Overview
The Siemens PROFIBUS Optical Link Module (OLM) converts electrical PROFIBUS DP/PA segments into optical fiber and supports line, star, and redundant ring topologies. A redundant optical ring uses both fiber ports (Channel 1 and Channel 2) of every OLM so that a single fiber break anywhere in the ring isolates the broken segment while the OLM loop-back function preserves communication on the surviving path. Unlike a line or star, the ring requires every OLM to be configured for "redundant optical ring" mode on both channels simultaneously, and the resulting slot time used by the PROFIBUS master (PLC/CPU) must be recomputed in STEP 7 / TIA Portal based on the number of OLMs and the total fiber length.
This reference consolidates the configuration rules, DIL switch maps, LED diagnostics, and STEP 7 parameters for the OLM/G12-1300 V4.0 family (for example 6GK1503-3CC00) and addresses the most common commissioning faults: flashing green/red system LEDs, ignored DIP switch changes, and mixed electrical/fiber connections inside the same ring segment.
OLM Hardware and Model Identification
OLMs are differentiated by fiber type, number of channels, and PROFIBUS segment type. The model 6GK1503-3CC00 is a dual-fiber OLM (two optical channels plus one electrical PROFIBUS segment) for plastic/HCS or PCF fiber depending on the variant suffix. The hardware release suffix "G12-1300" and firmware "V4.0" identify the device generation covered by the OLM/G12-1300 V4.0 manual set; behavior of the DIL switches and LED states described in this article applies to that release.
| Parameter | Value |
|---|---|
| Model (example) | 6GK1503-3CC00 |
| Hardware release | G12-1300 |
| Firmware | V4.0 |
| Fiber ports | 2 (Channel 1, Channel 2) |
| Electrical PROFIBUS port | 1 (D-sub, isolated) |
| Supported topologies | Line, star, redundant optical ring |
| Configuration interface | 8-position DIL switch S1 (S0..S7) |
Redundant Ring Topology Architecture
A redundant optical PROFIBUS ring is built by connecting Channel 1 of OLM 1 to Channel 2 of OLM 2, Channel 1 of OLM 2 to Channel 2 of OLM 3, and so on, closing the loop by connecting Channel 1 of the last OLM back to Channel 2 of OLM 1. Every OLM in the ring thus has fiber on both Channel 1 and Channel 2, and the electrical PROFIBUS segment of each OLM is used to attach the local PROFIBUS slaves (DP devices, ET 200 stations, drives) to that OLM's drop.
Each OLM acts as a repeater. When the ring is intact, token frames circulate one direction; the OLM ring manager logic is implemented identically in every node (no designated ring manager — the OLM ring is "self-healing" with distributed decision logic). On detection of a fiber break, the two OLMs adjacent to the break reconfigure their optical channels into a loop-back, creating a logical line out of the surviving path so that all nodes remain reachable.
DIL Switch Map for Redundant Ring Mode
The OLM is configured by a single 8-position DIL switch block (S0..S7). For a redundant optical ring, the relevant switch positions are S0 through S4. The remainder (S5..S7) remain off. The following table is the canonical setting for every OLM in the ring:
| Switch | Position | Meaning |
|---|---|---|
| S0 | 1 (ON) | Channel 1: optical channel operating mode (1 = line/ring; 0 = star/segment monitoring) |
| S1 | 1 (ON) | Channel 1: redundant optical ring mode |
| S2 | 1 (ON) | Channel 2: optical channel operating mode |
| S3 | 1 (ON) | Channel 2: redundant optical ring mode |
| S4 | 1 (ON) | Enable redundant ring supervision |
| S5 | 0 (OFF) | Reserved / channel-dependent option |
| S6 | 0 (OFF) | Reserved / channel-dependent option |
| S7 | 0 (OFF) | Reset to defaults on next power-up |
Critical rules:
- All OLMs in the ring must carry identical DIL settings for S0..S4. There is no designated "ring manager" OLM; the role is distributed across all nodes.
- If a single OLM is configured for redundant ring on only one optical channel (for example S1 and S3 off, but S0 and S2 on), the device cannot detect the loop-back condition on the unconfigured channel and a break on that channel will not be healed.
- Switch changes only take effect after a power-off / power-on cycle of the OLM. Re-power the device after every DIL change before reading the LED or evaluating bus communication.
