Two pictures to understand two-wire 4-20mA and four-wire 4-20mA

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Two pictures to understand two-wire 4-20mA and four-wire 4-20mA [Copy link]

For the module that receives 4-20mA analog signals, measure the voltage across the input when no analog signal source is connected. If there is a voltage of about 24VDC, it is a two-wire module, otherwise it is a four-wire module. This method is valid only when the other working conditions of the module are ready. For the instrument/transmitter/sensor (transmitting source) that sends 4-20mA analog signals, measure the voltage across the output when no analog signal receiving module is connected. If there is a voltage of about 24VDC, it is a four-wire module, otherwise it is a two-wire module. This method is valid only when the other working conditions of the analog signal sending source are ready. The two-wire 4-20mA analog signal receiving module can only be connected to a two-wire signal source; while the four-wire 4-20mA analog signal can be connected to a four-wire signal source, and by connecting a 24VDC power supply in series, it can also be connected to a two-wire signal source. For Siemens analog modules, the specific wiring is two-wire or four-wire, which can be changed in the module hardware configuration settings. The specific one to use depends on whether the external 4-20mA signal needs the Siemens analog module to provide 24VDC power to be connected in series to the 4-20mA loop. If the loop needs to be connected in series to the 24VDC of the analog module channel, choose the two-wire system, such as a transmitter, etc. (If the external 24VDC power supply is connected in series, choose the four-wire system, but this is generally not used, and Siemens does not recommend it for fear of burning its own module); there is another type, the direct incoming signal is a 4-20MA signal, and does not need to be supplied with 24VDC (generally secondary instruments are like this), in this hardware setting, choose the four-wire system. Siemens modules support both two-wire and four-wire systems, and the specific choice depends on whether the circuit needs the module to provide 24VDC. In fact, it is relatively easy to distinguish between two-wire and four-wire. Two-wire is the output signal of the field instrument, which needs to be powered by the PLC or DCS system. Four-wire is the power supply of the instrument itself, and does not require the power supply of PLC or DCS. Siemens AI module needs to define the category of channel signal. Two-wire 4-20mA yunrun.com.cn/tech/1135.html Here is a principle block diagram of the 4-20mA input card of the deltav system, I hope it will be helpful to everyone. Pay attention to the position of the 250 ohm resistor. Figure 1: Schematic diagram of the four-wire 4-20mA input card of the deltav system Figure 2: Schematic diagram of the two-wire 4-20mA input card of the deltav system