6.1        Data Logging

WebAccess collects each tags DATA in a separate file that allows remixing tags on a DATALOG trend without losing past data.

Data can be viewed in a Data Log Trend Group Display, up to 12 tags per Data Log Trend Group Display.

Data Log Trends can access trend data globally from any SCADA node in the project.  For example tag1 data logged on SCADANode1 can be viewed on a Data Log Trend display on SCADA Node 2 and mixed with tags from SCADA node 2 on the same trend display.

You must enable DATA LOGGING for each TAG in the Tag configuration using Tag Properties (see Analog Tag Properties or Discrete Tag Properties). 

Log Data in Tag Properties is Data Logging enabled.  A file will be created for this Tag and a record entered once a second, if the Dead Band is exceeded.

Data Log DB is the Dead Band used to reduce file size of the Data Log.  The Tag's value must change more than the Dead Band in order for a record to be added to the Data Log File. DATA LOG Deadband is in percent of SPAN (% of SPANHI - SPANLO).

You can Data Log Analog and Discrete (e.g. Digital) Tags.  You cannot Data Log Text-type Tags.

To view Data  Log Trends, you should create at least One Data Log Trend Group.  A maximum of 12 tags can be viewed in one Trend Group simultaneously; but, you can add or replace Tags "on the fly" in any Trend Group without losing historical data.  The Data Log Trend Group uses a pre-built template display.  You need only enter the Tag names in a list to build a Trend Group.

You should also check that the Deadband for Data Logging is set appropriately. Too large of a Deadband will prevent any data from being logged. Too small a Deadband will result in unnecessary recording of values that have not changed and will waste disk space.

The data log files are located on the SCADA node. Typically, the default path is


It is recommended to specify another path to reduce the time for re-installing software. This is done is SCADA Node Properties, Data Log Folder.  WebAccess modifies the security settings of all files in drive:\WebAccess during Node software installation.  A large number of Data Log files will greatly increase software installation.

A file is created for each tag allowing tags to be added or removed from Data Log Trends without loosing historical data.

It is highly recommended to use a Data Log deadband to reduce Disk Access. This will reduce the size of the Data Log Trend files and time used to write to the Hard Drive.  The Hard Drive is usually the bottle neck in Data Logging.  An IDE Hard Drive at 7200 rpm can handle about 800 Tags per second of Data Logging. Using Disable Data Log Seconds recording can also greatly reduce Disk Space and system throughput if one-second data is not needed.

Log Data Maintenance should also be used to delete and, optionally Archive, Data Log Trend files.  A large number of files in the Data Log Folder will reduce Disk Access time and slow you system greatly.  You should also Defragment your hard drive frequently (once per day is the Microsoft recommendation) if you are data logging


See also:

 4.2.15 Log Data

 4.2.16 Data Log DB

 3.2.23 Data Log Folder Disable Data Log Seconds recording

 3.2.22 Data Log to ODBC

 4.2.29 Log to ODBC Frequency

 6.2.3 Add Data Log Trend Group