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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.
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