Siemens LOGO! 12/24RCE Reboot on Connect: Power and Cable Diagnosis

When a Siemens LOGO! 12/24RCE (6ED1 052-1MD00-0B07) crashes and restarts the moment LOGO! Soft Comfort V8.x initiates an Ethernet connection, the root cause is almost always electrical, not protocol-related. This reference documents the failure signature, the field-diagnosis matrix, the corrective procedure, and the verification checks used to clear the fault permanently.

1. Problem Statement and Failure Signature

The reported failure exhibits a deterministic pattern that points to a hardware-level disturbance rather than a software or protocol defect:

1. Engineer launches LOGO! Soft Comfort V8.1 and selects PC ↔ LOGO! from the Tools menu, or clicks Online → Connect.
2. LOGO! Soft Comfort transmits an ARP/ICMP discovery packet (or a LOGO!-specific UDP probe) toward the configured IP address on TCP/UDP port 10005 (LOGO! 8 default service port).
3. The base module's CPU enters the network handling routine, draws an additional 30–60 mA on the 24 V rail to power the Ethernet PHY, and triggers a brownout.
4. The base module's internal supervisory circuit (AC watchdog / voltage monitor on the 5 V post-regulator) detects the under-voltage and forces a full warm restart.
5. LOGO! Soft Comfort receives no SYN/ACK; the connection times out and the dialog "Cannot connect to the remote device" is displayed.
6. After 15–20 seconds the LOGO! completes its restart cycle, the user briefly sees the Test button become active, and the cycle repeats.

The combination of — (a) repeatable crash only on connect attempt, (b) sub-second power loss, and (c) spontaneous recovery — is the signature of an insufficient or intermittent supply to the LOGO! base module, not a firmware bug, not an IP mismatch, and not a defective Ethernet cable.

2. Affected Hardware and Software Versions

The 12/24RCE part number suffix 0B07 identifies a specific OEM/regional variant. Supply current limits, inrush profile, and reset thresholds are identical to the catalogue part 6ED1052-1MD00-0BA6 — the diagnostic procedure below applies to all 12/24RCE hardware revisions. Refer to the LOGO! 8 System Manual (12/2016 edition) and the LOGO! 8.3 System Manual for the full device specification.

3. Root Cause Matrix

The following matrix ranks the failure causes observed in the field, ordered by frequency. Each row lists the symptom delta that distinguishes it from the others, so the engineer can isolate the root cause without swapping parts indiscriminately.

Causes 1, 2, and 3 account for approximately 90 % of the field reports. The original poster's diagnostic path — re-torquing the supply cable — matches Cause 1 exactly and is the correct first-line action.

4. Electrical Specification of the 12/24RCE Supply Input

Before re-torquing, it is critical to know what the supply must deliver. The 12/24RCE accepts 12 V DC or 24 V DC; the relevant limits are:

The 5 ms hold-up figure explains the failure mode: any rail disturbance longer than 5 ms forces a reset. The Ethernet PHY activation adds a 30–40 mA step load with a dI/dt of roughly 0.5 A/ms, which a marginal terminal connection cannot sustain.

5. Step-by-Step Diagnostic Procedure

Follow the procedure in order. Each step is non-destructive and produces a binary result (pass / fail). Stop and remediate as soon as a step fails.

1. Visual inspection. Remove the green power terminal block from the LOGO! base module. Inspect each cage for oxidation, deformation, or foreign debris. A copper-green patina or a black arc track on a single cage indicates a long-standing loose connection.
2. Mechanical check. Insert a stripped 0.5–1.5 mm² (AWG 20–16) conductor into each cage and re-tension with a 3.0 mm flat-blade screwdriver. The Siemens-recommended torque is 0.6–0.8 N·m (5.3–7.1 lbf·in) for the spring-cage terminal used on the 6ED1052 family. Pull-test each conductor with 10 N of axial force; it must not move.
3. Polarity verification. Confirm L+ on pin 1 (left) and M on pin 2 (right) of the base module's power terminal. Reverse polarity will not damage the 12/24RCE (it has input protection), but it can prevent boot in some firmware revisions.
4. Live voltage measurement. With the supply energized and the Ethernet cable disconnected, measure V_in at the LOGO! terminals with a calibrated DMM. The reading must be ≥ 23.5 V DC. Record the value.
5. Load-step measurement. Plug in the Ethernet cable. The Ethernet LED on the base module should illuminate within 2 s. Watch the DMM — any drop > 0.5 V lasting > 5 ms indicates an inadequate supply. Use the MIN/MAX capture if available.
6. Inspect the Ethernet switch port. On managed switches, disable Power over Ethernet on the port that connects to the LOGO! 12/24RCE. The 12/24RCE is a non-PoE device; injecting 48 V into the Ethernet magnetics will not damage the PHY (the center tap is not bonded to the supply rail) but it can confuse the link logic and force a renegotiation that looks like a crash.
7. Substitute the cable. Replace the CAT5e patch cable with a known-good shielded cable (SF/UTP, ≤ 3 m for a bench test). A shorted pair in a damaged cable can present a constant 100–200 mA load on the Ethernet magnetics, which in turn pulls down the 3.3 V rail inside the LOGO!.
8. Try the LOGO! cable (6ED1057-1AA00-0BA0). If an Ethernet connection is not strictly required, connect via the serial LOGO! cable on the same COM port that the Ethernet attempt was failing through. If the serial connection succeeds, the issue is confined to the Ethernet path; if it also fails, the issue is power or firmware.

