Solving LOGO! Ethernet Communication Failures on Long Cable Runs (150-250 m)

A LOGO! Basic Module configured as master loses its slave partner 150-250 m down a Cat5e run. The Ethernet link drops intermittently, the TDE freezes on diagnostic screens, and the operator suspects a bad module. The actual cause is the 100 m twisted-pair channel limit set by TIA-568 and the absence of an in-line repeater or managed switch. This reference covers the root cause, the field-proven fix using the LOGO! CSM (and, where required, a second CSM or industrial switch), cable/shielding practices, parameter settings in LOGO!Soft Comfort, and a step-by-step verification procedure that brings the link up reliably.

1. Problem Details

1.1 Reported Symptom Pattern

• Two LOGO! Basic Modules configured as master and slave on the same /24 subnet.
• A LOGO! TDE (text display, 6ED1055-4MH00-0BA0 or -4Ax00-0BA0 family) on the same network for HMI access.
• Single Cat5e UTP run, length 150-250 m, terminated in wall outlets or directly into the LOGO! RJ45 jack.
• Intermittent loss of the master/slave UDP/IP heartbeat. The slave appears offline on the TDE and in Web Server diagnostics.
• No physical damage to the cable, no tripped surge protection, no power fault at either cabinet.

1.2 Network Topology at Fault

Both LOGO! Basic Modules and the TDE sit on a single Ethernet segment with no intermediate Layer 2 device. The link budget is determined by the attenuation of the Cat5e channel alone, with no repeater to reshape and re-clock the signal. This is the textbook failure case for a copper Ethernet run that exceeds the 100 m horizontal cabling limit.

1.3 Affected Product Families

All LOGO! 8 generations with onboard Ethernet (6ED1052-1xxxx-0BA8, 6ED1052-2xxxx-0BA8, 6ED1052-1xxxx-0BA2 and later 6ED1052-1xxxx-0BA7/0BA8 variants) share the same 10/100 Mbit Ethernet PHY and the same 100 m reach limitation. The condition is independent of firmware version (LOGO!Soft Comfort V8.0 through V8.4) and is dictated by the IEEE 802.3 / TIA-568 copper channel.

2. Root Cause Analysis

2.1 The 100 m Cat5e Channel Limit

The 100 m maximum for a structured twisted-pair Ethernet channel is not a marketing figure. It is derived from:

• 90 m of permanent link (horizontal solid-conductor cabling between patch panel and wall outlet).
• 10 m of total patch cord (workstation + telecom room) per TIA-568-D and ISO/IEC 11801:2017.
• Insertion loss budget: 24 dB at 100 MHz for Cat5e, with crosstalk (NEXT/PSNEXT), return loss, and propagation delay skew consuming the remaining margin.

At 150-250 m, the insertion loss on a typical Cat5e run measures 28-44 dB at 100 MHz. The PHY's link monitor can no longer recover a valid MLT-3 (10BASE-T) or 4D-PAM5 (100BASE-TX) waveform above ~22-24 dB, and the link partner declares link-down. This is exactly what the LOGO! shows as a missing slave in the network status screen.

2.2 The Repeater/Segment Principle

Ethernet 10/100BASE-TX uses a Manchester / MLT-3 / 4D-PAM5 encoding that requires a clean received signal at the far end. A switch (or media converter, or any Layer 1/2 device) regenerates the signal: it re-amplifies, re-clocks, and re-transmits each packet on a new physical segment. Adding a switch at the midpoint of a long run splits the 250 m link into two compliant 125 m segments, both of which individually satisfy the 100 m rule when allowance is made for patch cords and the device's own internal ports.

2.3 Why the Problem Is Intermittent

The link only fails sometimes because the channel margin is not a hard cliff. Failures are triggered by:

• Temperature drift in PVC-jacketed Cat5e - attenuation increases ~0.4 % per °C.
• Connector contact resistance in outlet terminations and the RJ45 plug.
• Conducted/RFI noise from VFDs, contactors, and welding equipment installed since the original commissioning.
• Power events on either cabinet that briefly lower the PHY's link monitor sensitivity.

