Rockwell Automation 1756-IH16ISOE ControlLogix Sequence of Events Module - Manual (Page 19)

Publication 1756-UM528A-EN-P - April 2004
How Does the Sequence of Events Module Operate in a ControlLogix System? 2-3
Making Connections ControlLogix controllers can make direct or rack connections to digital I/O modules. The controller can only make
a direct connection to the Sequence of Events module. The controller cannot make rack connections to the
Sequence of Events module. Additionally, the controller cannot make listen-only connections to the
Sequence of Events module.
A direct connection is a real-time data transfer link between the controller and the device that occupies the slot
that the configuration data references. When module configuration data is downloaded to an owner-controller,
the controller attempts to establish a direct connection to each of the modules referenced by the data.
If a controller has configuration data referencing a slot in the control system, the controller periodically checks for
the presence of a device there. When a device’s presence is detected there, the controller automatically sends the
configuration data.
If the data is appropriate to the module found in the slot, a connection is made and operation begins. If the
configuration data is not appropriate, the data is rejected and an error message appears in the software. In this
case, the configuration data can be inappropriate for any of a number of reasons. For example, a module’s
configuration data may be appropriate except for a mismatch in electronic keying.
The controller maintains and monitors its connection with a module. Any break in the connection, such as module
faults or removal of the module from the chassis while under power, causes the controller to set fault status bits
in the data area associated with the module. RSLogix 5000 monitors connection status to annunciate the
modules’ failures.
Transmitting Data In the ControlLogix system, a controller does not poll the Sequence of Events module to obtain input status. When
you configure a Sequence of Events module to capture a CST value for a specific transition, the module produces
data whenever that input transitions and also at a user-configured rate. The type of input data transmitted and
transmission frequency depends on module configuration and where in the control system that input module
physically resides.
This is called the Producer/Consumer model. The Sequence of Events module produces the input data and the
controller consumes the data.
All Sequence of Events module inputs are updated asynchronous to the controller’s task execution. In other
words, an input may be updated in the controller at any time during the controller’s execution of the tasks it is
configured to run. The input device determines when the input is sent, based on its configuration.
Triggering Event
Tasks You can configure Sequence of Events modules to trigger an event task. The event task offers ControlLogix
controller users a task that executes a section of logic immediately when an event (i.e., receipt of new, unseen,
data) occurs.
Your Sequence of Events module can trigger event tasks. When using a Sequence of Events module to trigger an
event task, remember these considerations:
• Only one input module can trigger a specific event task.
• The input module triggers the event task based on the Enable CST Capture configuration for the module.
The Enable CST Capture configuration defines which points prompt the module to produce data if they turn
ON or OFF. This production of data triggers the event task.
• Typically, Enable CST Capture for only one point on the module. If you Enable CST Capture for multiple
points, a task overlap of the event task may occur.
For more information on using Event tasks, see the Logix5000 Controllers Common Procedures Programming
Manual, publication 1756-PM001.
Table 2.3 Ways that a Sequence of Events Module Behave Like Other ControlLogix Digital Input Modules
Concept: Description: