The short answer
Read the front of the module first: the display scrolls what is wrong and the OK, NET and LINK LEDs narrow it down. A red NET LED with “Duplicate IP” on the display means another device has the module's IP address, a flashing green OK LED with “BOOTP” or “DHCP” and a MAC address means the module has no IP configuration yet, and a steady red OK LED is a major fault – cycle power, and replace the module if it stays red. When the module looks healthy but a device behind it doesn't, read that device's connection code in Studio 5000: 16#0204 means it never answered, 16#0203 that it stopped answering, 16#0800 that a link in the path is offline.
This covers Allen-Bradley ControlLogix EtherNet/IP modules – the 1756-ENBT, 1756-EN2T, -EN2TR, -EN3TR and their relatives – in a Logix 5000 system. For DH+, DeviceNet and a wider view of a node that drops off the network, see lost communication.
Key takeaways
- The display and the LEDs tell you whether the module itself is faulted or only its connections are.
- A duplicate IP address puts a module into conflict mode – the display names the other device's MAC address.
- Slow or intermittent networks show up on the module's diagnostic web pages: CPU, duplex and error counters.
What the status display says
- The IP address (or OK and the address): normal operation.
- BOOTP or DHCP and the module's MAC address, with OK flashing green: no IP configuration – set one (below).
- OK, the address, Duplicate IP and another MAC address, with NET red (and OK flashing red): conflict mode.
- Image Update Needed, with OK red (dual-port modules): the main firmware image must be updated; then cycle power.
- Unsupported mode (1756-EN4TR): the mode rotary switch is in an unused position. The module answers on no port until the switch is corrected and power is cycled.
OK, NET and LINK LEDs
| LED | State | What it means (our words) |
|---|---|---|
| OK | Off | No power: check chassis power and that the module is fully seated. |
| OK | Flashing red / steady red | A recoverable minor fault – check the configuration (with NET red, it is an IP conflict) / an unrecoverable major fault – cycle power, then replace the module if it persists. |
| OK / NET | Flashing red and green | Power-up self-test in progress. |
| NET | Off | No power or no IP address. |
| NET | Flashing green | An IP address but no CIP connections, or all of them have timed out or closed: read the connection code at the controller that owns the connection. |
| NET | Green / red | At least one CIP connection is up / duplicate IP address. |
| LINK | Off | No link: cable, switch port or a port that has been disabled. |
| LINK | Flashing green / green | Traffic on the port / a link but no traffic. |
Older-series 1756-EN2TR and EN3TR modules have no NET LED. Each module also has diagnostic web pages: browse to its IP address and open the Diagnostics folder.
Duplicate IP: who keeps the address
The check runs when a device joins the network or its address changes. If one device was already running, it keeps the address and the newcomer goes into conflict mode. If both powered up at about the same time, both go into conflict: give one a new address, then power-cycle the other or unplug and replug its Ethernet cables. A device that can't detect duplicates always keeps the address – a 1756-ENBT detects them only from firmware revision 3.3. The MAC address on the display identifies the other device.
Setting or resetting the IP address
- Rotary switches: 001 to 254 gives 192.168.1.xxx with subnet mask 255.255.255.0. Any other setting (888 is the reset; modules ship at 999): the module uses its saved address, or asks a BOOTP/DHCP server if BOOTP/DHCP is enabled. Rockwell recommends setting them before the module goes in, and warns that changing them with power on can arc (ENET-AT003B, p. 9).
- BOOTP/DHCP server, RSLinx Classic or Studio 5000: after assigning an address with the server, click Disable BOOTP/DHCP – otherwise the module drops it at the next power cycle. On a subnet linked to a plant DHCP server, that server may answer first.
- Factory reset: set all three switches to 888, then power-cycle; the configuration, IP address included, returns to the factory value. The switches are read at power-up, so set them again and power-cycle once more.
Connection codes in Studio 5000
The code is on the device's Connection tab; the full list is Rockwell's fault code spreadsheet, 1756-RD001.
| Code | What it means (our words) |
|---|---|
16#0106 | Ownership conflict: another controller already owns the module, or it accepts only one connection. |
16#0113 | The module's connection limit is exceeded: reduce connections or add a module. |
16#0203 | The target is reachable but stopped answering, for example when multicast packets don't come back. The controller keeps retrying. |
16#0204 | The request timed out and the target looks missing: check its power, slot or address and network connection. |
16#0302 | A module along the path has run out of bandwidth: raise the RPI or spread the load over another bridge. |
16#0800 | A bridge in the path is offline from its network. |
Slow or intermittent: the diagnostic web pages
- Speed and duplex: 100 Mbps is normal through a switch. Duplex must match on the module and the switch port; it is set at power-up, so cycle power after a change. If autonegotiation fails, force both ends to the same setting.
