Communicating with plant machinery and instruments for control and measurement is the basis of industrial automation. The simplest communications technique – the earliest to be implemented — is a current loop with a normal range of 4 mA to 20 mA, employing a transmitter, a receiver and power supply. It allows transmission of process variable data. The current loop is reliable and highly immune to environmental interference over long communication distances. Little wonder, then, that it is still in wide use.
The obvious disadvantage, however, is that a single loop allows only one-way communication from a sensor, or to an actuator.
The HART™ (Highway Addressable Remote Transducer) protocol was developed in the mid 1980s to enable two-way field communication of additional information — beyond normal process variables — to and from a smart field instrument. The HART protocol uses the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital signals at a low level on top of the 4-20mA. Simply put, dc and low-frequency current signals are modulated by independent, higher-frequency signals that switch between a pair of frequencies.
Today, the HART protocol is the global standard for sending and receiving digital information across analog wires between smart devices and control or monitoring system, due, in no small part to the huge installed base of 4–20 mA systems.
The key to a successful HART implementation is the ability to accurately encode and decode HART communication signals in noisy, harsh industrial environments.
This application note describes methods for configuring the Texas Instruments’ DAC8750, DAC8760, DAC7750, and DAC7760 for HART™ communication. The DAC87x0 is a family of single-channel, 16-bit DACs that inherently meet the requirements of the “Carrier Transients” and the “Output Noise during Silence” tests that are required for a HART-compliant system.