1756-EN2TR vs EN2T, ENBT and EN4TR: which one your chassis actually needs

Somebody sends me a photo of a ControlLogix rack and asks what the Ethernet card costs. There are six of them in the current 1756 lineup, plus the one that went end of life, and four of those look close enough on a purchase order that the wrong one gets ordered regularly. The 1756-EN2TR is the default pick more often than any of the others. Sometimes that is right. Here is what actually separates these modules, which numbers are identical no matter what you pay, and the firmware detail that catches people buying used.

What the letters in the catalog number are telling you

The naming is not decoration. Read it once and you can size up an unfamiliar 1756 part number without looking anything up.

E is Ethernet, N is network. T means twisted pair copper, F means fiber. An R on the end means two ports with a switch built into the module. A P on the end means Parallel Redundancy Protocol instead. XT is the extended temperature build, K is conformal coating.

The digit is a generation marker, not a port count. So 1756-EN2TR reads as second generation, copper, two ports with an embedded switch. 1756-EN2T is the same generation with one port. ENBT is the old one, carrying a B where the generation digit sits on everything else, and that B ends up mattering more than it looks like it should.

The whole 1756 Ethernet lineup on one table

Every number below is per module, not per port. Unconn. msgs is the CIP unconnected message count, backplane plus Ethernet.

 

Three of these modules have identical connection budgets

Look at the middle of that table. EN2T, EN2TR and EN3TR carry the same three numbers: 128 TCP connections, 256 CIP, 128 + 128 unconnected messages. Only the EN4TR breaks out, and it breaks out hard, at 512 TCP with 1000 I/O connections and another 528 for messaging and HMI.

Which means if the I/O tree outgrew your EN2TR, an EN3TR is not the fix. Same ceiling, more money.

Running out of connections does not fail cleanly, and that is what makes it expensive. Everything is fine at a hundred devices. Then a few more go on and you start losing random drives or MCC buckets for a few seconds at a time, different ones each day, no pattern to it. Nobody suspects the comm module, because the comm module is sitting there green.

Two things get forgotten during the count. One device can take more than one connection, so a device count is not a connection count. And a message that only routes through the module on its way to another chassis still spends a connection on the module it passed through. I stop planning at around 80 percent of the ceiling and leave the rest alone, because there is always another expansion coming.

1756-EN2T. Same connection budget as the EN2TR, one port, no ring.

The second port on a 1756-EN2TR is a ring port, not a spare

Both RJ45s on a 1756-EN2TR hang off one embedded switch. One IP address, one connection budget, one module. It is not two networks, and it is not a standby port you land a second cable on for peace of mind.

What that switch buys is topology. The module can sit as a node inside a Device Level Ring, or partway along a linear daisy chain, with the network cabled through it instead of every device home running back to a panel switch. On a ring, a single break gets detected and routed around, and production keeps going while somebody hunts down the bad cable. Single break. Two breaks and you are down like everybody else.

Something that catches first ring builds: a DLR network needs at least one node configured as ring supervisor, and that function ships disabled on every device capable of running it. Cable a beautiful ring, leave every node on its default, and what you have is a loop with nobody in charge of it.

1756-EN2T cannot do any of this. One port. Neither can EN2F or the ENBT. Star only, every device needing its own path back to a managed switch. That is the real difference between an EN2T and a 1756-EN2TR. Not throughput, not connections, not speed.

Same four devices, two different failures. The single port on an EN2T has one path; the EN2TR has two.

1756-EN2TP is not a misprint for 1756-EN2TR

Two ports on that one as well, but no embedded switch and no ring. The EN2TP runs Parallel Redundancy Protocol, which comes at the same problem from the other end: two completely separate parallel networks, LAN A and LAN B, every frame sent down both, and whichever copy lands first is the one that gets used. Nothing reconfigures when a path dies, because the surviving path was already carrying the same traffic.

PRP is an international standard rather than something built specifically for controls, and inside Logix systems it stays uncommon. It shows up where somebody genuinely needs high availability and has the switches, the training and the budget to keep two full networks alive. For most plants a ring off an EN2TR covers it for a fraction of the effort.

There is a quick way to tell the two apart on a shelf, before anything gets powered up. The two jacks on an EN2TP are labeled LAN A and LAN B. On a 1756-EN2TR they are labeled 1 and 2, because they are two sides of the same switch rather than two separate networks.

So do not order an EN2TP expecting the ring version of the EN2TR. It will not ring.

Motion axes, and where EN3TR and EN4TR pull away

Connections are flat across the middle of the lineup. Motion is not.

A 1756-EN2TR tops out at 8 integrated motion axes over EtherNet/IP. EN3TR handles 128. EN4TR handles 256. That is the whole reason EN3TR exists, and the only reason to pay for one, given what its connection numbers already told you.

The EN4TR is a different animal in a few ways. Gigabit on the ports instead of 10/100. Four times the TCP connections. And it is the one with CIP Security, which costs throughput as you turn it up: the packet rate ceiling runs 50,000 without CIP Security, 25,000 with integrity, and 15,000 with integrity and confidentiality. If security gets added to a busy chassis late in a project, that budget needs recalculating rather than assuming.

1756-EN4TR. The only one in the family that runs gigabit, and the only one that does CIP Security.

The redundancy rule that catches people out

For a redundant chassis pair, meaning the primary and secondary racks holding the controllers, the rule is easy to say and surprising to apply. The communication module needs a 2 or a 4 in its catalog number. Modules with a B in the name are standard rather than enhanced, and they are not allowed in there.

