1. Problem Statement
A single I/O module fault inside an ET 200M station connected to a software-redundant S7-300 system (two CPU 315-2DP controllers, software-redundant via the SWR library) causes both IM 153-2 interface modules on the affected rack to transition to a fault state, and the PROFIBUS connection from both redundant CPUs to the rack is severed at the same time. The fault symptom is asymmetric to its trigger: a single-point failure (one I/O card) produces a system-wide disconnection of the distributed I/O from both controllers simultaneously, and the redundant partner cannot take over the I/O because the slave itself is no longer reachable.
1.1 Observed Behavior
- Both IM 153-2 modules report SF (red) and may show BF (red) on the bus side.
- Both CPU 315-2DP entries in the diagnostic buffer show "Distributed I/O: Station failure" with timestamps within the same DP cycle.
- OB 86 (Rack failure) is called on both CPUs in the same OB 1 scan.
- The SWR library flags the station as not available on both masters.
- Manual reset or power cycle of the IM 153-2 is required to recover.
1.2 Why This Is Critical in SWR
Software Redundancy (SWR) is built on the assumption that the DP slave stations used as the redundant I/O are individually accessible from each CPU. When a single failure mode can take down the slave from both masters' perspectives at once, the redundancy concept collapses and a single I/O card replacement can stop the process.
2. Hardware Identification
The configuration in the field report uses the catalog numbers below. Confirm the part numbers on the physical modules before drawing any conclusion: an order-number mismatch (for example a -2BA01 mixed with a -2BA82) is itself a fault cause.
| Component | Order Number (MLFB) | Role |
|---|---|---|
| CPU 315-2DP (master A and master B) | 6ES7315-2AG10-0AB0 or compatible | Redundant master pair |
| IM 153-2 | 6ES7153-2BA01-0XB0 | DP slave interface for ET 200M, metal housing, D-sub PROFIBUS connector |
| Active backplane BM PS/IM | 6ES7195-7HB00-0XA0 | Bus module for power supply / IM slot, leftmost position |
| Active backplane BM IM/IM | 6ES7195-7HD10-0XA0 | Bus module bridging the P-bus between two IMs |
| Active backplane BM IM/IO | 6ES7195-7HC00-0XA0 (typical) | Bus module bridging the IM slots to the I/O slots |
| PROFIBUS cable | 6XV1830-0EH10 | DP cable between CPU and IM |
The 6ES7153-2BA01-0XB0 is the metal-housing IM 153-2 with D-sub PROFIBUS connector. It supports DP-V0 and DP-V1, and is suitable for ET 200M with active backplane. The redundant version with hot-swap capability is the 6ES7153-2BA82-0XB0. The two are NOT interchangeable: the -2BA01 cannot hot-swap modules on the active backplane; attempts to do so place the P-bus in fault and produce the symptom described in section 1.1.
Siemens product support: IM 153-2 (6ES7153-2BA01-0XB0)
3. Software Redundancy (SWR) Architecture Background
Software Redundancy for S7-300 is implemented via the SWR library (typically SWR V2.x for CPU 315-2DP, or SWR V3.x for CPU 315-2 PN/DP and CPU 317-2 PN/DP, included in the STEP 7 optional packages). It is not a hardware-redundant S7-400H system; it is a software layer that keeps two CPU 315-2DPs in sync and uses standard DP slaves as the I/O interface.
3.1 Synchronization Links
A redundant pair needs:
- One synchronization link (MPI, PROFIBUS, or fiber) between the two CPUs. In the field report, this is MPI.
- One or more PROFIBUS subnets for the DP master interfaces. In the field report, the DP connections to the ET 200M stations are PROFIBUS.
3.2 SWR Library Blocks
| Block | Type | Purpose |
|---|---|---|
| FB 101 "RED_INIT" | FB | Initialize the SWR library on startup |
| FB 102 "RED_DEBOUNCE" | FB | Suppresses momentary slave failure spikes |
| FC 100 "RED_INF" | FC | Read SWR diagnostic information |
| FB 103 "RED_STATUS" | FB | Query current redundancy state |
| FB 104 "RED_CTRL" | FB | Force backup/primary role changes |
3.3 Required Organization Blocks
For SWR the user program must supply the OBs listed below. If any of them are missing, the CPU goes to STOP on the first event that would have called them.