- Interface counters: errors should be zero. Errors point to a duplex mismatch or electrical noise – run a continuous ping and look for lost replies or jitter over 10 ms.
- CPU, CIP and TCP connections: Rockwell recommends staying under 80 % of each. Too high: slower RPIs, rack-optimized connections, fewer messages and HMI reads, or another module.
- Packets per second: a 1756-ENBT is rated for 5,000 I/O and 900 HMI packets per second, and I/O traffic takes priority.
First checks
- Display message and the three LEDs, written down.
- Chassis power and the module fully seated.
- Cable, connector and switch port: the LINK LED and the switch's own port light.
- IP address: switches, BOOTP/DHCP, duplicates.
- The connection code on each faulted device.
- Web pages: duplex, errors, CPU and connection counts.
- Replacing a module: same catalog number and series, rotary switches set and firmware matched before it goes in.
Planning a network or controller upgrade instead? Our engineering team does PLC programming and network changes.
Safety first: only qualified personnel should work in the cabinet, and any measurement on energized equipment follows your site's live-work and arc-flash rules. A module that drops its connections can stop equipment, and one that reconnects can start it: put the machine in a safe state before you pull a module, reset an address or cycle chassis power, and follow Rockwell Automation's instructions and your site's lockout/tagout procedure. Rockwell warns that inserting or removing a module, changing switch settings or plugging cables with power on can arc – never in a hazardous location unless power is off or the area is confirmed non-hazardous. Never force outputs, jumper interlocks or bypass a safety circuit while a connection is down.
Parts that commonly fail
EtherNet/IP modules and the switches between them are the hardware behind many of these faults. The part numbers we have the most of in stock right now:
Stock changes every day. Every listing is photographed, so check the catalog number, series or product version and firmware on the label photo before you buy.
Not listed, or need several? Send us the part number and we'll check our shelves and our sourcing network and quote it in writing.
Sources
Every code, LED state and menu path on this page was checked against Rockwell Automation's own documents, listed below, and is described here in our own words. Manuals change between editions and firmware revisions: for your exact catalog or order number, check the current edition.
- Rockwell Automation, ControlLogix EtherNet/IP Network Devices User Manual, publication 1756-UM004F-EN-P, September 2024.
- Rockwell Automation, EtherNet/IP Network Devices User Manual, publication ENET-UM006C-EN-P, September 2025.
- Rockwell Automation, Troubleshoot EtherNet/IP Networks Application Technique, publication ENET-AT003B-EN-P, June 2014.
- Rockwell Automation, ControlLogix EtherNet/IP Bridge Module Installation Instructions (1756-ENBT), publication 1756-IN019E-EN-P, September 2021.
- Rockwell Automation, Logix 5000 Controller and I/O Fault Codes (spreadsheet), publication 1756-RD001F-EN-P, September 2025.
Frequently asked questions
What does Duplicate IP on a 1756-EN2T display mean?
Another device on the network has the module's IP address, so the module has gone into conflict mode with its NET LED red. The display scrolls the module's address and the MAC address of the other device, so you can find it. Give one of them a new address; if both started at the same time, both are in conflict – readdress one, then power-cycle the other.
Why does my 1756-ENBT show BOOTP and a flashing green OK LED?
The module has no IP configuration: its rotary switches aren't set to a valid 001 to 254 and BOOTP/DHCP is enabled, so it is asking a BOOTP/DHCP server for an address. Set the switches, or assign an address with the BOOTP/DHCP server, RSLinx Classic or Studio 5000; with the BOOTP/DHCP server, click Disable BOOTP/DHCP afterwards so the module keeps the address.
How do I reset a 1756-EN2T to factory defaults?
Turn the three rotary switches to 888 and power-cycle the module. That returns the module's configuration, including its IP address, to the factory value, so set the address again before the module goes back into service.
What does connection error 16#0113 mean?
The module's connection limit has been exceeded: more connections are asked of it than it supports. Reduce them, for example with a rack-optimized format for adapters, or add another module. Rockwell recommends keeping a module under 80 % of its rated CIP connections.