3. Cabling Standards and Limits

3.1 Reference Documents

3.2 Insertion Loss Quick Reference

For a typical 24 AWG UTP Cat5e solid-conductor cable at 20 °C:

4. The LOGO! CSM (Communication Module) Solution

The Siemens LOGO! CSM (catalog number 6ED1057-1EA00-0BA0, also offered as 6ED1057-1EA00-0BA2 in the current portfolio) is a 4-port unmanaged industrial Ethernet switch designed for the LOGO! control cabinet. It is the recommended fix for in-cabinet LOGO! networking and is an excellent choice for a midpoint repeater, although it is officially rated for cabinet mounting, not for the field midpoint of an outdoor cable tray. For a true outdoor/field midpoint installation, a SCALANCE XC-100 or third-party industrial switch in a NEMA 4/IP67 enclosure is more appropriate.

4.1 LOGO! CSM Technical Specifications

4.2 Will One CSM or Two CSMs Solve the Problem?

One CSM at the 100 m midpoint is sufficient and is the minimum recommended solution. A single CSM splits the original 150-250 m run into two segments, each of which must be ≤100 m end-to-end (including the 1-2 m pigtail into the switch).

Two CSMs are recommended in the following cases:

• The run is 200-250 m and a midpoint switch cannot be powered (no 24 V supply at the 100 m mark).
• You want to add a managed SCALANCE switch at the cabinet end for diagnostics, port security, and PRP/HSR redundancy later.
• Future expansion requires additional Ethernet drops along the cable path (e.g., a remote TDE).

4.3 Equivalent Alternative Hardware

5. Cable Selection and Shielding Practice

5.1 UTP, FTP, STP, S/FTP

For any industrial run approaching the 100 m channel limit, the recommendation is shielded twisted pair (FTP or S/FTP) rather than unshielded (UTP). Shielding:

• Reduces alien crosstalk (ANEXT) in cable trays with multiple runs.
• Improves immunity to VFD common-mode noise (a leading cause of intermittent Ethernet dropouts on the plant floor).
• Improves the headroom on the receive signal, helping when the run is close to 100 m.

5.2 Shield Grounding Rule

Ground the shield at one end only, and specifically at the cabinet that supplies the 24 V (the "source" of the system ground). Reasons:

• Prevents ground-loop current from the 50/60 Hz potential difference between the two cabinets.
• Prevents 50/60 Hz induction from being shorted through the shield, which would heat the cable and inject common-mode noise into the signal pairs.
• Maintains the high-frequency shield connection through the device's RJ45 metal shell, which bonds to chassis ground at the LOGO! and CSM.

5.3 Cable Construction Recommendations

• Conductor: Solid 24 AWG (0.51 mm) for fixed runs; 26 AWG stranded only for patch cords ≤5 m.
• Jacket: PUR (polyurethane) for oil/abrasion resistance, LSZH where required.
• Shield: Overall aluminium/polyester foil with tinned-copper drain wire (F/UTP), or with additional braid (SF/UTP) for high-EMI areas.
• Rating: Cat5e minimum, Cat6A preferred for any new install to give 10 GB headroom.
• Bending radius: ≥ 4× cable diameter for fixed installation, ≥ 8× for flexed.

6. Topology Options

6.1 Option A - One CSM at Midpoint (Recommended)

LOGO! Master → 100 m Cat5e FTP → CSM (midpoint, weatherproof enclosure) → 100 m Cat5e FTP → LOGO! Slave + TDE on the same end (daisy-chain via the CSM's remaining ports).

[LOGO! Master]----100 m----[CSM at 100m]----100 m----[LOGO! Slave]
                                                  \
                                                   ----[LOGO! TDE]

6.2 Option B - Two CSMs (One at Each Cabinet, 250 m Run)

LOGO! Master → CSM-A → 125 m Cat5e FTP → CSM-B (midpoint or at slave cabinet) → 125 m → LOGO! Slave + TDE. This configuration gives two regenerations and maximises the use of cable before the next switch.

