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144 ControlLogix

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Analog HART Modules

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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 operationMaster-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 operationMultiple 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 modeBurst 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).