1. Description of the problem and scope of application

This guide is intended for diagnosing and troubleshooting communication failures of programmable logic controllers (PLCs) in industrial automation field networks. Typical symptoms include: complete or intermittent loss of communication with one or more network devices, I/O errors, slow or unstable control system operation, unplanned shutdowns of production processes, and communication error messages on operator panels (HMIs) or in PLC syslogs. The manual covers diagnostics of the most common industrial Ethernet-based protocols such as PROFINET, EtherNet/IP, as well as the serial protocol Modbus RTU/TCP.

Applicable equipment: PLCs from various manufacturers (eg, Siemens, Rockwell Automation, Schneider Electric), distributed I/O systems, industrial network switches, frequency converters, servo drives, sensors, actuators with field network support, as well as passive network components (cables, connectors, terminators).

Classification of severity of malfunctions:

• Critical: Complete loss of communication with the PLC or key devices, resulting in an emergency shutdown of the production line or significant financial losses. Requires immediate intervention.
• Significant: Intermittent communication failures causing unstable hardware, reduced performance, or frequent but short-term shutdowns. Requires urgent diagnosis.
• Minor: Sporadic communication errors that do not directly affect the production process, but may indicate initial problems or network degradation. Scheduled diagnostics is recommended.

2. Safety measures

CAUTION: SAFETY!

• Lockout and Tagout (LOTO): Before performing any work that requires tampering with electrical or mechanical parts of the equipment, you MUST apply lockout and tagout procedures (DSTU EN 1037, ISO 14118). Make sure there is no voltage using proven measuring tools.
• Electrical safety: Work with electrical equipment must be performed only by qualified personnel in accordance with the requirements of NPAOP 40.1-1.21-98. Always assume electrical circuits are live until proven otherwise.
• Personal protective equipment (PPE): Always use appropriate PPE (gloves, safety glasses, safety shoes, protective clothing) according to the workplace risk assessment (DSTU EN 340, DSTU EN 388, DSTU EN 166).
• Residual Energy: After the power is turned off, certain components (such as capacitors in power supplies, pneumatic or hydraulic accumulators) can store dangerous energy. Wait for complete discharge or reset of energy before starting work.
• Hot surfaces: Some components (eg PLC modules, power supplies) may have hot surfaces that can cause burns. Be careful.

3. Necessary diagnostic tools

The following set of tools is required for effective diagnosis of PLC communication failures:

4. Initial evaluation checklist

Before starting a detailed diagnosis, perform the following steps to collect initial information and visual assessment:

5. Systematic diagnostic algorithm

The following algorithm will help to consistently identify the source of the problem:

1. Symptom: No communication with one or more devices on the network.
   1. Checking the status indicators:
      • If the power (PWR) or status (STATUS/RUN) indicators of the device are off or flashing red:
        1. Check the power source of the device (voltage, fuses, connections).
        2. If the power is OK but the status indicators are faulty: Probable cause: device malfunction. Go to Device Fault Diagnosis.
      • If the device's power and status indicators are normal, but the communication indicators (LINK/ACT/COMM) are off or flashing red:
        1. Go to step 1b.
   2. Physical level check (cables and connectors):
      1. Visually inspect the cable connecting the problem device:
      2. Are there visible damages, bends, pinching?
      3. Are the connectors firmly inserted into the ports? Try reconnecting.
      4. Use a cable tester to check the integrity and routing of the cable:
         • If the test fails (open, short, cross-wire): Probable cause: damaged cable or connector. Go to Troubleshooting the Cabling System.
         • If the test is successful: Go to 1c.
   3. Check network settings and configuration:
      1. Connect the laptop to the same network (or directly to the device) and try executing the command ping <device IP address>:
         • If there is no response or packet loss: Go to 1c.ii.
         • If the answer is yes, but there is still no communication with the PLC: Probable cause: incorrect configuration of the PLC or device. Go to the "Network Configuration Diagnostics" section.
      2. Use PLC software (TIA Portal, Studio 5000, etc.) or a dedicated network analyzer:
         • Check the IP address, subnet mask, gateway of the problem device and make sure they match the project.
         • For PROFINET/EtherNet/IP: Check the device names and make sure they are unique and match the PLC configuration.
         • Check the speed and duplex settings (auto-adjust or fixed).
         • If the settings are incorrect: Probable cause: incorrect network configuration. Go to "Troubleshooting network configuration".
         • If the settings are correct: Go to step 1d.
   4. Node isolation and interference effects:
      1. If the problem persists, try to temporarily isolate the problem device by connecting it directly to the PLC or to a minimum network test segment:
         • If communication is restored: Probable cause: network infrastructure problem (switch, other device, interference) or IP address conflict. Go to section "Isolation and diagnostics of network infrastructure".
         • If the connection is not restored: Probable cause: malfunction of the device or its network interface. Go to the section "Diagnostics of the malfunction of the device".
      2. Use a thermal imaging camera to check for overheating network components or power supplies related to communication.
      3. If electromagnetic interference (EMF) is suspected: Use an oscilloscope or a specialized EMF analyzer to evaluate the noise level.
2. Symptom: Intermittent communication failures, packet loss, high jitter.
   1. Check section 1.a, 1.b.
   2. Use network analyzer to monitor traffic:
      • Are there collisions, looping packets, broadcast storms?
      • Are there an abnormally high number of erroneous packets?
      • Probable cause: network congestion, faulty switch, EMF, duplicate IP/names. Go to Network Infrastructure Isolation and Diagnostics or Interference Impact Diagnostics.
   3. Check the stability of the power supply for all network devices (voltage sags are possible).

6. Malfunction-cause matrix

The table below summarizes typical symptoms, likely causes, and methods of diagnosing them:

7. Root cause analysis for each malfunction

Understanding the root cause is critical to preventing repeated failures.

7.1. Damage to the cable system

• Explanation: Cables are the physical backbone of a network. They can be damaged mechanically (bends, pinches, cuts), under the influence of aggressive environments (chemicals, oils), high temperatures, vibration or rodents. Internal wire breaks, short circuits, or crosstalks can occur due to poor installation or insulation degradation.
• How to confirm: Cable tester (breaks, short circuit, incorrect wiring, high return loss), visual inspection, physical movement of the cable (may restore communication temporarily).
• Damage, if not corrected: Sporadic or continuous loss of communication, resulting in equipment shutdowns, data errors, controllability, and, as a result, significant production losses.

7.2. Incorrect network configuration

• Explanation: Each device in an industrial network must have a unique IP address (for Ethernet-based protocols), correct subnet mask, gateway, and for PROFINET/EtherNet/IP a unique device name. Mistakes in these settings (for example, duplicate IP addresses, wrong device name) lead to conflicts and the inability to establish communication.
• How to confirm: Network analyzer (detects duplicate IP addresses, name conflicts), PLC software (reads/writes device configuration), ping commands.
• Damage if not fixed: Defective devices, incorrect management, unpredictable network behavior, inability to integrate new hardware.

7.3. Problems with powering devices

• Explanation: Unstable, low or absent supply voltage leads to incorrect operation of network interfaces of devices or to their complete shutdown. Triggering of protections, interruptions in power supply circuits, malfunction of power supply units, overvoltage.
• How to confirm: Multimeter (measurement of the supply voltage at the device input, comparison with the nominal), visual inspection of fuses, power indicators. Standard: 24 V DC ±10% for industrial systems.
• Damage if not repaired: Malfunctions of powered devices, failure of other components due to unstable voltage, production stoppages.

7.4. Electromagnetic interference (EMF)

• Explanation: High-frequency noises caused by power cables, inverters, welding equipment, radio transmitters, electric motors can travel on signal cables and distort data transmission. Poor cable shielding, lack of or incorrect grounding can increase this effect.
• How to confirm: Oscilloscope (visualization of noise on signal lines), network analyzer (increase in number of CRC errors, decrease in throughput), ground check (multimeter).
• Damage if not fixed: Intermittent and unpredictable communication failures that are difficult to diagnose, data errors, network slowdowns that can lead to false positives and crashes.

