HART Communication Protocol

HART Communication Protocol: A Detailed Description

1. Introduction

2. Protocol Basics

• Hybrid Nature: HART combines the existing 4-20 mA analog signal with digital communication. The 4-20 mA current loop is used to transmit the primary process variable (such as pressure, temperature, or flow rate) in an analog manner, which is compatible with legacy systems. At the same time, digital signals are superimposed on this analog current loop, enabling bidirectional communication between field devices (like sensors and actuators) and the control system.

• Frequency Shift Keying (FSK): The digital communication in HART uses Frequency Shift Keying. In this method, two frequencies are used to represent digital bits. Typically, a frequency of 1200 Hz represents a logical "1", and 2200 Hz represents a logical "0". These frequencies are modulated onto the 4-20 mA current loop without interfering with the analog signal's ability to represent the process variable.

3. Network Architecture

• Topology: HART supports a point-to-point topology as its most common configuration. In this setup, a single field device is connected to a host system (such as a control system or a handheld communicator) via a dedicated wire pair. However, it can also be used in multi-drop configurations where multiple devices are connected to the same communication line, although with some limitations in terms of the number of devices and the available power for each device.
• Nodes and Devices: A wide variety of field devices can use the HART protocol. These include pressure sensors, temperature sensors, flow meters, level sensors, and actuators such as control valves. Each device has a unique address assigned within the network, allowing for specific communication with that device by the host system. The number of devices that can be connected in a multi-drop configuration usually ranges from a few to several dozen, depending on factors like the power supply and the cable length.