Troubleshooting S7-400H Redundant Profibus with CP443-5 and OLM

Problem Description

On a plant with an SIMATIC S7-400H redundant controller, two CP 443-5 Profibus communication processors are used to attach a single plant Profibus segment through a pair of RS485 repeaters and an active terminating resistor. The far end of the segment terminates in an OLM (Optical Link Module) that transports the segment to a remote HMI (with an APPLICOM S7-MPI/PROFIBUS card) over fiber. Two digital outputs of the S7-400H switch the 24 V supply of each repeater so that only the repeater belonging to the currently master CPU is energised.

Observed symptoms:

• While CPU 0 is the active master, the Profibus segment is healthy and the HMI exchanges data.
• After a master switchover to CPU 1, the OLM reports a Profibus-side error: CH2 LED flashes red (or red/yellow, depending on OLM firmware revision) and the HMI S7 connection is lost.
• Cabling has been verified, but the problem persists on every switchover.

Affected Hardware and Reference Catalog Numbers

Verify the exact MLFB on the device label before commissioning. H-system-relevant restrictions in older CP443-5 firmware (e.g. < V5.x) are documented in the CP 443-5 manual (entry ID 8773898).

Root Cause Analysis

The reported behavior is a classic combination of three independent issues, all of which must be addressed for a stable redundant segment.

Cause 1 — Energising/de-energising the repeater on switchover

An RS485 repeater is an active device. It needs a stable 24 V supply to bias the segment and to drive the RS485 transceiver. The current installation uses two digital outputs of the S7-400H to switch the 24 V of the two repeaters so that "only the repeater of the active CPU" is powered. The unintended consequences are:

• During the switchover window the segment sees a powered repeater, then a hard power-down, then a fresh power-up. This is a bus reset event on Profibus, not a clean redundancy hand-off. Master-capable stations on the segment time out their GAP update, the OLM reports "channel error" on its CH2 LED, and the HMI's S7 connection is torn down.
• RS485 idle-bias relies on the repeater's internal fail-safe. With the repeater unpowered, the bus is left floating — there is no defined recessive state, so the OLM's Profibus receiver sees a constant stream of framing errors and shows the channel-error indication.

Cause 2 — Topology inversion vs. the Siemens reference

The Siemens-approved H-system fiber Profibus topology (see Siemens entry ID 29715969) requires a fixed sequence from the OLM copper port toward the CPU:

1. Active RS485 terminating resistor (powered from the OLM side, 24 V fed from a non-switched source).
2. First repeater (powered from a non-switched source).
3. Second repeater (powered from a non-switched source).
4. CPU/CP 443-5 of CPU 0 and CPU 1 in parallel at the segment.

Reversing this sequence — placing the active terminator at the far end of the segment instead of at the OLM end — defeats the bias chain and produces the same "red CH2" indication on the OLM. The terminator must always be at the physical end of the segment closest to the OLM copper port, not at the CP end.

Cause 3 — External CP 443-5 vs. internal Profibus interface

In an H-system, the internal Profibus/MPI interface of the CPU is the one that is automatically switched active/standby by the H-sync mechanism. External CPs of the type CP 443-5, when used in H-systems, behave differently: the two CP 443-5 modules are typically both active in parallel on the segment (or, depending on the configured redundancy role, only one is the master with the other in standby). They are not under the same hot-redundancy control as the internal interface, and a master switchover does not perform a clean bus hand-off on the external CPs the way it does on the internal interface.

For a fiber topology that must survive a CPU switchover, Siemens references the internal Profibus interface in their H-system documentation, not an external CP 443-5 connected through switched repeaters.

