Date of issue 11/30/07 805639
15
HART Loop Converter KFD2-HLC-(Ex)1.D(.**)
Description of HART operations:
7 Description of HART operations
Dynamic variable acquisition
HART command 3 is always used to obtain the applicable dynamic variables, either directly or by
activating it in the Burst Mode.
Burst Mode operation
The HART Burst Mode is the HLC preferred one because it ensures a faster operation. The HLC will
therefore always try to set the connected device in the Burst Mode. The Burst Mode activation
command will be sent to the field device in the following situations:
• after an HLC unit power-up (or after an internally generated reset)
• after a field device power-up
• when the communication with the field device is recovered after a time-out.
"Command and answer" operation
If the field device doesn't support the Burst Mode, the HLC will use the normal "command and answer"
mode.
Burst mode recovery procedure
If the HLC stop receiving the burst frames, the following recovery procedure will be applied:
• The HLC immediately goes into the "command and answer" mode.
• The HLC then check if another HART master is sending messages on the loop; if another master
is found, the HLC will not try to re-enable the Burst Mode.
• If no other HART master is found the HLC will keep trying to re-enable the Burst Mode.
• As soon as any Burst frame is again received, the HLC immediately return to the Burst Mode.
This procedure makes possible for another HART master to temporary disable the Burst Mode, as
could be required for servicing reasons.
"Communication-loss" time-out
If the time-out is exceeded at missing communication, then all outputs switch in a defined safe state.
The time-out can be set.
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