Allen-Bradley EtherNet/IP Connection Timeout Fault — ControlLogix Fix: RPI, Port 44818, Switch QoS Root Cause

This Allen-Bradley EtherNet/IP connection timeout fault ControlLogix fix guide covers each root cause with a specific diagnostic test and corrective action. Approximately 40% of EtherNet/IP connection timeouts on industrial ControlLogix systems involve no actual hardware failure — the device is reachable, the cable is intact, and all link lights are green. The timeout is caused by one of three software or configuration conditions: an RPI (Requested Packet Interval) value set too low for the actual network latency, TCP port 44818 blocked by a firewall or industrial router, or a network switch that is not certified for industrial real-time traffic and drops EtherNet/IP packets during burst conditions. Each of these produces the same Type 3 fault in Studio 5000 as a physical cable disconnection. This guide provides the specific diagnostic test for each cause and the fix values for RPI settings by device type.

• EtherNet/IP Connection Timeout: What the Controller Detects
• Root Cause Table: Four Causes with Diagnostic Test and Fix
• Cause 1: RPI Value Too Low for Network Latency
• Cause 2: TCP Port 44818 Blocked
• Cause 3: Non-Industrial Switch Dropping Packets Under Load
• Cause 4: Duplicate IP Address on the Network
• Reference: Recommended RPI Values by Device Type
• Technical Validation
• Frequently Asked Questions

EtherNet/IP Connection Timeout: What the Controller Detects

The EtherNet/IP protocol uses TCP/IP for explicit messaging (MSG instructions, configuration, programming) and UDP/IP for implicit messaging (real-time I/O data). For I/O connections, the controller expects to receive an I/O update packet from the remote device within twice the configured RPI (the Connection Timeout Multiplier is 2 by default). If the controller does not receive the expected packet within that window, it declares a connection timeout and generates an I/O fault.

In Studio 5000, this appears as: – Type 3, Code 16 or 17 in the Faults tab (I/O connection fault) – The affected module in the I/O tree shows a red status indicator – The module’s Connection Status tag ([ModuleName]:C attribute) shows the connection error code

For the complete Type 3 fault code reference including sub-codes, see Allen-Bradley Controllogix Fault Codes Complete List Studio 5000.

Root Cause Table: Four Causes with Diagnostic Test and Fix

Cause 1: RPI Value Too Low for Network Latency

The RPI defines how often the remote device sends its I/O data to the controller. If the RPI is 2 ms but the actual network round-trip time is 3 ms, every I/O packet arrives late and the connection times out immediately.

Diagnostic test — measure network round-trip time: From the Studio 5000 workstation or any workstation on the same network segment:

ping -n 100 [device IP address]

The average RTT from the ping statistics is the network round-trip time. The RPI must be greater than RTT × 2 for reliable operation.

Safe RPI values: – Same LAN (no routers): minimum RPI = RTT × 4 (typical value: 4–10 ms) – Through one managed switch: minimum RPI = RTT × 4 (typical value: 5–20 ms) – Through a router or across subnets: minimum RPI = RTT × 10 (typical value: 20–100 ms)

Setting the RPI:

Studio 5000 → I/O Configuration → Right-click device → 
Properties → Connection → Requested Packet Interval (RPI)

Increasing the RPI does not reduce data freshness for most I/O applications — a digital input module reporting state changes every 20 ms (RPI = 20 ms) is more than adequate for process control applications with human-scale response requirements.

Cause 2: TCP Port 44818 Blocked

EtherNet/IP uses TCP port 44818 for connection establishment (the initial handshake before data transfer begins). If this port is blocked by a firewall, industrial router, or network segmentation policy, the controller cannot establish new EtherNet/IP connections. Existing connections (established before the firewall rule was applied) may continue; new connections or reconnections after a timeout fail.

Diagnostic test: From a workstation on the same network as the controller:

Windows: telnet [device IP] 44818
Linux: nc -zv [device IP] 44818

A “Connected” response means port 44818 is open. “Connection refused” or timeout means the port is blocked.

Fix: add a firewall rule to permit TCP traffic on port 44818 between the controller and the I/O device. For outbound rules: source = controller IP, destination = device IP, port = 44818, protocol = TCP. For inbound rules: source = device IP, destination = controller IP, port = 44818, protocol = TCP.

Note: EtherNet/IP implicit messaging (I/O data) uses UDP ports 2222 (EtherNet/IP) and 44818. If UDP 44818 is also blocked, implicit I/O data will not flow even after the TCP connection is established.

Cause 3: Non-Industrial Switch Dropping Packets Under Load

Consumer and commercial Ethernet switches prioritize bandwidth efficiency over determinism — under burst conditions, they may queue or drop packets that arrive faster than the switch can forward. Industrial EtherNet/IP traffic, particularly implicit I/O with RPIs of 2–10 ms across multiple simultaneous connections, can exceed the burst handling capacity of commercial switches.

