The short answer
A node drops offline when it loses power or its link, has a wrong or duplicated address, sits on a cable or terminator fault, or doesn't match the configuration. On a 1756 EtherNet/IP module, a red NET LED with “Duplicate IP” on the display means two devices share an IP address, and Studio 5000 reports a device that never answers as 16#0204 and one that has stopped answering as 16#0203. On DH+ the 1756-DHRIO display shows messages such as DUPL NODE or ONLY NODE, and on DeviceNet the 1756-DNB shows the problem node's address with a code such as 78 (not communicating) or 91 (bus-off).
This covers Allen-Bradley ControlLogix EtherNet/IP modules (1756-EN2T, -EN2TR, -ENBT and others), the 1756-DHRIO on DH+ and the 1756-DNB scanner on DeviceNet, programmed with Rockwell Automation Studio 5000. For I/O modules in the local chassis, see I/O module faults.
Key takeaways
- First find out how much is down: one node points at that node, its drop or its address; a whole network points at the trunk, terminators, power or a switch.
- All three networks detect a duplicate address, and each module says so on its display or LEDs.
- DH+ and DeviceNet need a terminator at each end and the same data rate on every node.
EtherNet/IP: the 1756 module's LEDs
| LED | What it means (our words) |
|---|---|
OK off | No power: check the chassis supply and that the module is fully seated. |
OK flashing green | Not configured; the display scrolls BOOTP or DHCP and the module's MAC address. |
OK flashing red / red | A recoverable minor fault (check the configuration) / an unrecoverable major fault: cycle power, and if it stays red, replace the module. |
NET off | No power, or no IP address yet. |
NET flashing green | Has an IP address but no CIP connections, or all of them have timed out or closed. |
NET green | At least one CIP connection is up. |
NET red | Duplicate IP: the display shows the module's address and the other device's MAC address. |
LINK off / flashing green | No link on the port – cable, switch port or a disabled port / traffic on the port. |
The three rotary switches on the module set the last part of the IP address: 001 to 254 gives 192.168.1.xxx with subnet mask 255.255.255.0, and a setting that isn't a valid number normally makes the module ask a BOOTP/DHCP server. A few settings do something else: 888 and a power cycle reset the module to factory defaults, and on modules with protected mode, 900 and 000 turn it on and off. Rockwell recommends setting them before you install the module. Each module also has diagnostic web pages: browse to its IP address and open the Diagnostics folder.
The connection code in Studio 5000
16#0204– the connection request timed out: the target never answered and looks missing. Check its power, its slot or address and its network connection.16#0203– the target can be reached but stopped answering its connection as expected, for example when multicast packets don't come back; the controller keeps trying to remake it.16#0800– a communication module on the path is offline from its network: not connected, or unable to forward.
The code is on the device's Connection tab in the I/O configuration. The full list is in Rockwell's fault-code spreadsheet, 1756-RD001.
DH+: the 1756-DHRIO
- Address: each DH+ channel has a node address from 00 to 77, set and shown in octal on the module's switches. The display cycles through each channel, its node address and its status.
- Messages: DUPL NODE – another node has the same address; ONLY NODE – no other node heard, check the cables; OFF LINE – the link is stopped, correct the configuration; CNFG FALT – routing table or configuration wrong, or the module is in the wrong slot.
- Data rate: every device on a link must use the same rate. Maximum cable length is 3,048 m (10,000 ft) at 57.6 kbps, 1,524 m (5,000 ft) at 115.2 kbps and 762 m (2,500 ft) at 230.4 kbps.
- Terminators: terminate both ends of the link with the resistors shipped with the module – 150 Ω at 57.6 and 115.2 kbps, 82 Ω at 230.4 kbps. Rockwell's cable is 1770-CD (Belden 9463); drop cables are limited to 30.4 m (100 ft).
