Resolving IM153-1 Bus Fault After Adding DP/DP Coupler to S7-300

A CPU 318-2 DP (MLFB 6ES7318-2AJ00-0AB0) running 19 PROFIBUS DP slaves - including several ET 200M IM 153-1 modules and Simovert Masterdrive units - drops a single IM 153-1 into BUS Fault the moment a DP/DP Coupler is added to HW Config and downloaded. The online view of HW Config shows the affected slave with a red crossbar and the diagnostic text "Slave not available." Rolling back to the previous configuration clears the fault instantly, which confirms the cause is configuration-level rather than a hardware failure of either the IM 153-1 or the coupler. This reference documents the four most common causes, the diagnostic sequence to isolate them, the corrective actions in STEP 7 V5.x (HW Config), and the verification procedure that brings the network back to a clean state without replacing hardware.

1. Problem Overview

The classic symptom set on the CPU 318-2 DP after the DP/DP Coupler is integrated is:

• One specific IM 153-1 (typically the one physically closest to the new device, but logically any slave can be affected) drops into BUS Fault immediately after the HW Config download.
• The CPU's diagnostic buffer records a "Station failure" or "Slave not available" event (event IDs 0x0101, 0x0108).
• The IM 153-1's BF (Bus Fault) LED is lit red and the SF (System Fault) LED is lit red; the slave's input and output modules stop exchanging process data with the CPU.
• The remaining 18 slaves continue to operate normally; only the slave at the conflicting address or with the conflicting timing is lost.
• Reverting to the previous HW Config brings the IM 153-1 back into RUN within one PROFIBUS cycle without any hardware intervention.

The reversibility is the diagnostic signature. A hardware failure of the IM 153-1, the bus cable, or the CPU's DP interface would not be cleared by a configuration rollback. The cause is in the configuration, the addressing, or the bus parameters of the new device.

2. Affected Components and Versions

The IM 153-1 with MLFB 6ES7153-1AA03-0XB0 is the DP slave interface for the ET 200M distributed I/O system. According to the Siemens product support page, the PM400 milestone is set for October 1, 2023. Continued service after that date depends on spare-part availability; if the BUS Fault recurs after months of clean operation, the lifecycle status of the affected IM 153-1 hardware revision should be reviewed against any migration plan to the IM 153-2.

Siemens Industry Online Support: ET 200M Interface Module IM 153-1 (6ES7153-1AA03-0XB0)

3. Initial Diagnosis: Reading LED Status and Diagnostic Buffer

3.1 LED Patterns on the IM 153-1 (6ES7153-1AA03-0XB0)

When both SF and BF are lit steady, the IM 153-1 has power but the master has either stopped token-passing to the slave, the slave's PROFIBUS address is unreachable, or the bus parameters calculated by the master exceed the IM 153-1's declared limits. This corresponds to the STEP 7 "Slave not available" diagnostic text.

3.2 Reading the CPU 318-2 DP Diagnostic Buffer

1. Open SIMATIC Manager and connect online to the CPU 318-2 DP.
2. Navigate to PLC > Diagnostics/Settings > Diagnostic Buffer.
3. Filter for events of class "I/O Fault" or "Station Fault."
4. Open the detail dialog of the most recent fault entry. The detail lists the affected logical base address, the PROFIBUS address, the diagnostic function (DF), and the manufacturer-specific diagnostic bytes.
5. Common entries on this symptom set:

• Station failure (event ID 0x0101) — the master lost the response from the slave.
• Slave not available (event ID 0x0108) — the master did not receive a valid response within the watchdog.
• Parameter assignment error on the DP slave (event ID 0x0110) — the master sent parameterization frames but the slave rejected them.
• Configuration error on the DP slave (event ID 0x0111) — the master's configuration does not match the actual slot count or module type.

6. Capture the diagnostic buffer to a text file (Save As > Text File) before clearing it. The entries document the fault history and are required when escalating to the Siemens Technical Support team.

3.3 Online View of the Slave in HW Config

In HW Config, switch to the online view (View > Online or press F5 with the DP master system selected). The DP slave objects change color to reflect their state:

• Green check mark: slave is in cyclic data exchange.
• Yellow warning: slave is configured but has diagnostic pending.
• Red crossbar: slave is not reachable on the bus ("Slave not available").

