Instrument refers to the on-site measuring instrument

The concept of a transmitter is an instrument that converts a non-standard electrical signal into a standard electrical signal. A sensor is a device that converts a physical signal into an electrical signal. In the past, physical signals were often used, and other signals would also appear. The primary instrument refers to the on-site measuring instrument or the base control table. The secondary instrument refers to the instrument that uses the primary signal to complete other functions: functions such as control and display.