HART field communications protocol (Computer network protocol)

The HART Communication Protocol (Highway Addressable Remote Transducer) is a hybrid analog+digital industrial automation open protocol. Its most notable advantage is that it can communicate over legacy 4–20 mA analog instrumentation current loops, sharing the pair of wires used by the analog-only host systems. HART is widely used in process and instrumentation systems ranging from small automation applications up to highly sophisticated industrial applications. Based on the OSI model, HART resides at Layer 7, the Application Layer. Layers 3–6 are not used. When sent over 4–20 mA it uses a Bell 202 for layer 1. But it is often converted to RS485 or RS232. According to Emerson, due to the huge installation base of 4–20 mA systems throughout the world, the HART Protocol is one of the most popular industrial protocols today. HART protocol has made a good transition protocol for users who wished to use the legacy 4–20 mA signals, but wanted to implement a "smart" protocol.