Allen-Bradley 1794-IE8H, User Manual, Page: 99 / 146

1 Publication 1794-UM063A-EN-P - March 2006
Appendix
C
Additional HART Protocol Information
What This Appendix
Contains
This appendix discusses the HART protocol and provides references for
additional information about the protocol. The appendix provides:
•
HART protocol background information
•
Command practice command sets
•
Extended command sets
•
References to additional information
Message Structure
This section describes the transaction procedure, character coding, and
message structure of the HART protocol. These correspond to layer 2, the
data-link layer, of the OSI protocol reference model.
Master-slave Operation
HART is a master-slave protocol. This means that each message transaction is
originated by the master; the slave (field) device only replies when it receives a
command message addressed to it. The reply from the slave device
acknowledges that the command has been received, and may contain data
requested by the master.
Multiple Master Operation
The HART protocol allows for two active masters in a system, one primary
and one secondary. The two masters have different addresses, therefore each
can positively identify replies to its own command messages.