Siemens SIMATIC ET 200SP Manual page 25

Each request is stored on a channel-specific basis, and the corresponding request data record
is disabled. Another writing of the same request data record is thus not possible and is
acknowledged with BUSY.
The disable of the request data record is reset after the termination or completion of the
requested HART command.
SHC sequence
If a HART command with a set SHC bit is sent to the module, this channel is reserved for 2
seconds for HART commands. That is, if the module detects a HART command with SHC bit
set for a channel, the complete HART interface of the analog module is reserved for this
channel for approximately 2 seconds. There is no processing of internal HART requests, and
there is no HART command processing for all other channels of the analog module during this
time.
Each time a HART command with a set SHC bit is sent, the analog module reserves this
channel again for another 2 seconds for HART commands. If a HART command without SHC
bit set is detected for this channel or if no other command for this channel occurs within
2 seconds after the previous HART command, the analog module returns to "normal" HART
operation. This means all HART channels are processed again.
Analog Output Module AQ 4xI HART HF
Manual, 05/2019, A5E35098013-AA
HART function
4.3 HART communication interface
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