ValueState retrieval - how it works

The following illustration shows how ValueState retrieval returns values for a discrete tag.

This example has a start time of TC0 and an end time of TC3. The resolution has been set in such a way that the historian returns data for three complete cycles starting at TC0, TC1, and TC2, and an incomplete cycle starting at TC3. Time is measured seconds.

A gap in the data occurs in the third cycle due to an I/O Server disconnect.

The state calculation is based on each cycle, and the values returned at the query start time are not regular initial values, but are the resulting values that occur after applying the algorithm to the last cycle preceding the query range. The returned points are PC0, PC1, PC2 and PC3, shown in green at the top to indicate that there is no simple relationship between the calculated values and any of the actual points.

Assume the query is set so that the total time (wwStateCalc = ‘Total’)in the two states are returned. The timestamping is set to use the cycle end time.

• For TC0, the query returns two rows (one for the "on" state and one for the "off" state), calculated as a result of the "phantom" cycle that precedes the query start time. The values have a timestamp of the query start time.

• For TC1, one row is returned for the "on" state. The "on" state occurred twice during the cycle--one time for four seconds and again for two seconds before the cycle boundary, and the total time returned is six seconds. The state was "off" twice during the cycle for a total time of four seconds, and one row is returned with that value.

• For TC2, one row is returned for the "on" state, and one row is returned for the "off" state. The "on" state occurred for a total of nine seconds between the cycle boundaries, and the "off" state occurred for a total of one second.

• For TC3, one row is returned for the "on" state, and one row is returned for the "off" state. The "on" state occurred for a total of four seconds between the cycle boundaries, and the "off" state occurred for a total of three seconds. An additional row is returned for the NULL state occurring as a result of the I/O Server disconnect.

Using the same data, if you queried the total contained time for the states, the following is returned:

• For TC0, the query returns two values (one for the "on" state and one for the "off" state), calculated as a result of the "phantom" cycle the precedes the query start time. The value has a timestamp of the query start time.

• For TC1, one row is returned for the "on" state, and one row is returned for the "off" state. The "on" state occurred one time for four seconds within the cycle. The two seconds of "on" time that crosses the cycle boundary does not contribute to the total time. The state was "off" one time during the cycle for two seconds completely within the cycle boundary.

• For TC2, the state was not "on" for any contained time between the cycle. Both occurrences of the "on" state cross over a cycle boundary, so no rows are returned for this state. One row is returned for the "off" state. The state was "off" one time during the cycle for one seconds completely within the cycle boundary.

• For TC3, one row is returned for the "on" state, and one row is returned for the "off" state. The state was "on" for a single contained time of two seconds between the cycle boundaries. The state was "off" three times during the cycle for three seconds completely within the cycle boundary. An additional row is returned for the NULL state occurring as a result of the I/O Server disconnect. The state was NULL for a total of three seconds. The I/O Server disconnect that "disrupted" the off state is treated as its own state, thereby changing what would have been a single "off" state instance of five seconds into two instances of the "off" state for one second each.