Common Misconfiguration Scenarios
Three patterns recur in commissioning of OLM rings. Diagnose against the symptoms observed on the system LED before changing any DIL switch.
| Scenario | DIL State Observed | Symptom | Resolution |
|---|---|---|---|
| 1. Mixed mode inside one ring | One OLM in redundant ring mode, others in line/star | System LED flashes green/red on the misconfigured OLM; ring does not self-heal on break | Set S0..S4 to ON on every OLM; verify the change by repowering each OLM and observing solid green system LED on all units |
| 2. Mixed electrical and fiber between adjacent OLMs | DIL set to redundant ring, but a PROFIBUS copper cable also runs between OLM 1 and OLM 2 in parallel with the fiber | System LED faults; token cannot traverse; sporadic slave timeouts | Remove the electrical PROFIBUS cable between OLMs that already have a fiber link. Electrical segments are only permitted on the OLM "drop" side, not as a parallel path between ring nodes |
| 3. Mode change not re-powered | Engineer changes DIL switches on a powered OLM | LED continues to indicate the previous mode; ring appears to "work" with random settings | Power off the OLM, wait at least 5 s, power on, then re-evaluate. The OLM reads the DIL block only at boot |
LED Diagnostics and Status Codes
Every OLM has at minimum a system status LED and per-channel LEDs for the optical ports and the electrical PROFIBUS port. The behavior of the system LED is the primary diagnostic for redundant ring misconfiguration.
| System LED | Meaning | Action |
|---|---|---|
| OFF | No supply voltage or device defect | Check 24 V supply on the OLM terminal block; verify polarity |
| Solid green | Normal operation; ring intact (in redundant mode) or line OK (in line mode) | No action required |
| Flashing green | Bus activity detected; no fault | No action required |
| Flashing green/red | Transmission rate detected, but slot time not yet determined. Three typical causes: HSA in STEP 7 set too low and no node is transmitting on the segment; one optical channel set to "redundant optical ring" but the other is not; or slot time parameter in the master set too low for the ring length and node count | Increase HSA so that it covers the highest PROFIBUS address actually used; ensure both channels of every OLM are in redundant ring mode; recompute slot time from ring length and OLM count in STEP 7 / TIA Portal |
| Solid red | Hardware fault or ring break that the OLM cannot heal (for example, both channels broken on the same OLM) | Inspect fiber connectors, splices, and patch panel; verify signal with a fiber optic power meter |
STEP 7 / TIA Portal Parameters (HSA, Slot Time, Ring Properties)
The PROFIBUS master parameters in STEP 7 V5.x (NETPRO) or TIA Portal must be recomputed for a redundant optical ring. Three values matter:
- HSA (Highest Station Address). Set HSA to 126 unless your network contains addresses above the highest DP slave address. A common commissioning error is leaving HSA at the default of 126 but with the DP master set to a lower bus parameter profile that does not include the active nodes, which makes the OLM unable to sense any transmitter. With active DP slaves attached, the OLM will eventually measure a valid slot time; with no transmitter, it remains in the green/red flash state indefinitely.
- Number of OLMs in ring. In NETPRO (STEP 7 V5.x), open the Properties of the PROFIBUS subnet and enter the OLM count. STEP 7 will recompute slot time from the standard OLM signal delay. In TIA Portal the equivalent is the "Optical ring" / "Number of OLM" parameter in the subnet properties.
- Fiber ring length. Enter the total fiber length in km. Slot time scales with propagation delay; the longer the ring, the larger the slot time the master must allow. For a 6-node ring of plastic fiber the total length is typically < 1 km and slot time defaults are sufficient; for glass-fiber backbones of several km, recompute explicitly.
The approximate slot time contribution of an OLM is 12.8 tbit per device in the ring, where tbit is the bit time of the configured PROFIBUS baud rate. At 1.5 Mbit/s, tbit ≈ 0.667 µs, so a 6-OLM ring adds 6 × 12.8 × 0.667 µs ≈ 51 µs of OLM-internal delay, plus the fiber propagation delay. STEP 7 / TIA Portal adds these contributions automatically once the OLM count and ring length are entered; do not override the computed slot time with a smaller manual value.
