HART Communication is a bi-directional industrial field communication protocol used to communicate between intelligent field instruments and host systems. HART is the global standard for smart process instrumentation and the majority of smart field devices installed in plants worldwide are HART-enabled. The global installed base of HART-enabled devices is the largest of all communication protocols at more than 20 million. HART technology is easy to use and very reliable.

There are several reasons to have a host communicate with a field instrument. These include:
• Device Configuration or re-configuration
• Device Diagnostics
• Device Troubleshooting
• Reading the values of additional measurements provided by the device
• Device Health and Status
• And much more!

A host system can be a Distributed Control System, PLC, Asset Management System, Safety System or a handheld device.

HART is a master-slave field communications protocol developed in the late 1980's to facilitate communication with Smart field devices. HART stands for Highway Addressable Remote Transducer. The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA:

This enables two-way field communication to take place and makes it possible for additional information beyond just the normal process variable to be communicated to/from a smart field instrument. The HART protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a field device. As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal.

HART is a master/slave protocol which means that a field (slave) device only speaks when spoken to by a master. The HART protocol can be used in various modes for communicating information to/from smart field instruments and central control or monitoring systems.

HART provides for up to two masters (primary and secondary). This allows secondary masters such as handheld communicators to be used without interfering with communications to/from the primary master, i.e. control/monitoring system.

The HART protocol permits all digital communication with field devices in either point-to-point or multidrop network configurations:

Point- to Point Configuration

Multidrop Configuration

There is also an optional "burst" communication mode where a single slave device can continuously broadcast a standard HART reply message. Higher update rates are possible with this optional digital communication mode and use is normally restricted to point-to-point topologies.

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