Rockwell Automation 1756-OF8H ControlLogix HART Analog I/O Modules - Manual (Page 40)

40 Rockwell Automation Publication 1756-UM533C-EN-P - February 2011
Chapter 3 ControlLogix Module Operation
Direct Connections 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.
ControlLogix analog I/O modules use direct connections only.
When module configuration data is downloaded to an owner-controller, the
controller attempts to establish a direct connection to each of the modules the
data references.
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 first detected, the controller automatically sends the configuration
data and one of the following events occurs:
• 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
code displays in the software. For example, a module’s configuration data
can be appropriate except for a mismatch in electronic keying that prevents
normal operation. For more information about error codes, see
Module
Configuration Errors on page 160 .
The controller maintains and monitors its connection with a module. Any break
in the connection, such as 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. You can use ladder logic to monitor this data area and detect module
failures.
Input Module Operation In the ControlLogix system, the owner-controller does not poll analog input
modules after a connection is established. The modules multicast their data
periodically. Multicast frequency depends on the options chosen during
configuration and where in the control system that input module physically
resides.
An input module’s communication, or multicasting, behavior varies depending
upon whether it operates in the local chassis or in a remote chassis, based on the
network type. The following sections detail the differences in data transfers
between these setups.