General instruments because many of them are 4 ~ 20mA analog signals, sometimes they are widely distributed in the field. If you go to the field to adjust the meter in the traditional way, it is very troublesome. At this time, the intelligent instrument will produce communication requirements. As we all know, the general 4 ~ 20mA signal transmission is relatively far (within 1000m), so it is better to superimpose the signal on it. A resistor of 250 ohms or more (about 230~600) can be connected in series at the PLC end, and then connected to the terminal, the current information of the instrument can be read remotely, and the instrument can be set with parameters, zero point and designated point calibration. The traditional serial signal is limited by the distance, generally not used, the use of words is also the use of DCS.
some intelligent valve positioner can also use HART communication.
If necessary, you can choose when selecting the instrument. Generally, you can choose hart or pulse, but pulse is not very useful. E H's devices basically support the HART protocol.