OLM LED Decoding

Reference Topology per Siemens Entry ID 29715969

The Siemens Application Note "PROFIBUS DP on S7-400H with two repeaters" defines the only approved wiring of a fiber Profibus segment driven by an H-system. The signal flow from the OLM copper port to the CPUs is:

OLM CH2 (copper) ── A1/B1
   │
   ├── [Active Terminator 6ES7972-0DA00-0AA0]  (powered, never switched)
   │
   ├── [Repeater #1, segment channel 1]        (powered, never switched)
   │         A2/B2 ─────────── A1'/B1'
   │
   ├── [Repeater #2, segment channel 1]        (powered, never switched)
   │         A2'/B2' ───────── A1"/B1"
   │
   └── CP 443-5 #0  (CPU 0)        CP 443-5 #1  (CPU 1)
       segment off channel 2         segment off channel 2

Two critical wiring rules are easy to violate in the field:

1. The 24 V supply of the active terminator and of both repeaters must come from a non-switched, redundant 24 V source (typically the plant's buffered DC bus, not a DO of the PLC).
2. The active terminator sits at the OLM end of the segment, not at the CP end. Its job is to define the bus state when the segment is otherwise idle.

Step-by-Step Resolution

Step 1 — Re-power the repeaters and the active terminator

1. Pick a non-switched 24 V rail (e.g. a SITOP PSU or the plant's buffered DC bus, fused at 1 A per repeater + 0.5 A for the active terminator).
2. Disconnect the 24 V wires from the two PLC digital outputs.
3. Wire both repeaters and the active terminator to this common 24 V rail.
4. Confirm the green "PWR" LED on each repeater and the LED on the 6ES7972-0DA00-0AA0 are continuously lit.

Step 2 — Re-order the bus terminator

1. Move the active terminator to the segment connector closest to the OLM copper port (A1/B1 side of repeater #1).
2. Verify by ohms measurement with the 24 V off: 220 Ω between A and B at the OLM end, > 2 kΩ between A and B at the CP end.

Step 3 — Re-check the CP 443-5 redundancy role

In HW Config, on the CP 443-5 properties, verify:

• Operating mode: DP master on both CP 443-5 (H-system mode).
• For each DP slave on the segment, both CP 443-5 must list the slave. With the H-redundancy S7-400H firmware, only the active CPU's CP 443-5 is the actual Profibus master; the standby CP 443-5 mirrors token traffic. The slaves must not see two distinct masters with different addresses.
• If you cannot configure this cleanly in HW Config, the segment cannot use two CP 443-5 in parallel; either switch to the internal Profibus interface of the CPU, or install a Y-Link downstream of the CPs.

Step 4 — Use the internal Profibus interface instead of CP 443-5

Where the H-system's internal Profibus interface is available (CPU 41x-5H family), prefer it. The internal interface is under the H-sync's direct control: a master switchover performs a clean bus hand-off and the segment is not interrupted. Configuration in HW Config:

1. Open the CPU rack. The slot "IF1" (internal Profibus interface) appears automatically.
2. Mark the interface as DP master, assign a Profibus address, and download the HW Config to both CPUs.
3. Wire the two repeaters to this single internal interface connector. The two CPUs' internal interfaces are wired in parallel electrically but only one is physically driving the bus at any time.

Step 5 — Y-Link fallback for DP slave integration

If you must keep a single DP-master segment and the H-system cannot expose its internal Profibus (e.g. CPU 412-5H not in scope), the Y-Link (6GK1500-0AA50) provides a clean, Siemens-approved coupling of one Profibus segment to the redundant H-system. The Y-Link accepts two redundant Profibus lines and presents one fault-tolerant segment to the DP slaves downstream. The HMI's APPLICOM card in this case lives downstream of the Y-Link as a DP slave, and the S7-400H couples to it through the Y-Link's master interface.

Verification Procedure

1. Power-cycle the whole segment, then the CPUs, and confirm the OLM CH2 LED returns to green within 5 s of CPU 0 reaching RUN.
2. Force a switchover from CPU 0 to CPU 1 with the HMI connection active. Use SIMATIC Manager: PLC > H-System > Switch Master (or PG function "Switch Over Master"). The HMI S7 connection must remain established; the OLM CH2 LED must not flash red.
3. Force a switchover back to CPU 0. Same expectation: no OLM alarm, no HMI connection break.
4. Disconnect one of the repeaters from 24 V (simulate a single repeater failure). The other repeater must continue to drive the segment, the OLM CH2 LED must remain green, and the HMI connection must remain alive. (This is a destructive test on production plants — perform on a maintenance window.)
5. Read the diagnostic buffer of both CPUs: there must be no DP master system fault, no station failure, no bus fault on the redundant interface.
6. Check the diagnostic repeater's statistics (if you have installed the 6GK1500-0AA10 instead of the standard 6GK1500-0AB00): live-list of stations, retry counters, and per-station diagnostic must be readable in STEP 7 via the diagnostic repeater's GSD.

