The Net-ENI was intended to allow an HMI and an RSLogix session to connect to non-Ethernet MicroLogix controllers. Loading it up with two HMIs and a data collector plus something else.. you run out of connections even when everything goes right. Your plan to get an SLC-5/05 into this application is a good one.
Remember that the Net-ENI should never be in the I/O tree of a ControlLogix
Some people trying to connect Net-ENI and ControlLogix somehow decide to put the Net-ENI into the ControlLogix I/O tree as a generic module, which doesn't do anything because that's not how they work.
Or, I should say "doesn't do anything good", because it does cause the ControlLogix to attempt to create an I/O connection, which fails repeatedly but uses up connection resources in the Net-ENI and the ControlLogix.
you'll have to enter the CIP Path by hand
One of the reasons I've seen people put the Net-ENI into the I/O tree is so that they would have an easy-to-click-on target for the Path tab of the MSG configuration window.
Instead, you have to enter the CIP Path by hand.
In general, it's "1, Slot, 2, IPAddress"
Where Slot is the slot number of the 1756-EN_T module in the ControlLogix, and IPAddress is the IP Address of the 1761-NET-ENI.
The 1 means "from the CPU out the backplane port" and the 2 means "from the Ethernet module out the network port". The commas are part of the syntax.