PLC current loop transmission

Application of the current plc temperature control module loop is started in the early 1900s teletype, the earliest use of the 0-60mA loops and later changed to 0-20mA loop, PLC system is the first to use the 4-20mA loop.
4-20mA current loop has many advantages. Need to be carefully calculated in the traditional discrete component designs, and compared with the current integrated 4-20mAIC, circuit take up a lot of space.
Any current values ​​measured are representative of certain information. Practical applications, 4-20mA current loop in 0mA to 24mA current range. But 0mA to 4mA and 20mA to 24mA current range for diagnostics and system calibration. Due to lower than 4mA and above 20mA are used for diagnostics, it is believed readings between 0mA and 4mA could indicate a broken wire in the system. Similarly, between 20mA and 24mA between readings can indicate a potential short circuit occurred in the system.
Plus 4-20mA communications Highway Addressable Remote Sensor, which is backward compatible with 4-20mA instrumentation. In HART system uses microprocessor-based intelligent integration of field devices enables two-way communication. The HART protocol capable of carrying additional digital information on the same line 4-20mA analog current signal for process control.
PLC functions can be divided into several functional groups. Many plc IO module vendors will integrate these functions as separate modules, each module with features vary with the specific application. Many modules have multiple functions, it can interface with a variety of sensors. However, for specific applications will design special modules or expansion modules, such as in most cases: resistance temperature detector (RTD), sensor, or thermocouple sensor. In general, all modules have the same core functions: analog input, analog output, distributed control (eg fieldbus), interface, digital input and output (I / O), CPU and power supply.