| OB | Name | Required? | Reason |
|---|---|---|---|
| OB 1 | Main cyclic | Yes | Calls the SWR FBs |
| OB 10 / OB 11 | Time-of-day interrupt | Optional | Used by SWR for periodic checks |
| OB 70 | I/O redundancy error | Yes | Called on redundancy loss |
| OB 72 | CPU redundancy error | Yes | Called when CPU redundancy is disturbed |
| OB 80 | Time error | Yes | Handles scan-time overruns |
| OB 82 | Diagnostic interrupt | Yes | Called by I/O faults (module or channel diagnostic) |
| OB 85 | Program execution error | Yes | Handles update errors |
| OB 86 | Rack failure | Yes | Called by DP slave failure |
| OB 100 | Warm restart | Yes | Required for SWR startup sequence |
| OB 102 | Cold restart | Yes | Required for SWR startup sequence |
4. Active Backplane Bus Module Selection
The 6ES7195-7HA00, -7HB00, -7HC00, -7HD10, and -7HD80 bus modules are physical backplane segments that route the PROFIBUS address and the P-bus (peripheral backplane bus) across the active ET 200M rack. Each module occupies one slot and physically passes the address and P-bus through to its neighbour.
4.1 Bus Module Layout for a Two-IM ET 200M
Slot assignments:
- Slot 0: BM PS/IM (6ES7195-7HB00-0XA0) — carries the 24 V power supply contact through to the active backplane.
- Slot 1: IM 153-2 (master A) — the first DP interface.
- Slot 2: BM IM/IM (6ES7195-7HD10-0XA0) — bridges the P-bus between the two IMs.
- Slot 3: IM 153-2 (master B) — the second DP interface.
- Slot 4: BM IM/IO (6ES7195-7HC00-0XA0) — bridges from the IM slots to the I/O slots.
- Slot 5+: I/O modules.
4.2 Why the Bus Module Matters
A bus module inserted in the wrong slot, or a bus module of the wrong type, breaks the address chain. If the address chain is broken, the IM 153-2 cannot enumerate the I/O modules downstream of the fault and reports a configuration error. The fault then bubbles up to both masters because the slave itself is no longer in a valid state.
The 6ES7195-7HD10-0XA0 is the BM IM/IM module for a two-IM ET 200M station. It is keyed differently from the BM PS/IM (7HB00) and the BM IM/IO (7HC00) so the modules cannot be physically interchanged at the slot type. However, two bus modules of the same type in two adjacent slots are treated as a "spare" segment, which is also a configuration error.
5. Root Cause Matrix
The reported symptom (single I/O fault → both IMs fault → both masters lose the rack) has multiple candidate causes. The table below maps each to the diagnostic signature and the verification step.
| # | Candidate Cause | Diagnostic Signature | Verification |
|---|---|---|---|
| 1 | Backplane bus module wrong type or wrong slot | SF on both IMs, OB 86 on both CPUs, IM diagnostic buffer "Configuration error" | Pull each BM and check order number on the side label |
| 2 | Active backplane termination missing (no 7HB00 at slot 0) | BF on both IMs intermittently, PROFIBUS diagnostics show line reflection | Verify 7HB00 is the leftmost module |
| 3 | Shared PROFIBUS segment between master A and master B without proper multi-master configuration | OB 86 with "Station failure" on both masters within the same cycle | Check DP topology: each CPU should have its own DP subnet or proper multi-master tokens |
| 4 | SWR library not loaded or version mismatch | OB 86 not called or called too late; OB 82 stops the CPU | Cross-check SWR library version in both CPUs' program folders |
| 5 | OB 82 and/or OB 86 not present | CPU 315-2DP goes to STOP on first I/O fault | Insert dummy OB 82 and OB 86 and re-test |
| 6 | I/O module not configured in HW Config | SF on IM 153-2, "Module does not exist" in diagnostic buffer | Compare HW Config with physical rack |
| 7 | PROFIBUS connector rotated 180° on one IM | Only one IM appears online; the other is in "Station failure" | Check connector orientation; DP connectors must be inserted with the cable exiting downward |
| 8 | Terminating resistor on the IM 153-2 PROFIBUS connector missing on one segment | Slave appears, drops out, reappears in diagnostics | Verify the on-board terminator on the last slave is ON and intermediate slaves are OFF |
| 9 | Duplicate PROFIBUS address | Both IMs come online; one drops out with "Address conflict" | Run "PROFIBUS → Diagnostics" in STEP 7 |
| 10 | I/O module defective (true field fault) | OB 82 called, channel diagnostic data identifies the channel | Replace the I/O module after diagnostic dump |
| 11 | 24 V supply undervoltage (< 20.4 V) on the active backplane | Both IMs report "24 V supply: low voltage" | Measure the 24 V at the BM PS/IM terminal block |
| 12 | Mixed IM 153-2 variants (-2BA01 and -2BA82) on the same rack | P-bus fault after first hot-swap attempt | Verify both IMs are the same order number |
S7-300 diagnostic buffer event list
6. Diagnostic Procedure
The following procedure identifies the root cause in a controlled order, starting with the most common and easiest to verify.
