Allen-Bradley ControlNet Compact GuardLogix 1769, User Manual, Page: 15 / 100

ControlNet Overview Chapter 1
In traditional I/O systems, controllers poll input modules to obtain their input
status. In a Logix system, digital input modules are not polled by a controller.
Instead, they produce (multicast) their data either upon a change of state (COS)
or periodically. The frequency of update depends upon the options chosen during
configuration and where on the network the input module resides. The input
module, therefore, is a producer of input data and the controller is a consumer of
the data.
The controller can also produce data for other controllers to consume. The
produced and consumed data is accessible by multiple controllers over the Logix
backplane and the ControlNet network. This data exchange conforms to the
producer/consumer model.
A ControlNet network link’s most important function is to transport time-
critical control information, such as I/O data and control interlocking. Other
information that is not time-critical, such as program uploads and downloads, is
also transported but does not interfere with time-critical messages because a
ControlNet network can transmit scheduled and unscheduled data.
On a ControlNet network link, nodes transfer information by establishing
connections. Each message sent by a producer contains a Connection ID (CID).
Nodes that have been configured to recognize the CID consume the message,
becoming consumers themselves.
Media access to the network is controlled by a time-slice access algorithm,
Concurrent Time Domain Multiple Access (CTDMA), which regulates a node’s
opportunity to transmit in each network update interval (NUI). You configure
how often the NUI repeats by selecting a network update time (NUT) in
milliseconds.
Rockwell Automation Publication CNET- UM001E-EN-P - June 2016 15