VTS makes data logging easy. A standard VTS tag type is the "Logger". Logger tags may be attached to any other tag in the system, such as analog tags, digital tags or calculated value tags.
Each logger tag records the data from its associated tag to an encrypted binary data file. The logging rate and the maximum number of records per file are both under the control of the developer. Each log record contains a time stamp, and validity flags, along with the data values. Validity flags are used to mark bad data when reading back from the files. You also have the option to introduce manually entered data to be logged by the system. This is a useful feature during times when it is known that automatically read values are in error (such as an instrument failure).
Logging to the data file is performed in a circular manner. New records are created at each log event until the file reaches its maximum size (specified by the user), then the oldest information in the file starts to be overwritten. This logging method is well suited to continuous processes in that the last "n" logged records (where "n" is a number set by the user) are always available, and the maximum disk space required for data logging is predictable. The length of the alarm logging file defaults to 100,000 events, but may be changed by the user. The only practical limitation on the length of the alarm event log file is the available disk space.
Multiple data files can be specified for each group of values so that data may be kept in separate files on a daily, or on a batch basis.
Along with the normal data logging requirements, the VTS system can also be configured to log various alarm events. All of these events include a time and date stamp. The events can include the setting of the alarm, the acknowledgment of the alarm, the clearing of the alarm, and the enabling and disabling of the alarm. Other information may also be logged, such as the value of the tag when the alarm occurred, and the operator who acknowledged the alarm.
Logged data can be displayed on a graph using VTS's built-in Historical Data Viewer page.