7.5. Network equipment/interface failure

• Explanation: Malfunction of the industrial switch (burnt port, faulty board), PLC network adapter or field device network interface. This can be due to overheating, short circuit, overvoltage or natural wear of components.
• How to confirm: Node isolation (direct connection), switch/device replacement with a known good one, thermal imaging camera (detection of overheating), switch event log (notification of port failures).
• Damage, if not repaired: Total or partial failure of the network, resulting in production stoppage.

8. Step-by-step troubleshooting procedures

For each identified root cause, perform the following corrective actions:

8.1. Troubleshooting the cable system

1. CAUTION: SAFETY! Apply LOTO before working with cables that may be live.

2. Damage identification: Use a cable tester to pinpoint the location and type of damage (break, short circuit, crosstalk).
3. Cable Replacement: If the damage is significant, replace the entire length of cable with a new one that meets industry standards (eg CAT5e/CAT6A for Ethernet, shielded, with copper cores). Use cables with the appropriate protection class (IP) and resistance to external influences.
4. Replacing connectors: If only the connector (eg RJ45) is damaged, carefully cut it and install a new one using a special tool (crimper). Make sure the wiring is correct (T568A or T568B).
5. Checking the shielding and grounding: Ensure that the cable shielding is correctly connected to grounding on both sides (for shielded cables) or on one side (for some configurations). Check ground resistance with a multimeter (should be <4 ohms).
6. Verification: After replacement or repair, retest the cable with a cable tester. Make sure all parameters are standard (eg ISO/IEC 11801). Restore the power and check the communication with the device.

8.2. Troubleshooting network configuration

1. CAUTION: Be careful when changing network settings, it may affect the entire system.

2. Determining the correct settings: Refer to the project documentation or to the current configuration of other similar devices.
3. Change IP address/device name: Using PLC software (eg TIA Portal, Studio 5000) or specialized utilities (eg Siemens Primary Setup Tool, Rockwell BOOTP/DHCP Server), set the correct IP address, subnet mask, gateway and device name. Make sure these parameters are unique.
4. Checking speed/duplex settings: If fixed settings are set, make sure they match the corresponding switch port settings. It is recommended to use auto-reconcile if possible.
5. Rebooting the device: After changing the settings, as a rule, you need to reboot the device to apply them.
6. Verification: ping to the device. Check the communication through the PLC software. Verify that the device appears in the network topology without errors.

8.3. Restore power to devices

1. CAUTION: SAFETY! Apply LOTO. Before taking voltage measurements, make sure the multimeter is set to the correct range.

2. Voltage measurement: Using a multimeter, measure the supply voltage directly at the terminals of the problematic device. Make sure it is within acceptable limits (eg 24 V DC ±10%).
3. Check fuses: Inspect and check the fuses protecting the device's power circuit for integrity. Replace blown fuses with new ones of similar rating and type (EN 60127).
4. Power Supply Diagnostics: If voltage is low or absent, check the output voltage of the power supply supplying power to the device. If necessary, replace the faulty power supply unit.
5. Checking the power cables: Check the power cables for breaks, insulation damage, loose contacts.
6. Verification: After restoring stable power, check the device's power indicators. Reconnect to the PLC.

8.4. Reducing the influence of electromagnetic interference

1. CAUTION: SAFETY! Grounding work must be performed by qualified personnel.

2. Ground Check: Make sure the control cabinets and all components are properly grounded according to EN 60204-1. Check the integrity of the ground loop with a multimeter (resistance should be minimal).
3. Cable separation: Separate signal cables and power cables. They should be laid in separate channels or at a sufficient distance (minimum 20 cm).
4. Use of shielded cables: Use only shielded industrial Ethernet cables (eg PROFINET Type B/C, EtherNet/IP ODVA Industrial Ethernet) with proper shield connection.
5. Direction filters: Apply ferrite rings or EMF filters to the power cables of devices that generate interference (such as frequency converters).
6. Verification: Network traffic monitoring using an analyzer to reduce the number of CRC errors and improve connection stability.