Signs that a switch is causing EtherNet/IP timeouts: – Timeouts occur only during peak production (maximum I/O activity) and clear during idle periods – Multiple devices timeout simultaneously (suggesting a common switch is overloaded) – The switch is a commercial or consumer-grade unmanaged unit (Netgear, TP-Link, Linksys) rather than an industrial-grade unit (Stratix, Scalance, ProSoft)

Diagnostic via switch port statistics: For managed switches: access the switch management interface → Port statistics → check Rx Errors, Tx Errors, and Dropped Frames counters for the port connecting to the affected controller or device. Increasing error or drop counters confirm packet loss.

Fix: replace commercial switches with industrial Ethernet switches rated for EtherNet/IP — Rockwell Automation Stratix series, Siemens Scalance X series, or equivalent. Industrial switches support hardware-based Quality of Service (QoS) that prioritizes EtherNet/IP implicit messaging traffic over lower-priority network data.

Cause 4: Duplicate IP Address on the Network

If a second device on the network is assigned the same IP address as the ControlLogix I/O device, the EtherNet/IP connection establishment process (which uses IP addressing) may reach the wrong device, fail the CIP handshake, and generate a connection timeout.

Diagnostic test: From a command prompt on the network: arp -a [device IP]. The ARP table response should show exactly one MAC address for the IP. If two different MAC addresses appear at different times, or if the MAC address does not match the expected device’s MAC (printed on the device label), a duplicate IP exists.

Fix: identify which device has taken the duplicate IP by checking all networked devices. Use the management interface of a managed switch to identify which physical port the duplicate MAC is connected to. Assign a unique static IP to the duplicate device and configure all Rockwell I/O devices with static IPs (not DHCP) to prevent future address conflicts.

For the systematic 8-step diagnostic framework that provides the process context for EtherNet/IP fault diagnosis, see Plc Diagnostic Troubleshooting Systematic 8-Step Guide Industrial. For RSLinx Classic communication path errors that arise alongside EtherNet/IP faults, see Rslinx Classic Communication Path Not Found Error Fix Studio 5000.

Reference: Recommended RPI Values by Device Type

Technical Validation

EtherNet/IP connection mechanism and RPI specification from the Allen-Bradley EtherNet/IP Application Guide. Type 3 fault code reference from Rockwell Publication 1756-PM014. Industrial network training from Cincinnati State Workforce Development Center PLC programs.

Frequently Asked Questions

The EtherNet/IP timeout occurs only on Monday mornings. What explains a weekly pattern for a network fault?

A Monday-morning-only EtherNet/IP timeout pattern strongly suggests a network infrastructure issue that correlates with weekend shutdown and Monday startup. Most likely causes: (1) a network switch that loses its ARP table entries during the weekend power-down — on restart, it must rediscover all devices via ARP, temporarily disrupting EtherNet/IP connections during the discovery period; (2) a DHCP address change on a device that does not have a static IP — the device received a different IP on Monday and the controller is trying to connect to the old IP address; (3) a firewall that resets its session table during a scheduled weekend maintenance window. Fix: verify all I/O devices have static IP addresses, and check the switch ARP table aging settings.

Multiple ControlLogix chassis on the same network all experience EtherNet/IP timeouts simultaneously. Is this a controller problem or a network problem?

Simultaneous timeouts across multiple controllers on the same network segment always indicate a network infrastructure problem, not individual controller issues. A single controller fault does not affect other controllers’ I/O connections. Simultaneous events point to: a common switch experiencing a failure or overload condition, a broadcast storm on the network (often caused by a loop in the physical cabling), a DHCP server assigning duplicate addresses to multiple devices simultaneously, or a firewall rule change that went into effect at the time of the simultaneous timeouts. Check switch error logs and look for broadcast packet rate spikes at the time of the simultaneous faults.

After fixing the root cause, how long does it take for the EtherNet/IP connection to re-establish automatically?

After the network condition is resolved, the ControlLogix controller re-establishes EtherNet/IP connections automatically without requiring a fault clearance or restart. The reconnection time depends on the Connection Timeout Multiplier configured for each device (default: 2) and the RPI: reconnection = RPI × 2 (minimum). For a device with RPI = 10 ms, the minimum reconnection time is 20 ms after the connection is re-established. For devices with longer RPIs, the reconnection may take longer. The controller Fault tab will show the Type 3 fault until the connection is fully re-established — the fault does not need to be manually cleared if the automatic reconnection succeeds.

The Connection Timeout Multiplier (CTM) is configurable per device in Studio 5000: Module Properties → Connection → Timeout Multiplier. Increasing CTM from 2 to 4 or 8 for devices on marginally reliable network paths prevents intermittent disconnections without changing the RPI — the device continues sending data at the original RPI interval, but the controller tolerates up to 4 or 8 consecutively missed packets before declaring a connection fault. This is the preferred adjustment for remote I/O on routed segments where occasional packet delays are expected but sustained loss indicates a real fault.