When Automation Communication Fails: Diagnosing PLC, Drive, and HMI Ne

In modern industrial facilities, a single communication failure can cost thousands per hour in lost production, even when every device still shows power. When PLCs lose I/O, drives drop offline, or HMIs stop updating, the system may appear operational—but it’s effectively blind. These failures often feel complex and unpredictable, yet most automation communication issues stem from a small set of well-defined causes.

This guide explains what communication failures really look like, the most common root causes behind them, which components are most often involved, and how to diagnose issues quickly—so you can restore control without guesswork or unnecessary part replacement.

What a Communication Failure Really Looks Like

Communication problems rarely announce themselves with a simple “network error.” Instead, they usually appear as secondary symptoms, including:

PLCs faulting or unexpectedly switching to stop mode
Drives displaying “No Communication,” “Fieldbus Error,” or reverting to local control
HMIs freezing, losing tag updates, or displaying stale values
Remote I/O modules intermittently dropping offline

Because control power remains present, teams often assume a PLC, drive, or HMI has failed—when the real issue lies in the communication path connecting the system.

The Most Common Root Causes

1. Loose, damaged, or poorly routed network cabling

Industrial environments are unforgiving. Vibration, heat, cabinet congestion, and years of maintenance activity all degrade cable integrity. Ethernet and fieldbus cables routed alongside high-power motor leads are particularly vulnerable to electrical noise and intermittent signal loss.

Red flag: If the issue appears only during startup, motion, or high load conditions, physical cabling should be your first inspection point.

2. Control power instability affecting communication modules

PLCs, network cards, drives, and remote I/O depend on clean, regulated control power. Even brief voltage dips can reset communication modules while the rest of the system appears healthy.

Power quality issues are frequently overlooked during troubleshooting, yet they are one of the leading causes of intermittent communication failures—especially in systems with aging power supplies or overloaded control circuits.

3. Network congestion or poor segmentation

As automation systems expand, multiple PLCs, drives, HMIs, historians, and remote access tools often share the same network. Without proper segmentation or managed switches, excessive traffic can introduce latency, dropped packets, or timeouts—critical failures in real-time control environments.

Watch for issues that appear immediately after adding new devices, software, or remote access connections.

4. Firmware, protocol, or configuration mismatches

Many communication failures follow maintenance or component replacement. Swapping in a “similar” device can introduce subtle incompatibilities such as:

Unsupported or mismatched firmware versions
Protocol version differences
Incorrect node addresses or device names

These problems are especially common in legacy systems or discontinued platforms where documentation is limited or incomplete.

Components Most Often Involved in Communication Failures

PLCs and Remote I/O

The PLC acts as the system’s traffic controller. A single failed CPU port, communication module, or remote I/O adapter can create widespread faults, making it appear as though multiple field devices have failed simultaneously.

Because PLC communication issues cascade quickly, they often cause the largest and most confusing outages.

Drives Using Fieldbus or Ethernet Control

Modern drives rely on network communication for commands, feedback, and diagnostics. When communication is lost, drives typically default to fault states or safe-stop conditions to protect equipment and personnel.

Intermittent drive faults are frequently network-related—not drive hardware failures—especially when multiple drives drop offline at once.

HMIs and Operator Interfaces

HMIs are the most visible part of the system, so they’re often blamed first. In reality, they are usually reporting a deeper problem upstream.

If an HMI freezes or loses tags intermittently, verify PLC communication, network health, and control power before replacing the interface.

A Real-World Example

A packaging line experienced repeated drive faults during morning startups. Operators replaced two drives and an HMI with no improvement. The root cause was eventually traced to a deteriorating 24VDC power supply that dipped during simultaneous motor starts—resetting the drive communication modules while the PLC stayed online.

Replacing the power supply resolved the issue permanently, preventing further unnecessary component swaps and downtime.

A Structured Approach to Troubleshooting

Instead of chasing symptoms, follow a disciplined process:

Determine whether the issue affects a single device or the entire network
Verify control power stability first
Inspect network cabling, routing, and terminations
Confirm firmware versions, addressing, and protocol compatibility
Substitute a known-good cable or communication module when possible

This method isolates the root cause faster and reduces the risk of replacing expensive components unnecessarily.

Why Having the Right Spares Makes the Difference

Communication failures are high-pressure events where every minute counts. Having tested spare PLC CPUs, communication cards, power supplies, and drive interface modules on hand can mean the difference between a quick recovery and a prolonged outage.

The most resilient maintenance teams don’t just react to failures—they prepare for them. Strategic spares and a structured diagnostic approach turn communication failures from production-stopping emergencies into manageable maintenance events.