Rockwell Automation 1794-APBDPV1 FLEX I/O PROFIBUS Adapter User Manual - Manual (Page 134)

121 Publication 1794-UM064B-EN-P - May 2014
Appendix
C
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
• Common practice and universal 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).
The master is responsible for controlling message transactions. If there is no reply
to a command within the expected time, the master should retry the message.