In many applications of PLC control, transmitters are used to convert the signals that PLC can not directly measure into electrical analog signals that PLC can process, such as current transmitter, pressure transmitter, temperature transmitter, flow transmitter, etc.

The early transmitters are basically voltage output type, that is, the measurement signal is converted into 0-5V voltage output, the signal power is less than 0.05W, and the digital signal is converted by analog / digital conversion circuit for PLC to read and control. However, when the signal needs long-distance transmission or in the environment where the power grid interference is large, the voltage output type Displacement sensor The two-wire current output transmitter has been widely used because of its high anti-interference ability.

The anti-interference ability of the voltage output transmitter is very poor, the line loss is damaged, and the accuracy is impossible to get anywhere. Sometimes, the output DC voltage is also superimposed with AC components, which makes PLC misjudge, control errors, and even damage the equipment. The output of 0-5V can not be transmitted remotely. After remote transmission, the line voltage drop is large, and the accuracy is greatly reduced. At present, many input signal ports of ADC, PLC and DCS are made into two-wire current output transmitter of 4-20mA, which proves the inevitable trend of voltage output transmitter being eliminated.