ControlLogix + ControlNet Flex I/O

sherman4

Member
Join Date
Jun 2002
Posts
46
Hello

I have been looking for answers to the next question on the Rockwell Automation site unsuccessfully:

If I have a ControlNet interface module attached to a ControlLogix controller, and I want to add several ControlNet-Flex I/O modules (Over redundant ControlNet)... how many 1794-ACNR15 (ControlNet adapter modules for Flex I/O) can I connect to the ControlLogix controller in one single segment?

I hope it is clear enough... otherwise I can write it this way: "How many 1794-ACNR15 can I connect to a single ControlNet adapter in a ControlLogix chassiss?"

|-----------------|
| | | |
| | | | ControlLogix
| | | |
|-----------------|
| |
| | redundant ControlNet
| |
°°°°
| | 1794 ACNR15 Module for Flex I/O +
°°°°
| |
| | redundant ControlNet
| |
°°°°
| | 1794 ACNR15 Modules for Flex I/O (how many more) ?
°°°°
THANKS !!!
 
As you might expect, this is an "it depends" answer. It depends on how much data you want, and how fast you want it.

1794-ACNR modules can attach to between 1 and 8 FLEX I/O modules, so the discrete I/O data connection size can vary.

1794-ACNR modules can also attach to analog and speciality FLEX I/O modules, so their individual I/O data connection sizes can vary.

ControlNet I/O connections can be configured for different periodic update intervals (called a Requested Packet Interval (RPI)), so amount of network bandwidth consumed by each connection can vary with the RPI.

Finally, the 1756-CNB(R) has a limited number of connections it can maintain at a given speed.

A 1756-CNB(R) can handle a theoretical maximum of 64 connections. I recommend a practical maximum of 48 I/O connections, so that connections will be available for configuration and diagnostics on the ControlNet link.

If you want these connections to run very fast, you're not going to want to jam too many onto the same 1756-CNB(R). If you want 10 millisecond updates, I'd limit the FLEX I/O connections to about sixteen per 1756-CNB(R).

In short; 16 connections for fast updates, 32 connections for average updates, and 48 connections for slow updates are good rules-of-thumb.
 

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