DeviceNet: the 1756-DNB and the trunk
The scanner's display alternates its own address and mode (for example A#01 RUN) and, when a node has a problem, that node's address and a code (N#33 E#72).
| Code | What it means (our words) |
|---|---|
70 | The scanner's own address is already used by another device. |
72 / 78 | A node stopped communicating / is still not communicating on the next scan: check its power and connection. |
73 | The device at that address isn't the one in the scan list (vendor, product code or type). |
77 | The device's data size doesn't match the scan list. |
79 | The scanner couldn't transmit: check it is on a working network, the cables and the data rate. |
91 | Bus-off, likely from cable or signal errors. |
92 | No 24 V network power at the scanner's port. |
- Terminators: one 121 Ω, 1 %, ¼ W (or larger) resistor at each end of the trunk, across the white (CAN_H) and blue (CAN_L) signal wires. With network power off, the two wires should measure about 50–60 Ω.
- Bus-off: Rockwell's checks are the same data rate on every device, no shorts between the CAN wires and the power or shield wires, terminators, grounding and noise from nearby high-voltage cables. One device set to the wrong rate can upset the others when it goes online.
- Network power: 24 V DC; the voltage at the middle and the ends of the network should be above 11 V and below 25 V DC. Ground the cable at one point only, near the middle.
- Node LEDs: solid red at power-up – two nodes share an address; solid red after allocation – bus-off, check the data rate; flashing red – no communication.
First checks
- Scope: one node down or the whole network?
- Physical layer: link LEDs, connectors seated, cables and drops undamaged, a terminator at each end of DH+ and DeviceNet trunks.
- Addresses: duplicate IP, DUPL NODE, code 70 or a node's solid red LED at power-up.
- Data rate: the same on every DH+ or DeviceNet node.
- Power: module and chassis power; 24 V on the DeviceNet trunk.
- Configuration: the connection code in Studio 5000 and the scanner's scan list.
- Replacing a module: set its address, data rate or IP switches to match the old one before you power it.
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. Network and power wiring often share a panel: switch off and lock out the panel supply before you open connectors or terminal blocks inside it, following Rockwell Automation's instructions and your site's lockout/tagout procedure. Never force outputs, jumper interlocks or bypass a safety circuit while a node is offline.
Planning a move off DH+ or an SLC 500? See the SLC 500 migration guide; our engineering team does PLC programming and network changes.
Parts that commonly fail
Communication modules, scanners, DH+ processors and DeviceNet media are the hardware behind many of these faults. The part numbers we have the most of in stock right now:
ControlLogix EtherNet/IP modules
PLC-5 processors (DH+ ports)
SLC 5/04 processors (DH+ port)
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, Logix 5000 Controller and I/O Fault Codes (spreadsheet), publication 1756-RD001F-EN-P, September 2025.
- Rockwell Automation, ControlLogix Data Highway Plus-Remote I/O Communication Interface Module User Manual, publication 1756-UM514C-EN-P, June 2014.
- Rockwell Automation, ControlLogix DeviceNet Scanner Module Installation Instructions, publication 1756-IN566D-EN-P, June 2008.
- Rockwell Automation, DeviceNet Media Design and Installation Guide, publication DNET-UM072C-EN-P, July 2004.
Frequently asked questions
What does a red NET LED mean on a 1756-EN2T?
The module is in conflict mode: another device on the network has the same IP address. The display scrolls the module's own address and “Duplicate IP” with the MAC address of the other device, so you can find it. Change one of the two addresses.
What is the difference between 16#0203 and 16#0204?
16#0204 means the connection request timed out: the target never answered and looks missing, so check its power, the slot or address and its network connection. 16#0203 means the target can be reached but has stopped answering the connection as expected; the controller keeps trying to remake it.
What does DUPL NODE mean on a 1756-DHRIO?
Another station on that DH+ channel uses the same node address. Choose an unused address – DH+ addresses run from 00 to 77 in octal – and set it on the module's node address switches. While a duplicate is detected, the DH+ channel's status LED flashes red.
What resistance should I measure on a DeviceNet trunk?
With DeviceNet power off, measure across the signal pair – white (CAN_H) and blue (CAN_L) in Rockwell's wire color tables. Rockwell expects about 50 to 60 ohms, from one 121-ohm terminator at each end of the trunk; it moves toward 50 ohms as more devices are connected.