8.5. Replacement of faulty network equipment/interface

1. CAUTION: SAFETY! Apply LOTO. Before replacing equipment, make sure you have a suitable replacement.

2. Fault identification: Confirm the fault by isolating the assembly or replacing with a known-good component.
3. Switch replacement: Replace a faulty industrial Ethernet switch with a new one with similar characteristics (number of ports, speed, protocol support).
4. Replacing the PLC module: If the PLC network module (eg Siemens CP module) is faulty, replace it according to the manufacturer's instructions.
5. Field device replacement: If the network interface of a field device (for example, an I/O module) is faulty, replace the entire device.
6. Configuration: After replacing the new equipment, it is necessary to restore its network settings and configuration according to the project documentation.
7. Verification: Check communication with all devices connected to the replaced hardware. Network stability monitoring.

9. Preventive measures

Prevention is more effective than elimination of consequences.

10. Spare parts and components

Availability of up-to-date spare parts is critical for quick recovery.

To order high-quality spare parts that meet industrial standards and have the necessary certificates (CE, UkrSEPRO), visit our electronic catalog UNITEC.

11. References

• DSTU EN 61784 (ISO 15745) – Industrial communication networks.
• ISO/IEC 11801 – Information technologies. Structured cable system.
• EN 60204-1 – Machine safety. Electrical equipment of machines. Part 1: General requirements.
• EN 61131-2 – Programmable controllers. Part 2: Equipment requirements and operational tests.
• NPAOP 40.1-1.21-98 - Rules for the safe operation of consumer electrical installations.
• ОЕМ посібники з програмування та діагностики для Siemens (PROFINET System Description), Rockwell Automation (EtherNet/IP CIP Networks), Schneider Electric (Modbus TCP/IP Communication).

1. Description of the problem and scope of application

This diagnostic guide is designed for engineers and service technicians who experience intermittent or no communication between programmable logic controllers (PLCs) and peripherals via industrial fieldbuses. The guide covers diagnosing and troubleshooting networks using Profinet, EtherNet/IP and Modbus (RTU/TCP) protocols.

Typical symptoms include:

• Lack of communication with one or more network nodes.
• Intermittent connection or periodic data loss.
• Slow response of devices or delays in data transfer.
• Communication errors displayed on the PLC or devices (eg CRC errors, timeouts).
• Reduction in productivity of the automation system.

These problems can lead to production stoppages, data loss, equipment damage and significant financial losses. Effective diagnostics and quick restoration of communication are critical for uninterrupted operation of industrial processes.

Classification of the severity of the malfunction:

• Critical: Complete loss of communication with key production nodes or PLCs, resulting in an immediate shutdown of production. Requires immediate intervention.
• Significant: Intermittent connectivity or loss of connectivity to non-critical nodes, causing reduced performance or intermittent crashes, but not a complete shutdown. Needs urgent elimination.
• Minor: Single communication errors or minor delays that do not affect production critically, but may indicate potential problems. Requires monitoring and remediation planning.

2. Precautions

CAUTION: Before beginning any diagnostic or repair work on industrial equipment, ALWAYS follow standard safety procedures. Failure to follow these instructions could result in serious injury or death, or damage to the equipment.

• Lockout/Tagout (LOTO): Before disconnecting or connecting cables, disassembling equipment, or working near moving parts, be sure to use LOTO procedures to isolate all sources of energy (electrical, pneumatic, hydraulic). Make sure there is no residual energy.
• Personal Protective Equipment (PPE): Always use appropriate PPE, such as safety glasses, gloves, protective clothing and protective footwear, according to the requirements of your business and the nature of the work.
• Working with electrical equipment: Make sure you are qualified to work with electrical networks. Always consider all electrical circuits live until proven otherwise by proven measuring equipment. Avoid direct contact with live parts.
• Residual Energy: Be careful with equipment that can store energy (eg capacitors, springs, accumulators, pressure in hydraulic/pneumatic systems). Make sure it is discharged or safely isolated.
• Hot surfaces: Some equipment components may be hot. Use heat-resistant gloves or allow the equipment to cool before working.
• Height: When working at height, use appropriate fall protection.

3. Necessary diagnostic tools

A set of specialized tools is necessary for effective diagnosis of communication problems.

4. Initial evaluation checklist

Before starting a detailed diagnosis, perform an initial assessment to gather the maximum amount of information about the malfunction. This will help narrow down the potential causes.

5. Systematic diagnostic algorithm

This algorithm offers a consistent approach to diagnosing communication problems. Follow it to isolate and troubleshoot effectively.

1. Determine the nature of the problem: single node or multiple?
   • If the problem is with a single node:
     1. Physical layer verification:
        1. Visual inspection of the cable and connectors to this node.
           • If visible damage: Go to 8. Step: Replacing damaged components.
           • If there is no damage: Continue.
        2. Checking the unit's power supply (LED indicators, multimeter).
           • If power is missing or incorrect (eg <21V DC): Check power supply, fuses, power cables. Restore power. If the problem persists, check the assembly itself for internal damage.
           • If power is normal: Continue.
        3. For Ethernet/Profinet/EtherNet/IP: check Link/Act indicator on device and switch (if present).
           • If Link is missing: Check the cable with a cable tester. Replace if defective.
           • If Link is present, but Act is missing or chaotic: Possible problem with auto-negotiation (speed/duplex) or malfunction of the device's network interface.
        4. For Modbus RTU (RS-485): measure the voltage between lines A and B (should be 0V for no data, >0V for transmission). Termination check (120 ohms between A and B at segment ends).
           • If the termination is incorrect: Correct it.
           • If the signal is distorted (oscilloscope): Possible noise, incorrect transmission speed, interface malfunction.
     2. Logical level check:
        1. Node configuration check (IP address, Profinet name, Modbus speed, slave ID).
           • If the configuration is incorrect: Correct in the PLC and/or on the device. Download the configuration.
        2. Ping the device (for Ethernet-based networks).
           • If Ping does not go through: Physical layer problem, IP conflict or network interface failure.
           • If Ping passes, but there is no communication with the PLC: Possible problem with the configuration of the protocol (for example, GSDML file Profinet, EDS file EtherNet/IP) or malfunction of the PLC/device itself.
        3. Rebooting the node (power off/on).
           • If the connection is restored: Possible temporary failure of the firmware. Test the stability.
2. If the problem is with several nodes or the whole network:
   1. Check the central equipment:
      1. Check the PLC: status, diagnostic log.
         • If the PLC is in the Stop state or has errors: Troubleshoot the PLC according to the manufacturer's documentation.
      2. Switch/router test (for Ethernet): status indicators, logs.
         • If the switch is faulty: Replace the switch.
      3. Checking PLC network modules: status indicators, fastening check.
         • If the module is faulty or incorrectly installed: Replace/reinstall the module.
   2. Checking common factors:
      1. Strong electromagnetic interference (EMF) near communication cables.
         • If there are EMF sources: Provide cable shielding, use ferrite filters, redistribute cable routes.
      2. Problems with the grounding of the network or equipment.
         • If the grounding is incorrect: Correct the grounding according to standards (for example, DSTU EN 60204-1).
      3. Network congestion (network analyzer).
         • If the network is overloaded: Divide the network into segments, optimize traffic, increase bandwidth.
      4. IP address conflicts (for Ethernet).
         • If a conflict is found: Reassign unique IP addresses.
3. Segment/Node Isolation:
   1. Sequentially disconnect nodes from the network (one at a time) while watching for reconnection with other nodes.
      • If the connection is restored after disconnecting a certain node: The problem is with the disconnected node or its cable. Focus on it.
   2. For Ethernet-based networks: Use the "Ping Flood" tool to detect downloads or "Ping" each device individually.
   3. For Modbus RTU: Split the segment into parts, adding devices one at a time to identify the problematic one.
4. Data analysis with a network analyzer:
   • Connect the network analyzer to the problem segment or near the node.
   • Analyze the data for:
     • CRC (Cyclic Redundancy Check) error - indicate data corruption during transmission, often due to noise or damaged cables.
     • Collision – for half-duplex networks (such as old Ethernet or Modbus RTU), indicate simultaneous data transmission.
     • Packet loss – can be the result of overload, noise or device malfunction.
     • Latency - may indicate network congestion or switching equipment malfunction.
     • Invalid frames/packets.
   • Compare the obtained data with the recommended values ​​(eg for Profinet RT: max packet delay <1ms, jitter <1μs).
5. Cable integrity check:
   • Use a cable tester to check all pairs, lengths, breaks, shorts, and crosstalk.
   • Permissible parameters of Profinet/EtherNet/IP cables (according to IEC 61784-5-3/5-2):
     • Segment length: Cat5e/Cat6 no more than 100 meters (without switch).
     • Losses (Attenuation): < 20 dB per 100 m (for 100 Mbit/s Cat5e).
     • Crosstalk (NEXT): > 30dB (for 100Mbps Cat5e).
   • For Modbus RTU (RS-485): cable resistance measurement, no short circuits.
6. Termination and Ground Check:
   • For Modbus RTU (RS-485): Make sure the 120 ohm termination resistors are installed only at the ends of the segment. Measure the resistance between A and B at the ends of the segment - should be around 60 ohms (for two terminators in parallel).
     • Incorrect termination: May cause signal reflection and communication errors.
   • Check the quality of equipment grounding and cable shields.
     • Incorrect grounding: May lead to ground loop and induced interference.
7. Compatibility and firmware check:
   • Make sure that the firmware versions of devices and drivers are compatible with the version of PLC and engineering software.
   • Verify that the GSDML/EDS files match the actual devices.
8. Replacement of damaged components:
   • If diagnostics revealed a faulty cable, connector, terminator, device network interface, switch or PLC module, replace it with a serviceable one.
   • Remember about LOTO!
9. Verification of communication recovery:
   • After troubleshooting, check the stability of the connection for a certain time.
   • Monitor the PLC diagnostic log and device status indicators.
   • Ensure that all production functions are restored.

6. Matrix of malfunctions and causes

This table presents the common symptoms of communication problems and their likely root causes, ranked by frequency of occurrence.

7. Root cause analysis for each malfunction

Understanding root causes is critical to troubleshooting as well as developing effective preventative measures. The most common causes of communication failures are discussed in detail below.

7.1. Damage to cables and connectors

Explanation: Mechanical damage (bends, stretching, crushing), vibration, aggressive chemical environments or simply aging of materials can lead to wire breaks, short circuits or loss of shielding. Poor-quality crimping of connectors or their oxidation is also a frequent cause of poor contact.

How to confirm: Visual inspection reveals obvious damage. The cable tester accurately identifies breaks, shorts, twisted pairs, incorrect lengths, or excessive loss/crosstalk. Intermittent contact when touching the cable also indicates a contact problem.

Potential consequences: Total communication loss, intermittent errors, increased CRC errors, signal degradation, loss of shielding resulting in EMF sensitivity.

7.2. Incorrect device power supply

Explanation: Lack of stable and correct power supply (under/over voltage, pulsations) can lead to unstable operation of the network interface of the device, its "freezing" or complete shutdown. This is especially critical for PoE (Power over Ethernet) enabled devices, where the problem may be with the PoE injector or switch.

How to confirm: Measure the supply voltage directly at the terminals of the device using a multimeter. Checking status indicators on power supplies. Measurement of ripples with an oscilloscope (must be minimal).

Potential consequences: Unstable connection, periodic disconnections of devices, firmware malfunctions due to incorrect disconnection, failure of device components.

7.3. Incorrect network/device configuration

Explanation: This includes incorrect IP addresses (conflicts, subnet mismatch), incorrect Profinet names, incorrect Modbus slave IDs, incorrectly set baud rate, duplex mode, missing or incorrect GSDML/EDS files in the PLC engineering software.

