Configuring S7-400H Profibus Fiber Optic Ring with Siemens OLM

Configuring S7-400H Profibus Fiber Optic Ring with Siemens OLM Modules

Engineers deploying a SIMATIC S7-400H (fault-tolerant) controller on a Profibus DP field network frequently need to extend the electrical RS-485 bus across plant floors, between buildings, or across long industrial yards. The standard remedy is to convert the electrical segment into an optical segment with Siemens OLM (Optical Link Module) repeaters arranged in a line or ring topology. This reference consolidates the rules for S7-400H dual-interface integration with the OLM V4.0 product family, including part numbers, fiber selection, STEP 7 network configuration, bus parameter rules, and commissioning verification.

1. S7-400H Profibus Interface Architecture

The S7-400H chassis contains two redundant CPUs (CPU 410-5H, CPU 412-3H, CPU 414-4H, or CPU 417-4H). Each CPU can host one or more Profibus DP master interfaces, giving the H-station a total of two physical Profibus DP ports. In redundant operation, the active CPU owns the master function; on failover the standby CPU takes over within the configured monitoring time without interrupting the bus.

Two valid cabling strategies exist for the Profibus side of an S7-400H:

1. Single network, both ports connected in parallel. Both Profibus DP ports are configured as members of the same Profibus subnet in STEP 7. The H-system uses MPA redundancy to present a single logical master token to the slaves; the ring is one logical subnet, not two.
2. Two physically separate networks. Each CPU's DP port is configured as a different subnet. Slaves are physically split between the two subnets, and each network must be terminated and parameterized independently. This is the conventional approach when the H-station controls a process where the I/O is also physically duplicated.

2. Profibus Ring Topology with OLM

An OLM ring provides cable redundancy: data is propagated in both directions around the ring. If one fiber segment breaks, OLMs on both sides of the break switch to the reverse path, restoring connectivity within milliseconds. The ring must be closed electrically - the last OLM in the chain loops back to the first - to enable the redundancy mechanism.

Optical OLMs (the G-series) contain a fiber switch that detects loss of light on the receive port. When the ring is healthy, the switch blocks the secondary path (avoiding a closed signal loop). When the ring is broken, the switch closes and traffic flows the long way around.

2.1 Logical vs. Physical Rings

3. OLM V4.0 Product Family

Siemens superseded the OLM V3 generation with the OLM V4.0 generation. OLM V3 modules were discontinued for new orders on 30 September 2007 and transitioned to spare-part supply at higher prices. OLM V4.0 modules are functionally compatible with V3, support the same Profibus baud rates (9.6 kbps to 12 Mbps), and add enhanced diagnostics.

3.1 OLM V4.0 Part Numbers

3.2 Optical Reach Reference (Profibus DP, 12 Mbps)

4. Topology Decision: One Ring or Two

The original question on the source thread asked whether both Profibus DP ports of an S7-400H can be landed on a single OLM ring. The answer depends on how the two ports are configured in STEP 7:

1. Same Profibus subnet (recommended): Open the S7-400H station in STEP 7, select the Profibus interface of CPU0 and the Profibus interface of CPU1, and assign the same subnet identifier (e.g. "Profibus(1)") and the same transmission rate to both. The H-system internally uses MPA redundancy to present one logical master. Both interfaces can be wired to the same OLM ring because the optical layer sees one Profibus token domain.
2. Different Profibus subnets: If the two DP interfaces are configured as different subnets (e.g. "Profibus(1)" and "Profibus(2)") with different node addresses or different baud rates, they cannot share an OLM ring. The optical layer cannot separate two logical buses, so the token handling will corrupt both subnets. Build two physically separate OLM rings - one per subnet.

4.1 When to Choose Two Rings

• The two Profibus subnets serve physically disjoint slave populations (e.g. subnet 1 = field devices in building A, subnet 2 = field devices in building B).
• One subnet runs at 1.5 Mbps (high noise immunity) and the other runs at 12 Mbps (high throughput). Mixing baud rates on a single OLM ring breaks the timing slots.
• Node addresses overlap between the two logical subnets (illegal in Profibus, but commonly attempted during commissioning). Two physical rings isolate the conflict.

4.2 When to Choose One Ring

• All DP slaves are in the same logical subnet, including redundant DP slaves (Y-link or MPA slaves behind a Y-coupler).
• Both S7-400H Profibus ports are configured for the same MPI/Profibus network ID and baud rate in HW Config.
• You want a single ring to carry the token to all slaves while retaining H-master redundancy.

5. STEP 7 Configuration Procedure

5.1 Prerequisites

• STEP 7 V5.5 SP4 or later, with the S7-400H option pack installed.
• S7-400H station assembled with two IF964-DP or CP 443-5 Extended modules (one per CPU rack or one in each half of the split rack).
• OLM V4.0 modules racked and powered (24 VDC, max 250 mA per module).
• BFOC (ST) patch cables pre-terminated and tested with a fiber fault locator.

5.2 Configure the Profibus Subnet

1. Open the S7-400H station in SIMATIC Manager and double-click Hardware.
2. Select the first IF964-DP / CP 443-5 in the rack. Right-click the DP interface and choose Object Properties > Interface > Parameter > Profibus.
3. Click New to create subnet Profibus(1). Set the transmission rate to the project value (commonly 1.5 Mbps for plant DP, 12 Mbps for tightly coupled I/O). The profile must be Standard or DP - not Universal, which can shift Tslot values on long fibers.
4. Set the highest Profibus address (HSA) to the maximum node ID + 1. For a 1.5 Mbps ring with 32 nodes, HSA = 64 is a safe default.
5. Apply the same subnet Profibus(1) to the second IF964-DP / CP 443-5 on the standby CPU. Do not create a new subnet.
6. Confirm both interfaces display the same network identifier and baud rate. STEP 7 raises a warning if a single H-station carries two DP networks with the same name but different parameters.

