For the older CompactLogix controllers, with embedded Ethernet ports, namely the 1769-L23E, 1769-L32E and 1769-L35E controllers, the I/O Configuration structure is quite unique and very particular. If you view the structure under the I/O Configuration, you will see the CompactLogix System is the higher level where basically all the key components of the CompactLogix System reside. The System represents a Virtual Chassis. Here you will see the processor and its embedded Ethernet port as separate instances. There is also the CompactBus instance where any Local 1769 I/O modules would reside...
Even though the Ethernet port is embedded in the processor, it is treated as though it is a Local Ethernet Bridge (ENB) module separate to the processor in the Virtual Chassis. As such, it must be assigned a Slot number unique from the processor. Because the CompactLogix processor, and hence its embedded ENB module, cannot be placed in any Slot, similar to a ControlLogix processor, and 1756 ENxT modules, they must be assigned fixed Slot numbers.
The CompactLogix processor is always assigned as virtual Slot 0. It's embedded ENB module is always assigned as a virtual Slot 1. What is fixed throughout all Logix 5000 systems is of course the backplane Port. For the CompactLogix System, this is known as the Virtual Backplane, and is always assigned as Port 1. Each device in a Logix 5000 system that can access the backplane will have a backplane Port number of 1. So the CompactLogix processor is in Slot 0 and can access the backplane via its Port 1. So the first port number in the path is 1 to the backplane, from the processor in Slot 0. The next entry needs to be an address, which would be the ENB Slot address, and in this case it is always fixed at Slot 1 for the Local ENB. It will not vary depending on where the ENB module is "placed" because it is physically embedded and using fixed slot addressing. It is a subtle but important distinction between the older CompactLogix, with "controller integrated communication ports", and the older ControlLogix "processor only" platform which, of course, allows communication modules be placed in any valid free chassis slot.
When you are typing the message path and you have entered the initial "1,1", this essentially represents the preset path to the Local ENB module. 1 to the backplane Port, and 1 to the LocalENB Slot address. To simplify (or confuse?) things, this entry will automatically change from "1,1" to "LocalENB" in the validation entry beneath, informing the user that they are communicating out the Local ENB...
Note how the validation is representing the "1,1" with "LocalENB". However, the path has not yet been applied proper. This is denoted by the asterisk beside "Communication*" on the tab and the fact that the "Apply" button is enabled. Once the path has been validated and then applied, it will then also change from "1,1" in the entry field to the "LocalENB" shortcut. If this does not happen immediately then close the MSG Configuration window and reopen it on the "Communication" tab. Both entries should now be identical...
Also note the "Browse" button option. Using this you can initially browse the CompactLogix System, or Virtual Chassis, to locate the Local ENB module instance. Once selected, it will automatically apply "LocalENB" to the beginning of the path in the entry field. You can of course always just quickly type "LocalENB" in the entry field and it will be just as valid. It is not case sensitive. "localenb" will still result in "LocalENB" in the validation. Once applied it will update the text to the correct casing.
It's possible they could have gone one step further here. The Ethernet port on the Local ENB is always fixed as Port 2. For any communications directed out these controller's embedded Ethernet ports they will always have to take this path. So we will always see...
1, 1,
2, <network>
Or...
LocalENB,
2, <network>
I don't see why the shortcut "LocalENB", which is of course representing the ENB module, could not have represented "1, 1,
2", reducing the path entries by another one. The Port 2 is, after all, also a part of the virtual ENB module.
Of course, for the newer 5370 and 5380 CompactLogix controllers, the embedded Ethernet ports are not treated as separate Local ENB modules in a virtual chassis. Instead they are treated as simply a direct embedded port out of the processor, for which they are. So we do not need to virtual hop from the processor Slot 0, to the backplane Port 1, to the ENB module Slot 1, to the ENB module Port 2...
1, 1, 2, <network>
Instead, we can now simply hop directly to the Ethernet Port 2...
2, <network>
K.I.S.S.
While I don't mind explaining the quirks of these older CompactLogix systems (I haven't even gone near the CompactBus) I will say that their architectural design does/did leave a lot to be desired.