So the ENBT is out. Nobody is shocked by that. EN2TR, EN2T, EN2F, EN2TP and EN4TR all pass.

EN3TR does not pass, and that one surprises people every time. Modern module, dual port, DLR capable, 128 axes, and Rockwell still does not allow it in a redundant chassis pair, because there is no 2 or 4 in the name. A single port EN2F is fine and the EN3TR is not. The rule tracks the module generation, not ports and not topology.

Keep one more thing separate from all of that. The EN4TR also has a redundant adapter pair mode, which solves a different problem: two EN4TR modules in slots 0 and 1 of a remote I/O chassis, one active and one standing by, the secondary sitting on primary IP plus one, taking over in 50 ms or less. That is adapter redundancy for remote I/O. It is not controller redundancy, and mixing the two terms in a spec conversation costs an afternoon.

Buying a used 1756-EN2TR: read the firmware before anything else

This is the part that matters when the module comes off a shelf instead of out of a box.

The 1756-EN2TR has not always been able to do what the table above says. On firmware 2.x it runs 10,000 packets per second for I/O and supports no integrated motion at all. From 5.x onward the same physical module does 25,000 and the 8 axes appear. Nothing changed in the hardware. Buy one that has been sitting in a panel since 2010, plan on motion, and you have bought a firmware upgrade as well as a module.

Same pattern across the family with different revision numbers. EN2T and EN2F want 3.6 or later for the higher packet rate and the axes. EN2TR wants 5.x.

There is one tell you can read off the faceplate before the module goes anywhere near a laptop. Newer series modules carry a NET status indicator in the LED row above the ports. The older series does not have that LED at all. It will not give you the firmware revision, but no NET indicator means you are holding an early unit and the revision is worth checking properly.

Three things worth doing to any used one before it goes near a live rack.

Read the revision. The module properties in FactoryTalk Linx Network Browser show catalog number, series, manufacture date and firmware revision, so you find out what you bought before you commission it instead of after.

Clear the previous address. The three rotary switches at the bottom of the module set the last octet: anything valid from 001 to 254 gives you 192.168.1.xxx with a 255.255.255.0 mask and 192.168.1.1 as the gateway. Set the switches to 888, cycle power, and the module returns to factory default, taking the previous plant's configuration with it.

Make sure it is not sitting in protected mode. A module in explicit protected mode refuses configuration changes, IP address included, and reads as broken when it is not. Switches to 000, power up, wait for "Unprotected Mode" to scroll across the display, power down, then set the switches back where you want them. 900 is the position that turns protected mode on, if you ever want it on deliberately.

The three rotary switches on a used EN2TR. 888 clears whatever the last plant left behind.

What to do with an ENBT you already own

The ENBT reached end of life on November 1, 2021. The engineered replacement is the EN2T, or an EN2TR if that slot needs ring topology.

The numbers explain why nobody is grieving: 64 TCP connections, 128 CIP, 64 + 64 unconnected messages, 5,000 packets per second for I/O and 900 for HMI and messaging. No motion. Roughly half of everything the second generation gives you.

I still do not pull working ENBTs out on principle. Sitting in a remote I/O chassis with a modest device count and no expansion planned, it does the same job it always did. Where it has to come out is a redundant chassis pair, anything doing motion, and any chassis whose I/O tree is still growing, because 64 connections vanish faster than people expect.

Quick pick

One copper port and nothing exotic: EN2T. Same thing on fiber: EN2F. Ring or linear topology out in the field: 1756-EN2TR, and that is the right answer for most new panels. More than 8 motion axes: EN3TR, as long as the chassis is not half of a redundant pair. Gigabit, 512 TCP connections, CIP Security, or redundant adapters on remote I/O: EN4TR. Two parallel networks under PRP: EN2TP. ENBT only if you already own one.

1756-EN2TR FAQ

What is the difference between a 1756-EN2T and a 1756-EN2TR?

Connections, motion axes and communication rate are identical: 128 TCP, 256 CIP, up to 8 axes, 10/100 Mbps. The EN2TR adds a second RJ45 and an embedded switch, which lets it sit inside a Device Level Ring or a linear chain. The EN2T is single port and needs an external switch for anything beyond a straight run to one device.

Does the 1756-EN2T support DLR?

No. One port, no embedded switch, no ring. Ring topology on a 1756 communication module means EN2TR, EN3TR or EN4TR, and nothing else in the family.

Does the 1756-EN2TR support PRP?

No. PRP belongs to the EN2TP, and to the EN4TR from firmware 4.001 onward. The EN2TR does Device Level Ring and linear topology. Those are the only two topologies it does.

Can a 1756-EN2TR go into a redundant chassis pair?

Yes. It is one of the enhanced modules, alongside EN2T, EN2F, EN2TP and EN4TR. The ENBT and the EN3TR are not.

How many devices will one module handle?

There is no device number, only a connection number, and a single device can take more than one. You have 128 TCP and 256 CIP to spend on an EN2TR, and messages passing through the module on their way somewhere else spend part of it too.

Where these come from now

Lead times on the current generation move around, and the ENBT is not coming back at all. We keep 1756-EN2TR, EN2T, EN2TP, EN2F, EN3TR, EN4TR and ENBT on the shelf at IQElectro, tested before they ship, with the firmware revision recorded so you know whether you are getting a 2.x or a 5.x before it lands on your bench rather than after. If you are not sure which one the chassis needs, send over the slot layout and what hangs off it and we will work it out.