Right-click the affected slave and select Module Information. The "DP Slave Diagnostics" tab shows the master PROFIBUS address, the slave's reported diagnostic bytes, and the slot-level status of the IM 153-1's installed S7-300 modules.

4. Root Cause Analysis: Common Triggers

The symptom "one IM 153-1 into BUS Fault immediately after a DP/DP Coupler is integrated" is produced by one of the following configuration-level causes. In order of observed frequency on CPU 318-2 DP networks of this size:

1. PROFIBUS address conflict: the DP/DP Coupler is configured with the same PROFIBUS address as the affected IM 153-1 (for example both at address 12). The master's request frames collide, the slave that loses arbitration is reported as "not available."
2. Bus parameter mismatch: the DP/DP Coupler's GSD file requests bus parameters (T_slot, T_set, max T_SDR) that exceed the values currently set on the master. The master recalculates the segment parameters to the union of all GSD files, and the IM 153-1 (not the coupler) becomes the slave that rejects the new timing.
3. Bus termination or cabling change: inserting the DP/DP Coupler physically changes the bus topology. If the terminator that was previously on the last IM 153-1 has been moved, or if the coupler is unpowered, the segment beyond the coupler sees reflections and the IM 153-1 loses frames.
4. I/O address space overlap: STEP 7 auto-assigned the diagnostic or process I/O addresses of the DP/DP Coupler overlapping with the existing range of the IM 153-1 modules. The S7-300 user program sees double-assigned inputs and OB 82 / OB 86 diagnostics are triggered.

5. Cause 1: PROFIBUS Address Conflict

5.1 Verifying the Address Conflict

PROFIBUS DP allows 126 device addresses per segment (0 is reserved for the master-class-1 device, 127 is reserved for broadcast). Every slave, including the DP/DP Coupler acting as a slave on the master side, must have a unique address on that segment. Address conflicts produce exactly the symptom described: a single slave becomes unreachable immediately after a new device is added, while the rest of the network operates normally.

1. Open HW Config, right-click the DP/DP Coupler object, and select Object Properties > PROFIBUS. Record the configured PROFIBUS address.
2. Right-click the affected IM 153-1 and record its PROFIBUS address.
3. If the two addresses match, the conflict is confirmed. Change the DP/DP Coupler's address to an unused value within the segment.
4. Recompile HW Config and download to the CPU only (target system: CPU 318-2 DP, not the entire station).
5. After the download, monitor the BF LED on the IM 153-1. It should clear within one PROFIBUS cycle (typically < 50 ms at 1.5 Mbit/s).

5.2 Programming the Physical DIP Switches

The DP/DP Coupler has a 7-position DIP switch on each PROFIBUS port that selects the address (1 to 99). The DIP switches must be set to the same value as the address configured in HW Config. If they differ, the master sees no slave at the configured address and the slave appears "not available."

1. Power down the DP/DP Coupler.
2. Set the DIP switches for both ports (master side and slave side) using the standard BCD code with positions 1 + 2 + 4 + 8 + 10 + 20 + 40. For example, address 64 is selected with position 64 only (DIP 7 ON, all others OFF).
3. Restore power. The coupler reads the DIP switches at startup and assigns them to both ports.
4. Verify the address in the project matches the DIP switch position.

Note: the address range supported by the DIP switches (1 to 99) is smaller than the full PROFIBUS range (1 to 125). When selecting a free address for the coupler, prefer a value above the highest existing slave address to keep the address map readable. For 19 slaves already addressed at 3 to 21, addresses 22, 32, or 64 are typical choices for the coupler.

5.3 Network Topology Reference

The diagram below shows the pre-change topology (top) and the post-change topology (bottom). The slave at PROFIBUS address 12 is the one that drops into BUS Fault after the change.

6. Cause 2: Bus Parameter Mismatch

PROFIBUS DP requires that the bus parameters calculated by the master are within the limits of every slave on the segment. The DP/DP Coupler's GSD file declares its required T_slot, T_set, min T_SDR, and max T_SDR values. If the master accepts the new device but its declared values push the calculated bus parameters above the IM 153-1's declared limits, the IM 153-1 (not the coupler) will be the one that fails to parameterize.

