In communication architectures using the ISOTCP protocol with a huge number of I/O Tags, over a thousand Tags, it can be interesting that a portion of these Tags be read only once when entering the In Advise status (please check the next note), as opposed to normal, which are read repeatedly with each scanning cycle. For performance improvement, ReadOnce Tags are removed from the block of reading Tags, decreasing the volume of information in future readings.

The ReadOnce Tag is only read again when leaving the In Advise status and then returning, or when some disconnection or communication failure occurs.

To configure a Tag as ReadOnce, add a "/readonce" String at the end of the syntax parameter Item, such as "DB2:DB0/readonce".

This feature is used exclusively for ISOTCP configuration with Extra Connections equal to or greater than 1 (one).

The way to change a Tag status to In Advise is by configuring its AllowRead property to True and by one of the next conditions:

•A Tag's AdviseType property is equal to 0 (zero), that is, it is equal to AlwaysInAdvise

•A Tag's AdviseType property is equal to 1 (one), that is, it is equal to AdviseWhenLinked, and it is linked to an active object on an open Screen

If one of these conditions is not available, the I/O Tag does not have its status set to In Advise, that is, it is not read.