Spectrum Controls 1756sc-OF8H Series A - Manual (Page 158)

144 ControlLogix ™ Analog HART Modules
Message Message Message Message Message
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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 Master-slave operation Master-slave operation Master-slave operation 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 Multiple master operation Multiple master operation Multiple master operation 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.
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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. After a few retries, the master should abort the transaction,
since presumably the slave device or the communication link has failed.
After each transaction is completed, the master should pause for a short
time before sending another command, to allow an opportunity for the
other master to break in if it wishes. This way, two masters (if they are
present) take turns at communicating with the slave devices. Typical
message lengths and delays allow two transactions per second.
Burst mode Burst mode Burst mode Burst mode Burst mode
To achieve a higher data rate, some field devices implement an optional
burst mode. When switched into this mode, a slave device repeatedly
sends a data message, as though it had received a specific command to do
so. Special commands (#107, #108, #109) are used to start and stop this
mode of operation, and to choose which command should be assumed. (If
burst mode is implemented, Commands #1, #2 and #3 must be supported;
other commands are optional.) There is a short pause after each burst
message, to allow a master device to send a command to stop the burst
mode operation, or to initiate any other single transaction (after which
burst messages will continue).