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FieldServer Technologies 1991 Tarob Court, Milpitas, California 95035 (408) 262-2299 fax: (408) 262-9042
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Page 19
4.4.7 Map Descriptor: Example 2 – Writing a Set-Point .
A Data Aire device cannot be written to until it has been read. This is a limitation of the Data Aire protocol. This means that you configuration cannot
consist only of wrbc map descriptors. It should consist of a at least a map descriptor like example 1for every unit that you wish to write to.
Map_Descriptor_Name, Data_Array_Name, Data_Array_Offset, Function, node_name, Address, Length, DA_Field , Scan_Interval
Write_SP_01 , DA_SETPOINTS , 0 , wrbc , unit1 , 0 , 1 , nom_h_setpt, 120.0s
Reccomendation
Use wrbx instead of wrbc. This causes the driver to generate the command message only when the setpoint changes which in turn minimizes
communications. Using this method the DART’s control mode is interrupted the least.
The setpoint is
obtained from this data
array.
Always leave the
address
set to zero and
the length as one.
Specify the data field you wish to write.
Pick from table 4.5.1. Ensure that the
keyword can be written to the unit type
identified as unit1.
Example. If unit1 is a Chiller then the
above map descriptor will not function
correctly as it can only be used to write
to unit types 7,9,14,15.
This is how often the data will be
written to the device.
Slow the scan interval down. The
dart will only take control of the
devices when the network has been
idle for at least 50 seconds. If you
generate polls too often then the dart
will never exercise control.