ABSTRACT
In the early 1980s, Honeywell introduced the concept of superimposing a digital signal upon the 4-20 mA current loop system for their field devices (sensors, actuators, etc.). This is believed to be the birth of the smart field devices. In manufacturing and process industries, the number of pointto-point connections between master (supervisory) and field devices has increased.2 Each new device has its own principles of communication in terms of transmission rate, messaging format, and medium of communication. Handling of interoperative communications has been more complex. The problem was then tackled by borrowing ideas from computer networking. There was a need to introduce one serial communication link to which the existing field devices could be connected. This was the concept behind the INS that led to the fieldbus standard in the mid-1980s.3
