FieldServer Data Aire (DAP) FS-8704-49 - Manual (Page 46)

FS-8700-49_Data_Aire_DAP Page 46 of 48
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Appendix E.5. Using DA_Assoc correctly with Writes/Toggling Values
Association of Map Descriptors can result in zero’s being written to the DAP device. The
following information can help to prevent this problem. When Map Descriptors are
associated using the same value for the DA_Assoc parameter then the associated Map
Descriptors are all processed before a write is sent to the DAP unit. A write to a DAP unit
writes a complex set of data – it is not possible to write a single variable’s value to a DAP
device. Each associated Map Descriptor is used to update the complex set of data that will
be written.
Consider the following Map Descriptor fragments
Data_Array, Data_Array_Offset, Function, Node_Name, Address DA_Field, DA_Assoc, Scan_Interval
DA_SETPOINTS, 0, WRBC, Unit1, 0, 1, nom_h_setpt,, 120s
DA_SETPOINTS, 1, passive, Unit1, 0, 1, t_setpt,, 120s
If an upstream device writes a value to DA_SETPOINTS Offset 0, when the write is done the
values of DA_SETPOINTS Offset 0 AND Offset 1 are both used to build the complex data
set since the Map Descriptors are associated. If the upstream device has never set the
value of DA_SETPOINTS offset 1, the value will be zero and hence a value of zero would
be written to the DAP’s Temperature Setpoint.
This could be avoided by not associating the Map Descriptors. This will, however, result in
extra write messages being sent to the DAP device. The extra messages could be avoided
by changing the wrbc to a wrbx. If you do this you must still consider the effects of
association and in this case it may be best to have each Map Descriptor defined as a wrbx
with a different value for DA_Assoc.