2018-04
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HART Multiplexer System KFD*-HM*-16
Operation
7.1.5 Constructing the Signal Loop/Scan (Software Function)
When the power supply to the device is switched on, the device searches the
multiplexer devices listed in the multiplexer table for the connected HART field devices
(command 0 and 4
1
) and in doing so generates the internally required access tables.
This process can also be performed by the maintenance station connected as is necessary, for
example, for communication with new HART-compatible field devices connected. The length
of this process depends:
• on the number of multiplexer slaves connected,
• on the number of connected HART-compatible field devices,
• on the loop search type, see chapter 9.1, command 153,
• on the number of message repeats in the case of a fault, or
• on whether the query has been answered.
With 16 field devices the duration is between 15 s and 30 s (e. g., for use by only one
multiplexer master without multiplexer slaves).
During the scanning process, only certain read commands
(commands 0, 1, 2, 3, 11, 12, 13, 48, 129) are accepted by the maintenance station.
All other commands result in the response Busy until the scanning process is concluded.
7.1.6 Cyclical Data Retrieval/Command Caching (Software Function)
The multiplexer device can read cyclical data from up to 31 field devices. To do this, the
scan option (command 137) must be set for the field device and the scan function in the
multiplexer device must be activated (command 149, function 1). The Scan command option
(command 147) defines which data is to be transferred, see chapter 9.1.
If the scan function is activated, the field devices with the scan option activated are regularly
checked for data. To do this, the scan command that reads one or more variables from the
field device is executed. If a field device does not respond, it is designated as Disappeared
(command 129) but it remains in the search list. This means that the field device will be
searched for again during the next cycle. If a field device has disappeared but responds
correctly with one of the next search commands, it is designated as Appeared. If another
field device has responded instead, the Mismatched status appears.
1
Command 4 is also used for field devices that support only older HART specifications up to command 4.
Note!
In the DTM, the parameters for the signal loop construction can be parameterized in the
Communication menu in the Communication to field devices (as HART master) section.
For a detailed description, see chapter 6.7.3.
Note!
In the DTM, the scan function for cyclic data retrieval can be activated and parametrized in
the Command caching menu. For a detailed description, see chapter 6.7.4.
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