HART Protocol

HART Protocol

The HART (Highway Addressable Remote Transducer) Communication Protocol directly contributes to these business goals by providing cost savings in:

• Commissioning and installation
• Plant operations and improved quality
• Maintenance

Products that use the HART protocol to provide both analog 4-20A and signals provide flexibility not available with any other communication technology

The HART Communication Protocol is based on the Bell 202 telephone communication standard and operates using the frequency shift keying (FSK) principle.

The digital signal is made up of two frequencies: 1,200 Hz and 2,200Hz representing bits 1 and 0, respectively.

Sine waves of these two frequencies are superimposed on the direct current (dc) analog signal cables to provide simultaneous analog and digital communications as. Because the average value of the FSK signal is always zero, the 4-20mA analog signal is not affected.

HART devices can operate in one of two network configurations

• Point-to-Point or
• Multi-drop

Point-to-point

In point-to-point mode, the 4-20mA signal is used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digitally using the HART Protocol as shown in fig.

The 4-20mA analog signal is not affected by the HART signal and can be used for control. The HART Communication digital signal gives access to secondary variables and other data that can be used for operations, commissioning, maintenance and diagnostic purposes. 

Multi-drop

The multi-drop mode of operation requires only a single pair of wires and, if applicable, safety barriers and an auxiliary power supply for up to 15 field devices (HART 5) or 62 field devices (HART 7) as shown in fig.

All process values are transmitted digitally. In multi-drop mode, all field device polling addresses must be unique in a range of 1-63 (depending on the HART Protocol Revision) and the current through each device is fixed to a minimum value (typically 4mA).

Use multi-drop connection for supervisory control installations that are widely spaced such as pipelines, custody transfer stations, and tank farms.

HART Protocol Benefits

• Increase Plant Availability
• Proven in practice, simple design, easy to maintain and operate
• Compatible with conventional analog instrumentation
• Simultaneous analog and digital communication
• Option of point-to-point or multi drop operation
• Flexible data access via up to two master devices
• Supports multivariable field devices
• Sufficient response time of approx. 500 ms
• Reduce Maintenance Costs
• Worldwide support by leading suppliers