Rack Connection

When a digital I/O device 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 devices in that remote chassis.

An adapter and I/O devices are configured to use rack connections by choosing the Rack Optimization communication format when creating the device. 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 devices, this specifies that this device 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 devices, although direct connections to digital I/O devices are also allowed. In fact, the system can perform both direct and rack connection in the same chassis. Analog I/O devices cannot participate in the rack connection, so all connections to Analog devices are direct connections. Analog devices 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 devices 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 device 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 devices 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 devices in the chassis.

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

ControlLogix I/O Rack Connection Limitations

When using ControlLogix I/O with a remote ControlNet Adapter (catalog number 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 (catalog number 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 device that is configured to use Rack Optimization.

Suggestions for Rack Connection Usage

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

the majority of devices in the remote chassis are digital

your owner controller is running low on connections

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We suggest that you do not use a rack connection for diagnostic I/O devices. Diagnostic data will not be transferred over a rack connection. This loss of diagnostic data defeats the purpose of using I/O devices with diagnostic capabilities.

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