Allen-Bradley 1794-IE8H User Manual download pdf (Page 100)

Publication 1794-UM063A-EN-P - March 2006

C-2 Additional HART Protocol Information

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. 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 (not supported)

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, and 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.

Generally, burst mode is only useful if there is just one field device attached to

a pair of wires, since only one field device on a loop can be in burst mode at

any one time. In burst mode, more than three messages can be transmitted per

second.

The actual HART message between the FLEX I/O HART module and the

field device follows the standard HART messaging protocol.

Preamble Start

Character

Address Command Byte

Count

[Response

Code]

Data Checksum

1 2 ... 95 96 97 98 99 100 101 102 103 104 105 ... 145 146

No comments

Allen-Bradley 1794-IE8H User Manual Page 100