Siemens OLM Profibus Redundant Ring Fault: Step-by-Step Diagnosis

Problem Description

A PROFIBUS DP network built around Siemens Optical Link Modules (OLMs) is supposed to operate as a closed, redundant optical ring. Closing the ring—i.e. connecting the last OLM back to the first so that two physically independent light paths exist between every pair of segments—causes bus faults the moment the ring becomes a logical loop. The fault is independent of where the fibre is broken or how many nodes are between the break point and the master: the master reports station failures, the slaves drop in and out, and the diagnostic buffer fills with slave diagnostic or "bus fault" entries. When the ring is broken back to a stub or linear topology, the faults clear immediately.

This symptom pattern—faults only when the ring is closed—is almost always caused by one of three problems:

1. Bus timing parameters in the PROFIBUS master configuration are not extended for the OLM propagation delay.
2. OLM DIP switches are set so that segment monitoring is enabled in an inconsistent way across the ring.
3. One OLM in the ring is electrically damaged and only fails when it is required to forward traffic in the redundant direction.

Field data published by HMS Networks indicates that approximately 90% of all PROFIBUS problems occur during the design and installation stage, with poor physical connections being the dominant cause. The remainder—including redundant-ring faults of the kind described here—sit in the configuration and component layer and can be resolved systematically with the procedure below.

How a Siemens OLM Redundant Ring Operates

An OLM is a media converter between an electrical PROFIBUS DP segment (RS-485, 9.6 kbit/s up to 12 Mbit/s) and an optical PROFIBUS segment (plastic, PCF, or glass fibre). In a redundant ring, three to ten OLMs are chained so that each unit has two optical ports and one electrical segment. Each OLM monitors both optical ports for a healthy light signal. When both directions are intact, the OLM is passive on one port and forwards traffic through the segment on the other. When one direction fails, the OLM closes the loop locally and rebuilds the logical bus so that the master still sees all stations on a single physical segment.

Two consequences drive every parameter setting that follows:

1. Every OLM adds propagation delay. A single OLM hop is typically 11–12 bit times of additional latency on top of the segment's own turnaround. A 6-OLM ring therefore adds roughly 60–72 bit times end-to-end compared with a copper segment of the same length.
2. The PROFIBUS master and each slave expect deterministic timing. If the master uses bus timing values calculated for a copper segment, the slaves' replies arrive outside the master's receive window the first time traffic is forced through the longest possible path.

For these reasons a redundant OLM ring is not plug-and-play. It must be configured in two places: on the OLM hardware (DIP switches) and in the STEP 7 / TIA Portal bus profile (timing parameters).

Root Cause Categories and Symptoms

Pre-Diagnostic Checklist

Before changing any setting, capture the following so that any regression can be reversed:

1. Export the STEP 7 / TIA Portal project so the current bus profile is archived.
2. Read the diagnostic buffer of the PROFIBUS master CPU (S7-300 / S7-400 / S7-1500) and save the last 20 entries.
3. Photograph or list the DIP switch position of every OLM in the ring. Do not assume that two OLMs of the same order number have the same switch setting.
4. Record the baud rate configured in HW Config and the value of the Highest Station Address (HSA).
5. Note the firmware version of the master CPU and the article number of each OLM (for example 6GK1502-3CB10 for an OLM/G12 with glass fibre interface). The manual's chapter numbering depends on the model.

Step-by-Step Diagnosis

The procedure assumes the network is otherwise physically healthy (no broken fibres, no missing terminators, no swapped A/B cores) and that the ring has been demonstrated to fail when closed.

Step 1 — Open HW Config / Device Configuration

In SIMATIC Manager, open the SIMATIC 300 / 400 station and double-click the PROFIBUS subnet. In TIA Portal, open Devices & Networks, select the PROFIBUS subnet, and open Properties > Network settings > Bus parameters. Note the values for:

• Tslot (slot time)
• min TSDR (minimum station delay)
• max TSDR (maximum station delay)
• Tset (setup time)
• Tqui (quiet time)
• HSA (highest station address)
• Watchdog

Step 2 — Use the Automatic OLM Ring Profile

STEP 7 V5.5 and later, and TIA Portal, provide a one-click profile that automatically extends the bus timing for an OLM ring. In HW Config:

1. Open PROFIBUS > Properties > Options.
2. Activate “Activate OLM ring configuration” (also labelled “Activate diagnostic repeater / OLM ring” depending on version).
3. Enter the number of OLM hops in the ring and confirm.

The tool rewrites Tslot and max TSDR based on the configured baud rate and the number of hops. Download the new HW Config to the master CPU and test the ring again. If this step alone clears the fault, no further manual timing tuning is required.

