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Items are associated with values. Typically an item value provided by a DAIS server is read from a device and transferred
to one or more clients. In a distributed control system
involving remote devices (as indicated in Figure 4-3) communication failures might
make item values not available. To cope with communication failures item values are associated with a quality. The quality
indicates the reliability of the item value. Devices usually scan item values at a certain rate and item values will be transferred
to the DAIS server at this rate or some other. In the server, item values will appear as time stamped and quality coded samples.
A server that keeps item values in a local cache is expected to always hold the latest sample. Other item related informations
are access-rights and
scan-rate. This information is shown in Figure 4-4. The cache_value is the latest sample
received from a device, the cache_value_last_updated the time when the cache_value was last updated or validated, and cache_value_quality
when the value was last updated or validated.
For items, a DAIS server exposes the following information to clients:
• the value and its data type,
• the quality of the value,
• the time stamp for the value,
• the fastest scan rate with which the value can be expected to be updated, and
• Values delivered from the cache reflect the latest value considering update rate and update dead bands. Based on the agreed update rate (between a client and a server) a client can expect that the server will validate the values with devices with the agreed update rate.
• The dead band is expected to be checked at each update or validation. Values that don’t transgress the deadband will not be reported.
• Time stamps delivered from the cache shall reflect when the values were updated or validated with the devices according to the agreed update rate. The time stamp gives the time for the latest successful update or validation.
• The quality shall reflect how successful the server has been in keeping the values updated or validated.
• the access rights.
To make access of item values efficient and avoid reading the values from devices each time a client requests item values, a DAIS server is expected to have a local cache. The mechanism for keeping the cache up to date is server specific but a client shall expect the following from the server: