Spectrum Controls 1734sc-IE2CH - Manual (Page 61)

User’s Manual Pub. 0300257-01 Rev. A
Appendix B
Additional HART
Protocol Information
This appendix describes the HART protocol and provides references for additional
information about the protocol. Consult the HART protocol specification and vendor
provided documentation for specifics on HART commands.
This appendix provides the following:
•
HART protocol background information
•
Common practice command sets
•
Extended command sets
•
References to additional information
HART Field Communication Protocol is widely accepted in the industry as the standard
for digitally enhanced 4…20mA communication with smart field instruments. The
HART Protocol message structure, command set, and status are discussed in this
appendix.
The HART command set is organized into these groups and provides read and write
access to a wide array of information available in smart field instruments:
•
Universal commands provide access to information that is useful in normal plant
operation such as the instrument manufacturer, model, tag, serial number,
descriptor, range limits, and process variables. All HART devices must
implement universal commands.
•
Common practice commands provide access to functions that can be carried out
by many devices.
•
Device specific commands provide access to functions that can be unique to a
particular device.
Section B.1
Message
Structure
Read this section for a description of transaction procedure, character coding, and
message structure of the HART protocol. These correspond to layer 2 (datalink layer) of
the OSI protocol reference model.
B.1.1 Master-slave Operation
HART is a master-slave protocol. This means that each message transaction is originated
by the master; the slave (field) device replies when it receives a command message
addressed to it. The reply from the slave device acknowledges that the command was
received and can contain data requested by the master.
B.1.2 Multiple Master Operation
The HART protocol provides for two active masters in a system: one primary and one
secondary. The two masters have different addresses. Each can positively identify replies