6.1 Step 1: Capture the Diagnostic Buffer on Both CPUs
- Open STEP 7 and go online to CPU A.
- PLC → Diagnostic/Setting → Diagnostic Buffer.
- Filter for events with ID "Distributed I/O: Station failure" or "I/O error".
- Export the buffer to a text file.
- Repeat for CPU B.
- Compare the timestamps. If the two events are within the same DP cycle (typically 10 ms for a 1.5 Mbaud PROFIBUS segment), the fault is a hard rack failure, not a transient cable issue.
6.2 Step 2: Read the IM 153-2 Slave Diagnostics
- Right-click the DP slave in HW Config or NetPro.
- Select "Slave diagnostics" or "Station diagnostics". The structure follows EN 50170 / IEC 61158:
- Byte 0–5: Standard diagnostic
- Byte 6–7: Device-related status
- Byte 8–9: Module status
- Byte 10–13: Channel-related diagnostic (if present)
- Decode the module status byte: bit pattern
0x01= module OK,0x0F= module wrong,0x47= module not configured. - Cross-reference the channel diagnostic with the actual I/O module slot.
6.3 Step 3: Verify the Backplane Bus Module Layout
- Power off the ET 200M station.
- Open the door of the active backplane.
- From left to right, verify the order number on the side of each bus module: 7HB00 (PS), IM-A, 7HD10 (IM/IM), IM-B, 7HC00 (IM/IO).
- Any deviation is a hardware configuration error and must be corrected before the system can be brought back into service.
6.4 Step 4: Verify the PROFIBUS Topology
A software-redundant pair with shared I/O usually uses one of the following topologies:
Topology A (preferred): separate DP subnets
CPU A ---DP subnet 1--- IM A of ET 200M
\-- other slaves
CPU B ---DP subnet 2--- IM B of ET 200M
\-- other slavesTopology B (shared DP subnet with both masters attached):
CPU A ---+---DP bus--- IM A of ET 200M
| \- IM B of ET 200M
CPU B ---+ \- other slavesTopology B is the most common cause of the reported symptom if the bus is not configured for multi-master operation. In Topology B, both masters are on the same PROFIBUS segment, and a single physical break (connector removed, cable damaged, terminator missing) takes out both masters' view of the slaves at once.
For Topology A, the symptom described is harder to produce because a single physical break can only disconnect one CPU from the rack.
6.5 Step 5: Verify the SWR Library Version
The SWR library is version-sensitive. The S7-300 SWR V2.x library is for CPU 315-2DP and CPU 317-2DP. CPU 315-2 PN/DP and CPU 317-2 PN/DP require SWR V3.x. Mixing the two libraries, or using the wrong library version for the CPU, causes OB 86 to be called on both CPUs for the same DP event because the SWR state machine in each CPU interprets the slave's absence as a master-side loss.
- In SIMATIC Manager, open the CPU's S7 program.
- Look in Blocks for the SWR library FBs (FB 101, FB 102, FB 103, FB 104).
- Right-click → Object Properties → Version tab → confirm the SWR version.
- Confirm the SWR version is identical on both CPUs.
SWR V2.x download and documentation
6.6 Step 6: Verify OB 82 and OB 86 Are Loaded
A CPU 315-2DP without OB 82 in the S7 program goes to STOP the first time an I/O module generates a diagnostic interrupt. Without OB 86, the CPU goes to STOP the first time a rack (or a DP slave) is reported as failed.
For SWR, both OBs are required. The minimum content of OB 82 in a SWR system is:
FUNCTION_BLOCK OB82
BEGIN
// SWR handles the diagnostic interrupt via FB 101/102
// in OB 1. Do not add custom logic here unless you
// know the SWR state machine contract.
