Protocolo hart

© 2005 HART Communication Foundation

Copyright © 1999-2005 HART Communication Foundation. All rights reserved. HART® is a registered trademark of the HART Communication Foundation. Any use of the word “HART” hereafter in this document implies the registered trademark. All other trademarks used in this document are acknowledged to be trademarks of their respective companies.

For additionalinformation contact:
HART Communication Foundation 9390 Research Boulevard Suite I-350 Austin, Texas 78759 USA Tel: 512-794-0369 Fax: 512-794-3904

HART APPLICATION GUIDE

Preface
In today’s competitive environment, all companies seek to reduce operation costs, deliver products rapidly, and improve product quality. The HART® (highway addressable remote transducer) protocol directlycontributes to these business goals by providing cost savings in: Commissioning and installation Plant operations and improved quality Maintenance The HART Application Guide has been created by the HART Communication Foundation (HCF) to provide users of HART products with the information necessary to obtain the full benefits of HART digital instrumentation. The HART communication protocol is an openstandard owned by the more than 100 member companies in the HCF. Products that use the HART protocol to provide both analog 4–20 mA and digital signals provide flexibility not available with any other communication technology. The following four sections provide you with an understanding of how the HART technology works, insight on how to apply various features of the technology, and specific examplesof applications implemented by HART protocol users around the world: Theory of Operation3 Benefits of HART Communications Getting the Most out of HART Systems Industry Applications

© 2003 HART Communication Foundation

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THEORY OF OPERATION

Theory of Operation
The following sections explain the basic principles behind the operation of HART instruments and networks: CommunicationModes Frequency Shift Keying HART Networks HART Commands

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© 2003 HART Communication Foundation

THEORY OF OPERATION

Communication Modes
MASTER-SLAVE MODE
HART is a master-slave communication protocol, which means that during normal operation, each slave (field device) communication is initiated by a master communication device. Two masters can connect to each HART loop. Theprimary master is generally a distributed control system (DCS), programmable logic controller (PLC), or a personal computer (PC). The secondary master can be a handheld terminal or another PC. Slave devices include transmitters, actuators, and controllers that respond to commands from the primary or secondary master. Some HART devices support the optional burst communication mode. Burst mode enablesfaster communication (3–4 data updates per second). In burst mode, the master instructs the slave device to continuously broadcast a standard HART reply message (e.g., the value of the process variable). The master receives the message at the higher rate until it instructs the slave to stop bursting.
Use burst mode to enable more than one passive HART device to listen to communications on theHART loop.

BURST MODE

© 2003 HART Communication Foundation

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THEORY OF OPERATION

Frequency Shift Keying
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 2,200 Hz representing bits 1 and 0, respectively. Sine wavesof these two frequencies are superimposed on the direct current (dc) analog signal cables to provide simultaneous analog and digital communications (Figure 1). Because the average value of the FSK signal is always zero, the 4–20 mA analog signal is not affected. The digital communication signal has a response time of approximately 2–3 data updates per second without interrupting the analog...