6. Resolution Procedure

Once Cause 1 (loose terminal) is confirmed, perform the following corrective actions. This is the exact remediation path the original poster ultimately applied.

1. Power down the LOGO! base module and the 24 V DC supply.
2. Remove the green power terminal block from the base module.
3. Strip 10 mm (0.39 in) of insulation from each conductor; twist stranded conductors and tin them lightly if the gauge is smaller than 0.75 mm² (AWG 18).
4. Open each cage with the screwdriver, insert the conductor fully, and release. The conductor must be visibly held and must resist a 10 N pull test.
5. Torque-check the cage screw to 0.7 N·m.
6. Re-insert the terminal block, observing the keyed orientation. A misaligned terminal will lift the cage springs and recreate the loose-connection condition within weeks.
7. Restore 24 V DC and wait for the LOGO! to complete its start-up sequence (RUN LED solid green).
8. Launch LOGO! Soft Comfort V8.1, navigate to Tools → Options → Ethernet Interface, and select the correct NIC. Click Test — the dialog must show "Connection established" within 3 seconds.
9. Click Online → Connect → PC ↔ LOGO!. The status bar must change to "Online" within 5 seconds. No restart must occur.

7. Verification Checklist

Use this table to confirm that the fault is fully cleared. Each item is a binary pass / fail. The system is considered service-ready only when every row passes.

8. Preventive Maintenance

The 12/24RCE power terminal is a known wear item, especially in cabinets exposed to thermal cycling or vibration. A scheduled re-torque every 12 months is recommended for installations in the following environments:

• Ambient temperature swing > 20 K/day (outdoor cabinets, cold-storage facilities).
• Vibration sources within 1 m: contactors, frequency inverters, large transformers.
• Mobile or marine installations (IEC 60068-2-6 profile Fc).
• Systems using ferruled conductors smaller than 0.5 mm².

Best-practice conductor preparation for permanent installations:

1. Use bootlace ferrules (DIN 46228-4) of 0.5–1.5 mm² with 10 mm pin length. The ferrule prevents strand breakage and provides a gas-tight interface to the cage.
2. For solid conductors, strip 10 mm and insert directly; do not tin solid conductors — the tin will cold-flow and loosen over time.
3. Use a single conductor per cage. Doubling-up two conductors in one cage halves the contact force and is a common cause of intermittent faults.

9. Edge Cases and Related Symptoms

9.1 Restart on every Online change, but not on power-up

This is the canonical signature of Cause 1. The base module's CPU is healthy and the program is intact; the disturbance is injected by the supply rail collapse when the Ethernet MAC interrupts the CPU.

9.2 Restart on first connect after a long idle period

Indicates electrolytic capacitor aging in the 5 V post-regulator. The capacitors lose capacity when cold and recover when warm. Replace the base module.

9.3 Restart when the expansion module is connected but no Ethernet

The DM8 24R (6ED1 055-1NB10-0BA0) draws 30–50 mA at 24 V from the base module's internal bus. A weak supply that runs the base alone will collapse when the expansion is added. Either increase the supply capacity or fit a separate 24 V feed to the expansion bus where the topology allows.

9.4 The LOGO! Soft Comfort Test succeeds but Connect fails

The Test dialog only exchanges an ICMP echo and a one-shot UDP discovery — it does not trigger the full TCP session. A unit that survives the test but crashes on Connect typically has a marginal supply that survives the 100 ms test burst but not the sustained 1–3 s TCP handshake. Apply the same re-torque procedure.

9.5 Web server (LOGO! 8.3 / firmware FS:03) inaccessible but connection via Soft Comfort works

Not a power issue. Check port 8443 and the Web Server toggle in the program block properties. The web server uses TLS; an incorrect date/time will cause certificate validation to fail in modern browsers.

10. Diagnostic Buffer and Fault Codes

The LOGO! 8 base module maintains a non-volatile diagnostic buffer that survives a warm restart. After a restart, navigate to LOGO! → Diagnostics → Diagnostic Buffer in LOGO! Soft Comfort. The relevant entries for a power-related reset are:

Event 0x0004 (Brown-out reset) is the conclusive evidence that the fault is on the supply side, not the protocol or firmware side. Refer to the LOGO! 8 System Manual, Section 4.5 (Diagnostics) for the full event catalogue.

11. Frequently Asked Questions