6.3 Option C - Industrial Media Converter (Coax, Fibre, or 10 Mbit TP)

If cable replacement is impossible, run 10BASE-T media converters to legacy Cat-3 twisted pair, coax, or single-mode fibre. Siemens offers the OLM (Optical Link Module) family and third parties offer 10/100 Mbit Cat5-to-fibre media converters in IP67 form factors. Note: 10 Mbit is fully adequate for LOGO! master/slave traffic, which is well under 100 kbit/s of payload.

7. Configuration Procedure (LOGO!Soft Comfort V8.x)

7.1 Prerequisites

• LOGO!Soft Comfort V8.3 (or later V8.4) installed; the project matches the hardware in both cabinets.
• Both LOGO! Basic Modules have onboard Ethernet and a known IP address in the same subnet. Factory default: 192.168.0.1 (master) and 192.168.0.2 (slave).
• A 24 V DC supply at the midpoint (for the CSM) and at each cabinet.
• A shielded Cat5e (or better) cable plant terminated in shielded RJ45 plugs.

7.2 Step-by-Step

1. Power down both LOGO! Basic Modules. Note the IP, subnet mask, and master/slave role from the existing configuration.
2. Install the CSM at the 100 m midpoint (or at the slave cabinet, see Option B). Mount on a DIN rail inside an IP54+ enclosure, even if the run is indoors, to allow for cooling and service access. Apply 24 V DC to the CSM's power terminals (positive to "+", negative to "M"). Observe the green power LED.
3. Cut or re-terminate the existing run at the midpoint, install shielded RJ45 plugs on both halves, and crimp with a Cat5e/Cat6 pass-through crimper. Verify each plug with a continuity tester.
4. Connect one half of the run from the master LOGO! into Port 1 of the CSM. Connect the other half into Port 2, leading to the slave LOGO!. If the TDE is at the slave cabinet, plug it into Port 3 of the CSM (the CSM is unmanaged, so any port works).
5. Power up the CSM first, then the master LOGO!, then the slave LOGO!.
6. Verify link LEDs: Each port of the CSM should show a steady green link LED and a flashing yellow activity LED. If a port stays dark, re-seat the RJ45 or re-terminate the plug.
7. On the master LOGO!, navigate to Network → Master/Slave and confirm the slave IP is reachable. The status page should show "Slave: connected" within 2-3 seconds of power-up.
8. On the TDE, cycle to the network diagnostic screen. Both LOGO! modules should be visible with stable IP, subnet, and "online" status.
9. Run a 24-hour soak test with operator-grade I/O toggling to confirm no intermittent dropouts under load. Monitor the LOGO! system log (Diagnostics → Ethernet) for any late collisions, CRC errors, or drop events.

8. Verification and Acceptance Test

8.1 Physical Layer Acceptance

• Each end-to-end segment (CSM ↔ LOGO!) measures ≤100 m including the patch cords.
• All RJ45 plugs use Cat5e/Cat6 pass-through crimp and are within the cable's 4× bending radius limit.
• Shield drain wire is bonded at the powered-cabinet end only.
• CSM and each LOGO! are bonded to the cabinet's protective earth (PE) bar via the DIN-rail contactor clip or a dedicated ground wire.

8.2 Functional Acceptance

8.3 KPI / Telemetry (Optional)

For a more thorough validation, attach a managed switch (SCALANCE XB-200) at one end and monitor per-port counters via SNMP or the Web UI. Look for:

• CRC errors: 0 over 24 h (any value > 0 indicates marginal cable or connector).
• Late collisions: 0 (indicates duplex mismatch, very rare on a 100 Mbit link with auto-neg).
• Drop events: 0 (indicates buffer overflow; could indicate a runaway broadcast, e.g., a misconfigured device flooding the segment).

9. Troubleshooting Matrix

10. Frequently Asked Questions