Specifications

B - 14 RSLinx – Training Guide
There are two types of groups, public and local (or ‘private’). Public is for sharing across
multiple clients, local is local to a client. Refer to the section on public groups for the
intent, purpose, and functionality and for further details. There are also specific optional
interfaces for the public groups.
Within each Group the client can define one or more OPC Items.
Item 1
Group
Item 2
Item 3
Figure 0-3 - Group/Item Relationship
The OPC Items represent connections to data sources within the server. An OPC Item, from
the custom interface perspective, is not accessible as an object by an OPC Client.
Therefore, there is no external interface defined for an OPC Item. All access to OPC Items
is via an OPC Group object that “contains” the OPC item, or simply where the OPC Item is
defined.
Associated with each item is a Value, Quality and Time Stamp. The value is in the form of
a VARIANT, and the Quality is similar to that specified by Fieldbus.
Note that the items are not the data sources - they are just connections to them. For
example, the tags in a DCS system exist regardless of whether an OPC client is currently
accessing them. The OPC Item should be thought of as simply specifying the address of
the data, not as the actual physical source of the data that the address references.
OPC Alarm and Event Handling Overview
These interfaces provide the mechanisms for OPC Clients to be notified of the occurrence
of specified events and alarm conditions. They also provide services which allow OPC
Clients to determine the events and conditions supported by an OPC Server, and to obtain
their current status.
We make use of entities commonly referred to in the process control industry as alarms and
events. In informal conversation, the terms alarm and event are often used interchangeably
and their meanings are not distinct.
Within OPC, an alarm is an abnormal condition and is thus a special case of a condition. A
condition is a named state of the OPC Event Server, or of one of its contained objects,
which is of interest to its OPC Clients. For example, the tag FC101 may have the following
conditions associated with it: HighAlarm, HighHighAlarm, Normal, LowAlarm, and
LowLowAlarm.
On the other hand, an event is a detectable occurrence which is of significance to the OPC
Server, the device it represents, and its OPC Clients. An event may or may not be
associated with a condition. For example, the transitions into HighAlarm and Normal
conditions are events which are associated with conditions. However, operator actions,
system configuration changes, and system errors are examples of events which are not