Commissioning Checklist

• ☐ OLM fiber ports show green link on both ends; FO cable attenuation < 3 dB margin to OLM sensitivity.
• ☐ Active terminator 6ES7972-0DA00-0AA0 powered from a non-switched 24 V; its green LED is on.
• ☐ Both repeaters powered from a non-switched 24 V; the segment between them is terminated only at the OLM end (single termination).
• ☐ All Profibus connectors are 9-pin D-sub, with the terminating switch ON only at the physical end stations (here: OLM copper port, and the last DP slave on the segment if the HMI sits at the end).
• ☐ Shield of the Profibus cable bonded to ground at both repeater housings and at the OLM copper connector, not to the shield bar of a non-grounded cabinet.
• ☐ Baud rate consistent on all stations: 1.5 Mbps is the typical ceiling for a long fiber-to-copper Profibus segment; drop to 500 kbps if the diagnostic repeater reports retries.
• ☐ Bus addresses unique. H-system internal Profibus uses two consecutive addresses (e.g. 2 and 3) so that both CPUs' interfaces have a unique bus address.
• ☐ APPLICOM card on the HMI has the correct GSD for the configured DP master, and the S7 connection is configured as a "fault-tolerant S7 connection" (S7-REDCONNECT) in the HMI project.

Troubleshooting Matrix

Power and Sizing Notes

Repeater + active terminator current draw, useful for sizing the non-switched 24 V supply:

• 6GK1500-0AB10 RS485 repeater: 24 V DC, 200 mA typical (relay contact open), 250 mA with relay energised.
• 6GK1500-0AA10 diagnostic repeater: 24 V DC, 200 mA typical, up to 280 mA with two segments loaded.
• 6ES7972-0DA00-0AA0 active terminator: 24 V DC, 60 mA.

Total continuous draw for the recommended configuration (two repeaters + one active terminator): roughly 0.5 A. Fusing at 1 A slow-blow per device is appropriate.

Configuration Best Practices Summary

• Never switch the 24 V of a Profibus repeater as a way of "selecting" the active CPU. Power comes from a redundant, non-switched DC bus.
• Place the active terminating resistor at the OLM end of the segment, not at the CP end.
• Prefer the CPU internal Profibus interface over an external CP 443-5 for H-system redundancy; only the internal interface is under clean H-sync hand-off control.
• For DP slave integration to a redundant H-system from a single-master segment, use a Y-Link (6GK1500-0AA50); do not wire two CP 443-5 in parallel with switched power.
• On the HMI side, configure the S7 connection as a fault-tolerant S7 connection (S7-REDCONNECT) so that the S7-400H switchover is invisible to the HMI application.
• Use a diagnostic repeater (6GK1500-0AA10) to gain a live station list, retry counters, and per-station diagnostics in STEP 7 — invaluable when troubleshooting a segment that fails only on switchover.

Documentation and Standards References

• Siemens Entry ID 29715969 — "PROFIBUS DP on S7-400H with two repeaters" (Application Note).
• Siemens Entry ID 8773898 — CP 443-5 manual, including H-system notes.
• S7-400H Automation System, System Manual
• Diagnostic Repeater 6GK1500-0AA10 manual
• Y-Link 6GK1500-0AA50 manual
• IEC 61158 / IEC 61784 — PROFIBUS DP fieldbus standard.
• PROFIBUS Nutzerorganisation e.V. (PNO) installation guide for PROFIBUS DP/FMS (order number 2.141).