T_bit is the bit time, equal to 1 / baud rate. At 1.5 Mbit/s, T_bit = 667 ns; at 12 Mbit/s, T_bit = 83 ns; at 187.5 kbit/s, T_bit = 5.33 us.

6.1 Reading the Bus Profile in STEP 7

1. In HW Config, right-click the DP master system line (the icon to the right of the CPU 318-2 DP) and select Object Properties > PROFIBUS > Bus Parameters.
2. Switch the profile from "DP" to "User-defined" to view the raw values.
3. Note the T_slot, T_set, max T_SDR, and T_TR currently in effect.
4. Compare against the values reported for the DP/DP Coupler in the same dialog. The master will display the calculated "maximum values" it has derived from the union of all GSD files on the segment.

If the DP/DP Coupler's max T_SDR is higher than the IM 153-1's, the master will use the higher value for the segment and the IM 153-1 will see longer slot times than it accepts. The fix is one of:

• Select a lower baud rate (500 kbit/s or 187.5 kbit/s) on the master, increasing T_bit so the slot time fits in the longer window.
• Add a PROFIBUS repeater (6ES7972-0AA01-0XA0) to segment the network so the IM 153-1 sits on a sub-network where its max T_SDR value is the controlling value.
• Replace the older IM 153-1 hardware revision with one that supports the same max T_SDR as the DP/DP Coupler.

6.2 Baud Rate vs Segment Length

6.3 Reducing the Baud Rate as a Quick Mitigation

If the IM 153-1 is not reaching the parameter assignment phase after the coupler is added, drop the baud rate to 187.5 kbit/s as a diagnostic step. The longer bit times absorb the wider timing window of the additional device. If the IM 153-1 re-enters RUN at the lower baud rate, the cause is timing, and the decision is whether to keep the segment at the lower baud rate, accept the timing margin loss, or install a repeater to keep the higher baud rate on the rest of the segment.

7. Cause 3: Power Supply and Bus Termination

A DP/DP Coupler is an active device: it requires a 24 V DC power supply on both ports, draws approximately 250 mA per port (500 mA total with both ports active), and regenerates the bus signal on each side. When the coupler is inserted physically between the CPU and the affected IM 153-1, three physical-layer conditions can produce the BUS Fault:

• Unpowered coupler: the coupler passes neither frames nor power. The IM 153-1 sees a dead segment, identical to a cut cable.
• Missing or moved termination: PROFIBUS DP requires active bus termination at both ends of each segment. Inserting a device in the middle of a previously terminated segment leaves one segment with two terminators and the other with none. The standard terminator is 220 ohm in series with +5 V bias on each signal line.
• Shield grounding: the shield of the new segment must be bonded to ground at one point using PROFIBUS connectors with grounding tabs and a grounding bar. Missing bonding exposes the bus to common-mode noise that can drop the IM 153-1 intermittently.

1. Verify the 24 V supply at the coupler's power terminals. The voltage must be within 20.4 V to 28.8 V under load.
2. Verify the PROFIBUS connectors: the bus terminator switches on the connector at the upstream end of segment A and at the downstream end of segment B must be ON. All connectors in the middle of the segment must have the terminator OFF.
3. Verify the shield continuity with a multimeter on the shield contact from one end of the segment to the other. The reading should be less than 1 ohm after subtracting the cable's shield resistance (typically 12 to 25 ohm/km for PROFIBUS violet cable 6XV1830-0EH10).
4. If the coupler's 24 V supply is shared with the IM 153-1 (PS 305 or PS 307 with a common busbar), verify the supply is not in current limit. A temporary sag during coupler startup can drop the IM 153-1 into BF.

8. Cause 4: I/O Address Space Overlap

STEP 7 HW Config assigns a default input and output address to every DP slave's diagnostic interface and process image. When the DP/DP Coupler is inserted, STEP 7 may auto-assign its process I/O addresses (default 4 bytes input, 4 bytes output) into a range that overlaps with the IM 153-1's existing addresses.

STEP 7 will avoid the overlap if "Automatic address assignment" is enabled. The issue appears only if the user has manually placed the DP/DP Coupler's address in the same range as the IM 153-1.

1. Right-click the DP/DP Coupler in HW Config and select Object Properties > Addresses.
2. Note the Start address for inputs and outputs.
3. