ControlNet Connection Count?

I have a question regarding C-Net topology and device number limits.

At the moment, I am counting the number of "connections" (not sure of the correct term) on each 1756-CNBR in our C-Net system (we have 3 networks).

I am to tally the number of connections from all the 1794-ACNRs under each bridge, and their repsective IO:
-All AI / AO modules count as 1 connection.
-All DI / DO modules are rack optimized, so any and all DI/DO modules on an ACNR are considered 1 connection, together.
-Add 1 connection for the ANCR itself.
-We are tieing in D-Net, so CN2DN modules count as 1 connection.

Am I correct in understanding that per network there can only be a max of 99 connections, after which another CNBR card is needed?

What would happen if the digitial modules were not rack optimized (would each card count as a connection)? I know this enables the status info to be sent, but no one has broken down for me how this affects the connections count.

Thanks

You're pretty close, and have the basic ideas correct for counting CIP Connections.

A "Rack Optimized" connection to a 1794-ACN(R)15 provides status for the adapter itself as well as a scheduled connection that will include any Rack Optimized module connections. You do not need a Connection "for the adapter itself".

Another Connection is used by any analog or specialty module, or a discrete module for which you define a "Module Connection".

Adapters like 1788-CN2DN or 20-COMM-C use one Connection each.

Next you have to talk about Connection capacity, which can be a trickier subject because it involves variables like unscheduled messaging Connections, ControlNet bandwidth, and the CPU capacity of the 1756-CNB.

The value "99" refers to the maximum number of Nodes possible on a ControlNet network. This is far in excess of the practical number of Connections that can be bridged through a single 1756-CNB.

The 1756-CNB has a theoretical maximum of 64 connections that it can bridge. Depending on the NUT of the network and the RPI's of the connections, this can be much lower for a given 1756-CNB. The 1756-CNB Series E modules have superior connection capacities compared to earlier modules, and the 1756-CN2 modules have double the connection capacity of any 1756-CNB.

There are several software tools and publications to help you with this kind of assessment.

In RSLinx, you can right-click and select "Module Statistics" on a 1756-CNB. The third tab, labeled "Connection Manager" will tell you the current number of Connections the module is handling, as well as its CPU utilization.

The RSLogix 5000 Task Monitor Tool (included with RSLogix 5000) can also read out Connection and CPU loading on a module, and adds some handy graphic trendign features.

And RSNetworx for ControlNet provides this information, both online and when you perform an offline schedule of a proposed configuration.

There are several Knowledgebase documents about 1756-CNB capacity calculation, as well as the excellent overview of Controlnet Connections in Publication CNET-UM001, ControlNet Modules in RSLogix 5000 Control Systems.

Thanks for the info Ken, I'll read up on the CNET publication.

Ken,

To clarify:

Say I have a controller with a 1756-CNBR, connected to a remote 1794-ACNR15 node. The comm format is Rack Optimization.

I have filled 6 slots at the ACNR15 with digital IO
3 1794-IB16
and
3 1794-OA16

The total connections from that node would be 2, or 1?

Conversely, Say I have another 1794-ACNR15 (same CNBR) with no comm format. I have 4 analog modules
2 1794-IF8IH
and
2 1794-OF4I

Connections from this other node would be 4, or 5?

Thanks again.

The first 1794-ACNR15 adapter with a Rack Optimized connection to the adapter and six Rack Optimized module connections included in it will use 1 CIP connection.

The second 1794-ACNR15 adapter with no Rack connection and four analog modules connected to the adapter will use 4 CIP connections.

If you perform an offline schedule of the ControlNet you can view the Properties of your 1756-CNB bridge and see that this configuration results in 5 total CIP connections.



Again, remember there's more to the story than CIP connections; the 1756-CNB needs to be able to handle the volume of connections at the RPI rates you designate.

Thanks again Ken.

One additional question:

Say I have two controllers on a rack, and both are set up to include the aforementioned CNBR bridge in their IO Configuration. The first controller is the owner of the IO from the remote ACNR15 nodes (controlling the IO). The other is set up to Listen-only. Am I essentially doubling my CIP connections used, since both controllers are looking at the same node through a the same CNBR? Or are the CIP connections on the network the same regardless of how many controllers are looking at the nodes?