Power-Cycle Requirement After Mode Changes
The DIL switches on OLM/G12-1300 V4.0 are sampled only at power-on. A live edit of the switches is held in a shadow register and applied on the next boot. If the system LED or bus diagnostics appear inconsistent with the DIL switch state just changed, the most likely root cause is a missed re-power. The required procedure is:
- Set DIL switches S0..S4 to ON (S5..S7 OFF) on every OLM in the ring.
- Disconnect the 24 V supply to the OLM.
- Wait at least 5 seconds (capacitor discharge on internal logic).
- Re-apply 24 V.
- Read the system LED after the boot phase (typically < 5 s for OLM/G12-1300 V4.0) and confirm solid green or flashing green.
Topology Constraint — No Parallel Electrical and Fiber Between OLMs
A redundant ring is realized purely with the optical channels of the OLMs. The electrical PROFIBUS port of an OLM is a drop port and must not be used to interconnect two OLMs that are already joined by fiber. Adding a copper PROFIBUS cable between OLM 1 and OLM 2 (in parallel with the fiber ring segment between them) creates a parallel segment that the OLM ring logic cannot resolve; the system LED of the affected OLMs will fault and tokens will be lost intermittently.
The correct usage of the electrical port is:
- Terminate the electrical port with the bus terminator that is integrated in the OLM (switch "TERM" on the D-sub) if that OLM is the end of an electrical drop.
- Attach DP slaves to the electrical port of the OLM.
- Connect the PROFIBUS master (PLC CPU) to the electrical port of one OLM in the ring.
Verification Procedure
After applying the DIL map and STEP 7 parameters, validate the ring with the following sequence:
- LED check on every OLM. Solid green or flashing green on the system LED. No green/red flash, no solid red.
- Per-channel LED check. Channel 1 and Channel 2 LEDs of each OLM should show link (typically a green LED per channel). On a 6-OLM ring, every OLM has fiber on both channels and both channel LEDs must be on simultaneously.
- DP slave online in STEP 7 / TIA Portal. All slaves attached to the electrical drop of each OLM must be reachable (no "slave not found" or "bus fault" diagnostics).
- Ring-break test. Disconnect one fiber at a patch panel. The two adjacent OLMs must enter loop-back within < 1 s (system LED pattern changes to indicate a healed ring, channel LEDs on the broken side go off, the other channel remains active). All DP slaves remain reachable. Reconnect the fiber; the ring returns to the normal state automatically.
- Live diagnostics in STEP 7 / TIA Portal. No bus faults in the diagnostic buffer of the PROFIBUS master; no retries on the diagnostic counter page of any slave.
Troubleshooting Matrix
| Symptom | Probable Root Cause | Corrective Action |
|---|---|---|
| System LED on one OLM flashes green/red; other OLMs solid green | That single OLM in mixed mode, or DIL change not re-powered | Verify S0..S4 are all ON on the affected OLM; power-cycle the OLM |
| All OLMs flash green/red | HSA too low in STEP 7 / TIA Portal, or no DP master transmitting on the segment, or slot time manually set below the OLM ring's computed value | Set HSA = 126; enter OLM count and ring length in subnet properties; remove any manual slot time override |
| Ring works when DIL is left all-OFF, fails when S1..S4 are set ON | Electrical copper segment runs in parallel with fiber between two OLMs; loop-back logic conflicts with the parallel segment | Remove the parallel copper segment between ring OLMs; keep copper only on the drop side of each OLM |
| Ring works at 1.5 Mbit/s but fails at 12 Mbit/s or higher | Slot time contribution of fiber length and OLM count is too large for the chosen baud rate | Recompute slot time from total fiber length in STEP 7; either reduce ring length, lower the baud rate, or split the ring into two segments |
| One OLM in the ring is solid red after a break | Both fiber channels on that OLM are broken or disconnected; loop-back cannot be established | Restore at least one fiber channel; check patch panel, splices, and connectors on that OLM |
| Slave reachable in STEP 7 but Go-Online fails intermittently | HSA / slot time borderline for the configured ring; propagation delay margin too small | Recompute OLM count and ring length in subnet properties; verify the baud rate matches the slowest DP slave |
| DIL change appears to have no effect | OLM was not re-powered | Power off the OLM for at least 5 s, power on, re-evaluate LED |
Field-Commissioning Notes and Edge Cases
- Mixing OLM generations. OLM/G12-1300 V4.0 firmware is compatible with earlier OLM/G11 and OLM/G12 hardware for ring operation as long as every OLM supports the redundant ring mode. If one OLM is a non-ring variant (for example an OLM with only one fiber channel), the ring cannot be closed through it. Replace with a dual-fiber ring-capable variant (6GK1503 series, 6GK1502 series for PROFIBUS PA).
