The Highway Addressable Remote Transducer (HART®) protocol is used in many factory automation and control systems. This protocol uses the established 4-20mA loop to send digital signals between a smart transmitter and a host for data that can be used for control, monitoring, or safety. This application note starts with an overview of 4-20mA systems and HART transmitters and discusses the protocols for developing HART enabled devices. This application note also discusses different TI devices with HART connectivity.
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The Highway Addressable Remote Transducer (HART) protocol is a backward-compatible enhancement to 4-20 mA instrumentation that allows two-way communication with smart, microprocessor-based field devices.
For factory automation and control (FA&C) applications, 4-20 mA current loop communications are a common method to send data from a remote transmitter measuring a factory variable back to a host that receives and processes the data. The value of current is the primary variable sent by the remote transmitter. This primary variable represents some measure of temperature, flow, or pressure.
The HART signal is superimposed onto this 4-20 mA current loop and is modulated for two-way digital communications. The 4-20 mA loop is already a widely used standard in FA&C applications. Because this technology is backwards compatible and can be used with already-existing infrastructure means that HART is easy to adopt and cost effective.
HART is a command/response protocol where a host sends commands and a remote transmitter returns standardized responses. The data received by the commands can communicate device status and diagnostics. Data can also include the device measurement floating-point digital values, the engineering units of the primary variable, and other information about the remote transmitter.
The HART protocol can also be used for standardized operating procedures such as testing the current loop, range setting of the current loop, and calibration information from the transmitter. HART modifies the 4-20 mA system that sends only the primary variable as a current value and adds digital communication with more functionality.