Rack Connection

When a digital I/O module is located in a remote chassis (with respect to its owner), the user may select to use rack connections to communicate with the digital I/O modules in that remote chassis.

An adapter and I/O modules are configured to use rack connections by choosing the Rack Optimization communication format when creating the module. For the adapters, this specifies that the adapter should create a rack "image" and return I/O data in the rack image to the controller. For I/O modules, this specifies that this module will "participate" in the rack connection, and its I/O data should be returned in that manner.

Choosing a rack connection is only available to digital I/O modules, although direct connections to digital I/O modules are also allowed. In fact, the system can perform both direct and rack connection in the same chassis. Analog I/O modules cannot participate in the rack connection, so all connections to Analog modules are direct connections. Analog modules never offer a communication format of Rack Optimization.

Benefits of Rack Connections

A rack connection economizes connection usage between the owner and digital I/O modules in the remote chassis. Only one connection is made from the controller to the ControlNet adapter for the rack image.

Limitations of Rack Connections

A rack connection transfers only input and output to/from the controller. Any other input data the module would return in a direct connection, such as status or diagnostics, are not returned in the Rack Image.

A rack connection only allows one RPI value to be used for all modules configured to communicate using rack communication. Rather than having several direct connections with individual RPI values, the owner has a single rack connection with a single RPI value. That RPI value accommodates all digital I/O modules in the chassis.

In the following example, the owner is communicating with all I/O in the remote chassis but has used only one connection. The data from all three modules is sent together simultaneously at the RPI rate specified for the ControlNet Adapter (1756-CNB) module. By using rack connections, the need for three separate connections has been eliminated.

ControlLogix I/O Rack Connection Limitations

When using ControlLogix I/O with a remote ControlNet Adapter (1756-CNB or 1756-CNBR), up to 5 controllers can create rack connections to the ControlNet Adapter. All subsequent rack connection requests will fail, with the exception of Listen-Only connections, which are virtually unlimited.

FLEX I/O Rack Connection Limitations

When using FLEX I/O with a remote FLEX I/O Adapter (1794-ACN15, 1794-ACNR15), one controller can create a rack connection to the FLEX ControlNet Adapter. All subsequent rack connection requests will fail.

You cannot inhibit the connection to a FLEX I/O module that is configured to use Rack Optimization.

Suggestions for Rack Connection Usage

In general, use a rack connection for digital I/O modules in the remote chassis if:

For more information about ControlLogix  analog modules, see the ControlLogix Analog I/O Modules User Manual.