Allen-Bradley 1756 ControlLogix - Direct or Rack-Optimized Connections

Chapter 4
Control I/O
62 Rockwell Automation Publication CNET-UM001F-EN-P - February 2018
Logix 5000 controllers use connections to transmit I/O data. These
connections can be direct or rack-optimized connections.
Term
Definition
Direct connection A direct connection is a real-time, data-transfer link between the controller and an analog or digital I/O
module that enables your controller to collect more data from an I/O module. For example, with a
direct connection, the controller can collect diagnostic status data from a 1756-
IA8D module that would
not be collected in a rack-optimized connection.
The controller maintains and monitors the connection with the I/O module. Any break in the
connection, such as a module fault or the removal of a module while under power, sets fault bits in the
data area associated with the module.
A direct connection is any
connection that does not
use the Rack Optimization
communication format.
Rack-optimized
connection
(Digital I/O modules only). A rack-optimized connection consolidates connection usage between the
controller and all the digital I/O modules in the chassis or DIN rail. Instead of a direct connection for
each I/O module, the entire chassis or DIN rail has one connection.
Anytime a remote chassis houses I/O modules that use rack-optimized connections, the remote
ControlNet communication module connecting these modules to their owner-
controller must also use a
rack-optimized connection. However, you can use both direct and rack-optimized connections to the
same remote chassis. For example, if your remote chassis houses six digital I/O modules and your
application requires direct connections for three and rack-optimized connections for the others, you
can choose direct connections for the first three and rack-optimized connections for the second three.
In this case, even though you must use a rack-optimized connection for the remote ControlNet
communication module, the owner-controller still makes direct connections with the three I/O modules
configured as such.
Each remote ControlNet communication module is limited to five rack-optimized connections.
Rack-optimized Connection.
Direct or Rack-
optimized Connections