Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 24)

24 Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
Chapter 2 Digital I/O Operation in the ControlLogix System
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. 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 that prevents normal operation.
The controller maintains and monitors its connection with a module. Any break
in the connection causes the controller to set fault status bits in the data area
associated with the module. Breaks in the connection can be caused by a module
fault or the removal of the module from the chassis while under power.
RSLogix 5000 software monitors fault status bits to annunciate module failures.
Rack-optimized Connections
When a digital I/O module is located in a remote chassis with respect to its
owner-controller, you can choose Rack Optimization or Listen-only Rack
Optimization during module configuration. The option you choose depends on
the communication module configuration. If the communication module uses
Listen-only Rack Optimization, then the I/O module must also use Listen-only
Rack Optimization.
A rack-optimized connection economizes bandwidth between owner-controllers
and digital I/O modules in the remote chassis. Rather than having several direct
connections with individual RPI values, an owner-controller has a single rack
connection with a single RPI value. That RPI value accommodates all digital I/O
modules in the remote chassis.