END_FUNCTION_BLOCKIf OB 82 is empty, it is sufficient. Adding custom logic to OB 82 can interfere with the SWR state machine.
6.7 Step 7: Check the Diagnostic Buffer of the IM 153-2 Itself
- In STEP 7, go online to the DP slave (IM 153-2).
- PLC → Module Information or Diagnostic Buffer.
- The IM's diagnostic buffer contains events that the CPU's diagnostic buffer does not, including PROFIBUS-level retries, configuration mismatches, and P-bus faults.
- Common entries to look for:
- "Module inserted/removed"
- "Incorrect module in slot X"
- "PROFIBUS DP: station failure"
- "PROFIBUS DP: configuration error"
- "P-bus: short circuit"
- "24 V supply: low voltage"
7. Configuration Verification
7.1 HW Config: IM 153-2 Properties
Open HW Config, double-click the IM 153-2. The following properties must match the physical installation:
| Property | Expected Value | Notes |
|---|---|---|
| Order number | 6ES7153-2BA01-0XB0 | Must match the label on the module |
| Firmware | 4.x or higher | Check with Module Information online |
| PROFIBUS address | 3 (typical) | Must be unique on the segment, set via DIL switch on the back of the IM |
| Diagnostic interrupts | Enabled | Required for SWR |
| Hot-swap | Disabled for non-redundant slots | The -2BA01 does not support hot-swap; the -2BA82 does |
| Slots | 1 to 12 | Match the physical module count |
7.2 STEP 7 Program: OB Configuration
The S7 program must contain:
- OB 1 (main cyclic)
- OB 82 (diagnostic interrupt) — empty is fine
- OB 86 (rack failure) — empty is fine
- OB 100 (warm restart) — required for SWR
- OB 102 (cold restart) — required for SWR
- FB 101 / FB 102 / FB 103 / FB 104 from the SWR library
- DBs for SWR data
7.3 PROFIBUS Network Configuration
In NetPro:
- The MPI network is the synchronization link between the two CPUs.
- The PROFIBUS networks (one or two) are the DP master networks.
- Verify that the DP master interface of each CPU is correctly bound to a PROFIBUS network.
- Verify the transmission rate (1.5 Mbaud is the safe default for ET 200M stations with 12 modules).
- Verify that the bus parameters (Tslot, max TSDR, etc.) are not overridden at the master level.
8. Common Pitfalls and Field Notes
8.1 The "Both IMs Fault at Once" Trap
The IM 153-2 does not have its own fault isolation. A bus module that is loose or has the wrong order number interrupts the P-bus for the entire rack. Both IMs then report the same condition to their respective masters. The CPU then attributes the loss of the slave to the rack, not to the bus module, which is misleading during diagnosis.
8.2 PROFIBUS Repeaters
If a repeater is used between the CPU and the IM, the segment after the repeater is electrically isolated. A short on the CPU side does not propagate to the IM side, and vice versa. Without the repeater, a single short can take down the segment for both CPUs.
8.3 Grounding and Shielding
ET 200M stations with active backplane require the grounding bar to be connected to the cabinet ground. A floating ground can cause the IM 153-2 to enter a fault state when the field wiring picks up noise. The shield of the PROFIBUS cable must be clamped to the grounding bar on both ends.
8.4 24 V Supply Quality
The 6ES7195-7HB00-0XA0 is the BM PS/IM. It carries the 24 V supply to the active backplane. If the supply dips below 20.4 V (the lower limit per the S7-300 system manual), the IM reports "24 V supply: low voltage" and can enter the fault state. Both IMs see the same supply and both report the fault, which matches the field symptom.
S7-300 system manual (power supply limits)
8.5 Module Hot-Swap on the Wrong IM Variant
The 6ES7153-2BA01-0XB0 does not support module hot-swap on the active backplane. The 6ES7153-2BA82-0XB0 does. If the user is attempting hot-swap on a -2BA01, the backplane will fault. Both IMs then report a P-bus fault, both CPUs lose the rack. The fix is to either upgrade the IM to the -2BA82 or to remove the I/O module only when the rack is in STOP.
9. Firmware and Version Considerations
The 6ES7153-2BA01-0XB0 is an older release. Siemens has since released:
- 6ES7153-2BA02-0XB0 — DP-V1 and DP-V2 support.
