Rockwell Automation 1794-OF8IH FLEX Isolated Input/Output HART Analog Modules - Manual (Page 113)

101 Publication 1794-UM065B-EN-E - September 2010
Appendix
B
Additional HART Protocol Information
Overview 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.
Transaction Procedure
HART is a half-duplex protocol; after completion of each message, the FSK
carrier signal must be switched off, to allow the other station to transmit. The
carrier control timing rules state that the carrier should be turned on not more
than 5 bit times before the start of the message (that is, the preamble) and
turned off not more than 5 bit times after the end of the last byte of the
message (the checksum).