ControlLogix Connected vs Unconnected Messaging

I'm looking at a system where two ControlLogix processors message each other across Ethernet. The messages continuously fires as the two systems need to be kept in the same state (they should probably use Produce/Consume, or an FSC to control firing the message, but that's another issue for another day.)

I'm trying to ensure the messages are configured as robust as possible. One thing I'm unsure of is the difference between "Connected" and "Unconnected" messages.

I've scoured the help file, I've looked at these manuals:
1756-pm012, 1756-rm003, Enet-at002, Enet-at003, Enet-um001

This document: INTEGRATIONWITH CONTROLLOGIX PROGRAMMABLE AUTOMATION CONTROLLERS (PACS) USING ETHERNET/IP

These Rockwell AnswerIDs: 19993, 21192

I even tried Rockwell Tech support and all they did was search through the same KB I did.

I understand (I think) that Connected messages use one of 256 connections on the processor (along with IO in the IO configuration). I understand that there's buffers for unconnected messages.

But no one will tell me when to use connected vs unconnected, and when to use cached vs. not cached. I think in this scenario given the frequency of the message, I want a connected, cached message, but I don't see anything telling me as much.

Can anyone explain when to use what type of message?

There are several different resources involved; the Connection resources in the CPU, the Connection resources (both CIP and TCP) in the Ethernet module, the buffers between the CPU and the Ethernet module, and the Unconnected Message Manager object.

For your application, use a Connected and Cached message.

Such a message uses the buffers between the CPU and the Ethernet module just once, to set up the connection. After that, the buffers are freed up to deal with other traffic, and the messages go through one of the 256 Connection objects instead.

Thanks Ken.

I made an educated guess earlier today to select connected and cached.

Would this be a fair statement on how messages should be configured based on how often they are sent?

Most frequent
Connected, Cached
Connected, Uncached
Unconnected (cached doesn't impact unconnected messages)
Less Frequent

That's an excellent summary of how I approach them.

I once read in a CIP manual in which they explained it like this:

Retrieving data requires a path to an object. Think of this path as a road.

When using an unconnected message, a new road is built that passes through the Unconnected Message Manager to go to the Message Router. After returning the data, that road is then destroyed. Since the unconnected message requires resources to build a new road for every request, there is a chance the Unconnected Message Manager could be busy and refuse the request.

When using connected messages, the Unconnected Message Manager will build a direct road to the Message Router. This road will remain and can be re-used for more reads. However if it is not used after a certain period (depends on the settings, but 30 seconds would be a typical value), then it will be destroyed.