Step 3 — Manual Bus Parameter Adjustment

If the automatic profile is unavailable, or if a “User-defined” profile is required for compliance reasons, compute the parameters manually. Use the following practical rules; the actual numbers must always be verified against chapter 5.10 of the OLM manual for the specific OLM model in use:

For each OLM hop, add approximately 12 bit times of latency. A safe minimum for a 4-OLM ring at 1.5 Mbit/s is therefore:

Tslot  = 100 + (4 hops × 12) ≈ 150 bit times
max TSDR = 60 + (4 hops × 12) ≈ 110 bit times
min TSDR = 11 bit times (default, do not change)

Round up, never down, and verify that the master Watchdog is greater than 2 × Tslot × number_of_slaves. Set HSA to 126 so that diagnostic addresses used by the OLMs and any RS-485 repeaters are within the master's polling range.

Step 4 — Verify OLM DIP Switch Settings

Each OLM exposes a 4- or 6-pole DIP switch under the front cover. Chapter 4.2 of the OLM manual describes the switch function per model. The two switches that matter for redundant ring operation are:

Walk the ring physically. Open each OLM, log the DIP positions against the photo taken in the pre-diagnostic step, and correct any deviation.

Step 5 — Validate the Fibre Path

Modern Siemens OLMs provide an in-built optical loss measurement (chapter 5.10 of the OLM manual). Trigger the diagnostic from the master CPU or from STEP 7 via the OLM's diagnostic address and read the measured attenuation in dB on each leg. A glass-fibre link of up to 3 km should report well below the OLM's saturation threshold (typically < -18 dBm receiver sensitivity plus a 3 dB system margin). If any leg reports near the limit, clean the connectors with a proper fibre-optic cleaning pen and re-test before continuing.

Step 6 — Test the Ring

With the corrected bus parameters downloaded and the DIP switches verified:

1. Break the ring by removing one fibre at a known point.
2. Confirm the network operates normally in linear mode.
3. Reconnect the fibre to close the ring.
4. Verify in the master diagnostic buffer that no new station-failure entries appear.
5. Pull each fibre in turn and confirm the ring recovers within one bus cycle.

Identifying a Faulty OLM

If the ring still fails after timing and DIP switches are corrected, isolate the suspect OLM using the segmentation method:

1. Number the OLMs 1–n in the ring order.
2. Insert a short fibre jumper between OLM 1 and OLM 2 so that a 2-OLM mini-ring is formed. The remaining OLMs are parked stub-style.
3. If the mini-ring works, add OLM 3, then OLM 4, and so on. The first OLM that breaks the ring is the suspect.
4. Swap the suspect OLM with a known-good spare of the same order number and re-test.

Common failure signatures on the suspect unit:

• Optical port LED dark on one side while the other side is bright, even after re-terminating.
• OLM internal diagnostic flags segment disturbed on a known-good fibre.
• OLM warm to the touch (failed internal regulator).

Bartec Ex-i PROFIBUS I/O Considerations

Bartec's intrinsically safe PROFIBUS-DP / PA couplers (for example the 17-7-72 and 07-7-72 series used in hazardous-area installations) connect to the OLM ring as ordinary PROFIBUS DP slaves. They do not introduce additional ring delay beyond the standard DP slave response time, but they do have three configuration items that frequently mimic ring faults:

1. Address DIP switches on the Bartec coupler must be unique on the segment and must match the configured address in HW Config / TIA Portal.
2. PA coupler baud rate on the PA side (31.25 kbit/s) is independent of the DP side baud rate; if the DP-side baud rate is forced up to 12 Mbit/s by the ring profile, verify the Bartec coupler's DP-side supports that rate (most modern units do).
3. Bus terminator on the Bartec coupler's DP segment must be OFF unless the coupler is the physical end of the segment.

If a single Bartec segment is the only thing that fails when the ring is closed, swap the Bartec coupler with a spare and re-test before suspecting the OLMs.

Diagnostic Toolkit and Best Practices

For networks that must stay in service, plan a recurring diagnostic routine using one of the following:

• Siemens SIMATIC PDM — reads slave diagnostics and OLM diagnostic buffers directly over the same PROFIBUS segment.
• PROFIBUS Diagnostic Repeater (Siemens 6GK1 500-0DA00 or later) — placed at the segment ends, it reports segment topology and per-station signal quality.
• Softing PROFINET/PROFIBUS Diagnostic Toolset — continuous bus monitoring with topology discovery.
• InPro COMM diagnostics from HMS Networks — applies the methodology described in the HMS Networks PROFIBUS diagnostics white paper: most faults are installation-layer, and continuous monitoring catches them before they become downtime.

Best practices that prevent the ring-fault class entirely:

1. Always set HSA to 126 in any PROFIBUS segment that includes a repeater, diagnostic repeater, or OLM ring.
2. Use the “User-defined” bus profile only when the automatic OLM profile does not match the physical topology.
3. Document the OLM DIP switch state of every unit on a panel drawing that lives next to the cabinet.
4. Carry at least one spare OLM of each variant in use on site.
5. After any fibre repair, run the OLM optical-loss diagnostic on the affected leg.

Verification Checklist Before Returning the Plant to Service

FAQ