Allen-Bradley Driver Tag Type Properties: DLG Tab
Certain Allen Bradley PLCs (of PLC Type "ControlLogix") are able to log time-stamped data into files in PLC memory.
The Allen Bradley driver tag can read records from these files (referred to here and in the PLC documentation as queues). In order for the Allen Bradley driver tag to make use of this feature, both the tag and the PLC must be configured properly. The DLG Read tab of the Allen Bradley driver tag can be used to configure the tag for this feature. Refer to the PLC's manual for instructions on how to configure the PLC (noting that up to 256 queues can be created and data from different PLC memory addresses can be logged into different fields in different queues).
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Use to select a queue (configured according to the PLC’s manual) from which the driver will be retrieving logged data.
The Length of Records from Queue edit field allows you to enter the length of the records in the queue currently selected in the Data Logging Queue drop list. This value is the length (in characters) of the records in the queue once they are converted into a string by the PLC for communication.
The valid range is from 1 to 80. The value must be calculated according to how the data logging queue was configured in the PLC itself:
• 11 characters for the date field,
• 9 characters for the time field,
• 12 characters for each long integer field
• 7 characters for each short integer field.
By default, the Allen Bradley driver tag will assume a length of 80 chars (the maximum allowed by the PLC). While default value will work, this may impose a penalty on the amount of communication between the driver and the PLC. Entering the exact length is therefore recommended.
Note: entering an incorrect value will most likely result in failure to read any records from a particular queue, therefore you should only change the default value if you know the exact value.
Important Notes
When configuring the data-logging feature on the PLC:
• Make sure that both date and time stamps are being logged into the relevant queues.
• Failure to configure a queue in this manner will result in the Allen Bradley driver tag simply ignoring all records from that particular queue. This behavior is by design since a list of logged values without time stamps is of little value.
• Make sure that a real-time clock is present and enabled in the PLC.
• For the same reason as above, if a real-time clock in not present in the PLC or if it is present but not enabled, the driver tag will ignore all records from all data logging queues.
• Make sure the PLC’s real-time clock configuration (date, time, time-zone, daylight savings time) matches the workstation’s clock configuration.
• Failure to do so will result in the Allen Bradley driver tag setting the wrong timestamps to all logged values read from the PLC.