Features of HART Communication Protocol

HART is an acronym for (High Addressable Remote Transducer). HART is implemented in two ways in industrial instrumentation networks:

• Point to point system
• Multidrop system

HART Point to Point System

Here serial communication data is superimposed onto a standard 4-20 mA loop signal as shown in figure 1(a):

Frequency shift keying (FSK) is used with frequencies of 1200 kHz for a ‘’1’’ and 2200 kHz for a ‘’0’’. These frequencies are far too high to affect the analogue instrumentation so the analogue signal is still used in the normal way. The system operates with one master (usually a computer, PLC or handheld programming terminal) and one slave (a transducer/transmitter or actuator). The advantage of this system is that allows HART to be retrofitted onto existing cabling and instrumentation structures. The disadvantage is that the full cabling benefits of a fieldbus system are not attained.

The serial data allows much more information to be conveyed in addition to the basic analog signal. The HART devices can all be remotely configured and monitored allowing very simple diagnostics and quick replacement after the failure.

In HART communication much more plant data can be passed from the transducer/transmitter for example a flow transmitter giving flow as 4-20 mA analogue signal can also give temperature and pressure via HART link along with diagnostic information about the flow transmitter status.

With HART programming terminal connected to the line, the transmitter can also be made to send fixed currents to aid in loop checks and fault diagnostics.

Related: Basic Features of Foundation Fieldbus (FF)

HART Multidrop System

In this system, each device has an identifying address and sends data to the master on request. This system is illustrated below in figure 1(b)

Normally devices and the relevant parameters are polled on a regular cyclic scan. The multidrop system brings all the cost savings from simple cabling and makes the system easy to modify and expand.

Related: How RS-232, RS-422 and RS-485 Networks are applied in Instrumentation

Shortcomings of HART communication protocol

HART communication protocol is rather slow when compared with other fieldbus systems like Foundation fieldbus and Profibus and this slow response time can cause difficulties in some industrial applications. The HART speed is generally adequate for simple monitoring systems where the process variables don’t change quickly.