ROC Plus Driver Guide - When to Scan (Database Objects)

When to Scan

NOTICE

UNEXPECTED HISTORIC DATA VALUES

To avoid potentially retrieving incorrect data, make sure that scans do not occur close to the time when new records are created, and try to keep the clock in the ROC tightly synchronized with the host.

Failure to follow these instructions may result in retrieval of incorrect historic data values.

The driver retrieves historic data from the ROC using multiple transactions. Because of this, and the fact that slow radios are sometimes used to retrieve data from ROC devices, it is important that the driver is not configured to scan a historic point very close to the time that its value is scheduled to change.

Example:

On a particular system, a point that retrieves historic data is set to be scanned exactly on the hour. However, the host clock is 2 seconds faster than the clock in the ROC. The scanner starts a scan at 9:59:58 according to the ROC clock, and the new hour hasn’t started. The driver retrieves some data about the pointers. The hour then rolls over in the ROC and new records are created. The driver continues retrieving data from the ROC, but the pointer values that were retrieved in the first part of the scan are now wrong. This potentially causes incorrect data to be pushed into the database, because the data changed as the driver was in the process of retrieving it.