RoundTrip retrieval - how it works

The following illustration shows how RoundTrip 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 contained time in the two states are returned. The timestamping is set to use the cycle end time. The RoundTrip retrieval mode returns values for states that are completely contained within the cycle boundaries. 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 that preceeds 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 round-trip for the "on" state occurred one time for four seconds completely within the cycle boundary. The round-trip for the "off" state occurred one time during the cycle for five seconds.

• For TC2, a round-trip did not occur for either state within the cycle boundaries. One NULL row is returned for this cycle.

• 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" one time during the cycle for one second completely within the cycle boundary. An additional row is returned for the NULL state occurring as a result of the I/O Server disconnect.

• 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 three seconds between the cycle boundaries. One row is returned for the "off" state for a total contained time of seven seconds. (The first round-trip for the "off" state includes the I/O Server disconnect for a length of four seconds. The second round-trip has a length of three seconds.) An additional row is returned for the NULL state occurring as a result of the I/O Server disconnect, and the returned value is NULL because there is no round-trip during the cycle for it. 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.