Rockwell Automation T8030S Trusted OPC Server Package - Manual (Page 25)

Trusted
TM
OPC T8030S
Issue 23 April 10 PD-T8030S 25
2.6. Configuring the IEC 1131 Application
2.6.1. Update Types
The OPC Server receives tag updates from a Controller using two methods: polled updates or
Sequence of Events / Process Historian updates (event updates).
A polled update is when the OPC Server pulls the value of a tag from the controller through an update
request. The value of the tag is its value at the time the update request is made, and the time-stamp
associated with a change is provided by the OPC Server at the time that the update request is
received.
An event update is when the Controller pushes the value of a tag to the OPC Server whenever its
value changes. Tag values are queued by the Controller before being sent to the OPC Server, and are
supplied in the order in which they were recorded by the controller. Unlike polled updates, the time-
stamp associated with an event update comes from the Controller. For certain tags associated with
Trusted
TM High Density I/O modules, the time-stamp comes directly from the I/O module.
2.6.2. Tag Configuration
In order to retrieve the value of a tag through a polled update, the tag must be given a
communications address. In order to retrieve the value of a tag through an event update, the tag
must have the “Enable SOE Logging” attribute set.
Only Boolean tags with the “Enable SOE Logging” attribute or tags that are connected to a special
SOE I/O board will be reported to OPC Alarm & Event clients. The defined TRUE/FALSE value strings
for each tag will be used when reporting events. If no TRUE/FALSE value strings are defined, the
events will be reported as “TRUE” or “FALSE”.
Details of how to set Extended Attributes, define TRUE/FALSE value strings and connect variables to
SOE or Process Historian boards can be found in the Product Description PD-8013 SOE & Process
Historian Software Package.
Whenever the application dictionary is changed, the OPC Server controller database must be reloaded
from the new Toolset files as described in section 2.4.4.
2.6.3. Read/Write Timing
If a client reads and writes the same point, and the point is assigned a Modbus address but is not
assigned to SOE, then on writing a new value, the old value may be briefly read back before the new
written value reflects from the system. This may appear, for example, as True, False, True on writing a
Boolean to True. This is because the OPC server stores its own copy of the written value and reports it
back to the client, but will not immediately write it to the system via a Modbus write until the next
scheduled write. A subsequent read request may arrive at the system first and return the old value.
This problem occurs on any polled data communications protocol with an intermediate buffer.
There are two ways to eliminate the problem:
• Do not read the same value that is used for writing. If an acknowledgement is required that a
value written has been received by the system, copy the value in the system application to a
different variable and change the OPC client to read from this second variable.
• Turn SOE on for the variable being written. This will prevent the OPC server reading the value
via Modbus, therefore no intermediate value will be seen.