How to confirm: Comparison of actual device settings (via web interface, DIP switches, specialized software) with the configuration in the PLC program. Using network scanning tools to detect IP conflicts. Checking PLC logs for configuration errors.

Potential consequences: Complete lack of communication with certain nodes, incorrect data processing, inability to connect the device to the network.

7.4. Electromagnetic interference (EMF)

Explanation: Powerful EMF sources (electric motors, welding machines, frequency converters, relays) can induce noise in communication cables, distorting digital signals and leading to data transmission errors (CRC errors, packet losses). Insufficient cable shielding or improper grounding exacerbates this effect.

How to confirm: Inspection of cable routes in relation to proximity to EMF sources. Using an oscilloscope to analyze waveforms on communication lines (especially RS-485 Modbus RTU) - presence of "noise" or distortion. A network analyzer will show a large number of CRC errors.

Potential consequences: Intermittent or unstable connection, "phantom" errors that are difficult to diagnose, reduced system reliability.

7.5. Incorrect termination (for Modbus RTU / RS-485)

Explanation: For RS-485 networks using the Modbus RTU protocol, the presence of 120 ohm (±5%) terminating resistors at both ends of the bus segment is critical. They prevent signal reflections that can cause distortion and communication errors. An incorrect number of terminators (more or less than two) or their incorrect location will lead to unstable network operation.

How to confirm: Turn off the power from the RS-485 segment. Measure the resistance between lines A and B at the ends of the segment with a multimeter. With two terminators, the resistance should be about 60 ohms. With disconnected devices and one terminator - 120 ohms. If there is no terminator - infinity or a very large value.

Potential consequences: Intermittent communication failures, high number of CRC errors, loss of communication with remote devices, unstable network operation.

7.6. Failure of network equipment or interfaces

Explanation: Internal failures of Ethernet switches, routers, PLC network modules or network interfaces of individual devices can lead to complete or partial loss of communication. Overheating, aging of components, voltage surges can cause their failure.

How to confirm: Checking the status indicators (LED) on the equipment. Replacing the suspect component with a working one. Analysis of switch/PLC logs for port or module errors. A thermographic camera can detect overheating.

Potential consequences: Complete stoppage of a network segment or the entire system, inability to communicate with individual devices, performance degradation.

8. Step-by-step troubleshooting procedures

The procedures below detail the steps to resolve identified root causes.

8.1. Troubleshooting cables and connectors

1. Identification: Using a cable tester (eg Fluke MicroScanner2) and visual inspection, pinpoint the location and type of damage (break, short, twisted pair, screen damage).
2. Safety: APPLY LOTO PROCEDURES! Isolate all power supplies to the damaged network segment and devices connected to it.
3. Replacement: Replace the damaged cable completely or repair the connector using quality components that meet industry standards (eg Profinet Type B/C cables, RJ45 IP67 connectors). Ensure proper crimping (T568B or T568A) and shielding.
4. Check: After replacing/repairing, retest the cable with a tester. Make sure that all parameters (length, wiring diagram, no open/shorts) meet the requirements.
5. Restore Power: After completion of work and security check, restore power and remove LOTO.
6. Verification: Check communication via PLC, Link/Act indicators on devices. Monitor stability.

8.2. Restoration of correct power supply

1. Identification: Measure the supply voltage directly on the device with a multimeter. Most industrial devices require 24V DC (allowable range 21.6V - 26.4V DC). Check the status indicators on the power supply.
2. Safety: APPLY LOTO PROCEDURES! Isolate the power supply serving the faulty device.
3. Elimination:
   • If the voltage is low: Check the load on the power source, its serviceability, the crossing of the power cables, the presence of loose contacts. Replace the power supply if it is faulty.
   • If there is no power: Check fuses, circuit breakers, integrity of power cables.
   • If the ripples are higher than acceptable (<100 mV): Replace the power supply.
4. Check: After eliminating the problem, measure the voltage again. Make sure it is stable and meets the requirements of the device.
5. Restore Power: After completion of work and security check, restore power and remove LOTO.
6. Verification: Check communication via PLC.