5.3 Configure Bus Parameters for Long Fibers

Optical segments do not change the Profibus electrical timing. The Tslot and quiet-time values in STEP 7 are based on the maximum number of repeaters and the cable propagation delay. Siemens calculates the defaults for typical copper runs; for plant segments that include 5+ OLM hops, you should re-check the values:

Accept STEP 7's recalculated values via Bus Parameters > Recalculate after entering the actual number of repeaters in the network. Manually overriding values that STEP 7 has calculated for the topology is the most common source of intermittent SF (station failure) and BF (bus fault) on long optical rings.

5.4 Lay Out the OLM Ring

1. Place one OLM/G12 (or G12-1300 for 15 km links) at each slave cluster. The number of OLMs in a ring is limited by the Profibus token timing, not by the optical layer: at 1.5 Mbps, 32-40 OLMs are typical; at 12 Mbps, 8-12 OLMs is the safe ceiling.
2. Connect the electrical port of each OLM to its local Profibus slave cluster using a standard Profibus DP cable with Profibus connectors that have terminating resistors enabled at the two ends of the cluster.
3. Connect the two optical ports of each OLM into the ring: optical port 1 of OLM[n] to optical port 1 of OLM[n+1], and the last OLM in the chain back to the first OLM. The optical direction (TX vs. RX) must be cross-over: TX[n] → RX[n+1], RX[n] → TX[n+1]. BFOC ST connectors are keyed to prevent mis-mating, but direction is still the operator's responsibility.
4. Connect the S7-400H station to the ring at one of the OLM/G11 or G12 nodes. The H-station's two Profibus ports are typically both connected to the same OLM at the plant entry, or to two adjacent OLMs in the ring (one to each, providing physical separation of the two redundant H interfaces).

6. Long-Distance Considerations (15 km Reference Case)

The original query specified a 15 km plant segment. The correct module family is the OLM/G12-1300 (single-mode, 1300 nm), order number 6GK1503-3CC00. Multi-mode OLM modules at 850 nm are limited to roughly 3 km per segment and will not close the optical power budget at 15 km.

6.1 Optical Budget for 15 km

For 15 km links, pre-terminated single-mode pigtails with fusion spliced patch panels are preferred over field-terminated ST connectors. Each patch panel is one additional connector pair in the budget.

6.2 Baud Rate vs. Distance on Single-Mode

The 15 km figure is independent of baud rate at the optical level - attenuation in single-mode fiber at 1300 nm is essentially the same from 9.6 kbps to 12 Mbps. The baud rate does affect the number of OLMs you can chain in a ring (token rotation time grows with each OLM) and may force you to run the ring at 1.5 Mbps or 500 kbps for a 15 km link to keep the token TTR within Profibus limits.

7. Verification and Commissioning Checks

7.1 Visual and LED Checks

7.2 STEP 7 / TIA Diagnostic Checks

1. In HW Config, right-click the Profibus subnet and choose Profibus > Diagnose. All DP slaves must appear with green status.
2. Open SIMATIC Manager > PLC > Diagnostics/Settings > Module Information on the active CPU. The DP master diagnostic buffer should be free of Station Failure entries.
3. In the H-system diagnostic view, confirm both CPUs report the same Profibus node list. Mismatched node lists after commissioning are an early warning of a single-port connection on a slave that should be redundantly cabled.

7.3 Ring Break Test

Procedure to confirm ring redundancy:

1. With the ring closed and all slaves online, disconnect one BFOC ST connector at any OLM.
2. All slaves should remain online (the ring recloses through the reverse path).
3. The disconnected OLM should show FD red on the affected channel.
4. Reconnect the fiber. FD clears within < 100 ms and the OLM resumes normal operation.
5. Repeat for the opposite channel and at a different OLM to confirm redundancy is symmetric.

8. Troubleshooting Matrix

9. Spare-Part and Migration Notes

OLM V3 modules (6GK1503-2AB00, 6GK1503-2AC00, 6GK1503-3AC00, 6GK1503-3CB00 prior to V4.0) were discontinued for new orders on 30 September 2007. After that date they remained available only as spare parts at higher unit prices. OLM V4.0 modules are drop-in replacements for V3 in most cases, with the following caveats:

• The diagnostic web interface (OLM V3.3 and later) is replaced by the OLM V4.0 diagnostics tool, which uses the same BFOC ports but a different command set. Existing diagnostic scripts must be updated.
• Firmware upgrades for OLM V3 are no longer released; if a V3 module fails, replace it with V4.0 rather than repairing it.
• OLM V4.0 modules require the same 24 VDC supply and the same BFOC ST fiber connectors as V3. Patch cables and termination resistors are reusable.

For new installations, the current Siemens PROFIBUS Optical Networking portfolio (which includes the OLM V4.0 family as the last generation of dedicated Profibus OLM hardware) should be specified directly. Newer Profibus installations using Profinet-to-Profibus proxies are outside the scope of this reference.

10. Frequently Asked Questions