire lengths, and there is no difference in accuracy caused by the unequal wire lengths , to achieve decentralized acquisition, the advantages of decentralized acquisition are: decentralized acquisition, centralized control.... 5. Using 4mA for zero level, it is very convenient to judge open circuit and short circuit or sensor damage (0mA state). 6. It is very easy to add one or two lightning protection and surge protection devices to the two-wire output port, which is conducive to safety lightning protection and explosion protection. In the two-wire transmission mode, the power supply, load resistance, and transmitter are connected in series, that is, the two wires transmit the power supply and output current signal required by the transmitter at the same time; in the three-wire mode, the positive terminal of the power supply and the positive terminal of the signal output are connected in series. terminal, they share a ground wire, and the current signal is obtained from the I+ and V- terminals. In the four-wire system, the power supply and the load resistance are connected to the transmitter respectively, that is, the power supply and the output signal of the transmitter are transmitted with two wires respectively. Three-wire and four-wire transmitters do not have the above advantages and will be replaced by two-wire transmitters. From the current transmitter to be installed on the power line of the field device when in use, and the monitoring system with the single chip as the core It is located in the monitoring room far away from the equipment site, and the two are generally tens to hundreds of meters or even farther away. The environment of the equipment site is relatively harsh, strong electrical signals will generate various electromagnetic interference, and lightning induction will generate strong surge pulses. Reliable transmission of small signals. The two-wire transmitter device converts the galvanic isolation of the power line of the field equipment into a standard current signal output of 4-20 mA that changes in a linear proportion, and then sends it to the input interface of the monitoring system through a pair of twisted pairs. At the same time, the 24V working power supply located in the monitoring system is also sent to the current transmitter. The measurement signal and power supply are simultaneously transmitted on the twisted pair, which saves expensive transmission cables, and the signal is transmitted in the form of current, which greatly enhances the anti-interference ability. Shaanxi Kaidi pioneered and successfully developed a two-wire externally attached liquid level gauge. The externally-mounted liquid level gauge and externally-mounted liquid level switch produced by the company are widely used in petroleum, chemical, pharmaceutical, electric power, metallurgy, military, storage and transportation and other industries. liquid level. Related product links: 31.html
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