- 6ES7153-2BA70-0XB0 — redundant version.
- 6ES7153-2BA82-0XB0 — redundant version with hot-swap.
If the user is mixing releases on the two IMs of the same ET 200M (one -2BA01 and one -2BA02, for example), the firmware versions can differ, and the diagnostic behavior can differ. The fix is to bring both IMs to the same firmware revision.
CPU 315-2DP firmware should be 3.x or higher for SWR V2.x. Earlier firmware releases do not support the SWR library.
10. Recovery Procedure
Once the root cause is identified, the following procedure brings the system back to a redundant state:
- Replace or correct the defective component (bus module, I/O module, cable, terminator, IM).
- Power-cycle the ET 200M station.
- Both IMs should come back online (BF LED off, SF LED off, ON green).
- Both CPUs should clear the OB 86 event in the diagnostic buffer.
- The SWR library should report the station as "Available" on both CPUs (read via FC 100 "RED_INF").
- Force a partner switch (FB 104 "RED_CTRL") to confirm that the redundancy logic is healthy.
11. Verification Checklist
Before declaring the system healthy, verify each item:
- Both IM 153-2 modules have green ON LED, no SF, no BF.
- HW Config matches the physical rack (slots, order numbers, addresses).
- OB 82, OB 86, OB 100, OB 102 are present in both CPUs.
- SWR library version is identical on both CPUs.
- Diagnostic buffer is empty on both CPUs.
- FC 100 "RED_INF" returns status "OK" on both CPUs.
- A partner switch via FB 104 succeeds without OB 86 entry.
- A simulated I/O fault (force a channel diagnostic) is handled without the second CPU losing the slave.
- PROFIBUS cable shields are clamped to ground at both ends.
- 24 V supply to the active backplane is within 20.4 to 28.8 V.
12. FAQ
Why does a single I/O module fault take down both IM 153-2 modules?
The IM 153-2 does not have fault isolation from the P-bus. A defective I/O module, a wrong bus module, or a P-bus short circuit places the entire ET 200M rack into a fault state, and both IMs see the same condition. This is by design of the active backplane architecture.
Is the 6ES7153-2BA01-0XB0 supported in software-redundant S7-300 systems?
Yes, the -2BA01 is supported. The redundant version with hot-swap is the -2BA82. The two are not interchangeable on the same rack because their P-bus behavior differs when a slot is hot-swapped.
How do I distinguish a P-bus fault from a true I/O module fault?
Read the IM 153-2 diagnostic buffer. A P-bus fault shows "P-bus: short circuit" or "P-bus: configuration error". A true I/O module fault shows "Module X: channel diagnostic" with the slot number. Use STEP 7 PLC → Module Information → Diagnostic Buffer on the DP slave to read these entries.
Can I run an ET 200M with only one IM in a redundant CPU system?
Yes, but the slave is then a single point of failure. If the IM fails, both CPUs lose the rack. With two IMs, the slave can be addressed by either CPU independently, which is the recommended topology for SWR and uses the 6ES7195-7HD10-0XA0 BM IM/IM bus module between the two IM slots.
What is the difference between 6ES7195-7HB00-0XA0 and -7HD10-0XA0?
The 7HB00 is the BM PS/IM module (carries the 24 V supply to the active backplane, leftmost slot). The 7HD10 is the BM IM/IM module (bridges the P-bus between two IMs in a hot-swap or redundant configuration). The 7HC00 is the BM IM/IO module (bridges from the IM slots to the I/O slots). The order number is printed on the side of each bus module; verify each one before commissioning.
Does the OB 86 need custom logic for SWR?
No. An empty OB 86 is sufficient for SWR. The SWR library installs its own logic via FB 101 / FB 102 in OB 1. Adding custom logic to OB 86 can interfere with the SWR state machine and cause false partner switches or stop the CPU during a transient slave failure.
How do I set the IM 153-2 PROFIBUS address?
The address is set on the DIL switch on the back of the module. Address 0 is illegal. Address 126 is illegal. Address 1–125 are valid. The address in HW Config must match the DIL switch, otherwise the slave is reported as "Station failure" on every cycle.
What is the maximum number of I/O modules per ET 200M with -2BA01?
12 modules with the active backplane. 8 modules with the passive backplane. The active backplane (7HB00 / 7HD10 / 7HC00) is required for the dual-IM configuration and for any slot-level diagnostic.