8.3. Network/device configuration adjustments

1. Identification: Use engineering software (TIA Portal, Studio 5000) to check the configuration of the PLC and devices. Make sure the IP addresses are unique, the Profinet names match, and the Modbus slave IDs match the master settings.
2. Adjustments:
   • For Ethernet/Profinet/EtherNet/IP: Change the IP address, name or configuration of the device according to the project. Download the new configuration to the PLC and/or device.
   • For Modbus RTU: Set the correct slave ID (with DIP switches or via configuration software). Check the baud rate and data format (eg 9600, 8, N, 1).
3. Verification: After downloading the configuration, check the connection and monitor the status of the devices.

8.4. Elimination of EMF influence and grounding problems

1. Identification: Identify sources of EMF (powerful motors, welding machines, frequency converters). Check the quality of the shielding of the cables and their grounding (one-sided grounding of the shields for Ethernet, two-sided for Modbus RTU).
2. Measures:
   • Remove communication cables from EMF sources.
   • Use industrial grade shielded cables (eg Profinet Type A/B, Cat5e/Cat6 SF/UTP) and metal cable trays.
   • Ensure correct grounding of cable screens and equipment in accordance with standards (DSTU EN 60204-1, DSTU ISO 21464). Check ground resistance (should be <4 ohms).
   • Use ferrite filters on cables to suppress high-frequency interference.
   • Install voltage stabilizers and filters to power sensitive equipment.
3. Verification: Monitor network traffic for CRC errors using a network analyzer.

8.5. Modbus RTU (RS-485) termination adjustment

1. Identity: APPLY LOTO PROCEDURES! Remove power from the RS-485 segment. Measure the resistance between lines A and B at the beginning and end of the segment. There should be one 120 ohm resistor at each end.
2. Fix:
   • If there are no terminators or more than two: Install/remove 120 ohm resistors so that they are only on the first and last device of the segment.
   • Check whether the internal terminators on the devices can be activated (often a DIP switch).
3. Check: After adjustment, measure the resistance between A and B. It should be about 60 ohms (parallel connection of two 120 ohm resistors).
4. Restore Power: After completion of work and security check, restore power and remove LOTO.
5. Verification: Check the stability of Modbus RTU communication.

8.6. Replacement of faulty network equipment/interfaces

1. Identification: Using PLC/switch diagnostic logs, communication tests, and visual inspection, identify the faulty component (switch, PLC network module, device network interface).
2. Safety: APPLY LOTO PROCEDURES! Isolate all power supplies to the component being replaced.
3. Replacement: Disconnect the faulty component. Install a new component that has the same or compatible specifications.
4. Configuration: If the new component requires configuration (IP address, name), do it according to the project.
5. Restore Power: After completion of work and security check, restore power and remove LOTO.
6. Verification: Check communication via PLC, Link/Act indicators. Monitor stability.

9. Preventive measures

The implementation of preventive measures significantly reduces the probability of repeated communication failures.

10. Spare parts and components

Availability of critical spare parts in stock is a prerequisite for rapid troubleshooting and minimizing downtime.

To order high-quality industrial components and spare parts that meet DSTU, EN, ISO standards, please refer to the electronic catalog UNITEC-D. We offer proven solutions to ensure the reliability of your automation systems.

11. Links

• DSTU EN 60204-1:2018 (IEC 60204-1:2016, IDT) Machine safety. Electrical equipment of machines. Part 1. General requirements.
• DSTU ISO 21464:2022 (ISO 21464:2020, IDT) Profibus and PROFINET networks for industrial automation systems.
• IEC 61784-5-3: Industrial networks. Profiles. Part 5-3: Profinet.
• ODVA Pub. 3: EtherNet/IP.
• Modbus Organization. Modbus Application Protocol Specification V1.1b3.
• Documentation from manufacturers of PLCs and field devices (Siemens, Rockwell Automation, Schneider Electric, etc.).
• UNITEC-D: Electrical Installation Safety Manual (internal document).