- Bus terminator on the OLM drop. The electrical PROFIBUS port of an OLM that ends an electrical drop must be terminated. The OLM provides an internal terminator activated by a switch adjacent to the D-sub. If the same OLM is the start of an electrical drop leading to other slaves (no termination at the OLM, slaves terminated at the far end), the internal terminator must be off. Mis-termination is a common cause of intermittent bus faults that are not related to the fiber ring.
- Diagnostic address reservation in STEP 7. STEP 7 / TIA Portal can reserve diagnostic addresses for the OLM ring. These addresses do not have to match the DP slave address range, but they must be inside the configured HSA. A reservation above HSA is silently ignored and contributes to the "no transmitter detected" condition that triggers the green/red flash.
- Number of OLMs vs. HSA. OLM count and HSA are independent parameters. The OLM count drives slot time, while HSA drives the address window the master polls. Both must be set correctly; setting OLM count to 6 and HSA to 32, for example, will leave any DP slave at address 40 invisible.
- Plastic vs. PCF vs. glass fiber. Maximum segment length per OLM hop: plastic HCS/PCF typically 50–400 m depending on the variant, multimode glass 3 km, single-mode glass up to 15–70 km (with the appropriate OLM variant). The total ring length entered in STEP 7 is the sum of the fiber segments, not the geographic distance. A long geographic ring with excess fiber coiled in a patch panel still counts as full length for slot time computation.
- Mixed baud rates. All DP slaves on a PROFIBUS segment must run at the same baud rate. In a ring, this rate applies to all OLM hops. If a slave cannot run at the master rate, it must be on a separate segment behind a PROFIBUS repeater or a second OLM, not on the same ring.
Reference Documentation
The following official Siemens resources are the primary references for the configuration described in this article:
- Siemens Support: How do you set up a redundant optical ring with PROFIBUS OLMs?
- PROFIBUS OLM product finder
- Network components in optical networks (OLM, OBT)
Which DIL switches must be ON for a redundant optical PROFIBUS OLM ring?
Set S0, S1, S2, S3, and S4 to ON on every OLM in the ring; leave S5, S6, and S7 OFF. All OLMs must carry identical DIL settings — there is no designated ring manager. Power-cycle the OLM after every DIL change so the new mode is sampled at boot.
What does a flashing green/red system LED on a 6GK1503-3CC00 OLM mean?
It means the OLM has detected the PROFIBUS baud rate but cannot determine a valid slot time. The most common causes are HSA set too low in STEP 7 / TIA Portal, no active DP master or slave transmitting on the segment, or the two optical channels of one OLM being configured in different modes (only one in redundant ring). The OLM holds the bus quiet until the slot time is resolved, which is why DP slaves cannot be reached.
Do DIL switch changes on a Siemens OLM take effect immediately?
No. The OLM/G12-1300 V4.0 DIL switches are read only at power-on. After changing the switches, disconnect the 24 V supply for at least 5 s and re-apply. The new mode is then active for both optical channels and the system LED reflects the new state after the boot phase.
Can a copper PROFIBUS cable run in parallel with the fiber between two OLMs in the ring?
No. The electrical PROFIBUS port of an OLM is a drop port for slaves or the master. Connecting two OLMs with both fiber and copper in parallel creates a parallel segment that the ring's loop-back logic cannot resolve and causes the system LED to fault. Use the electrical port only on the drop side of each OLM.
What STEP 7 / TIA Portal parameters control OLM ring timing?
Set HSA to 126 (or higher than the highest DP address in use), enter the number of OLMs in the ring, and enter the total fiber ring length in km in the PROFIBUS subnet properties. STEP 7 / TIA Portal uses these to recompute slot time; do not override the computed slot time with a manually smaller value or the OLM will report the green/red flash condition.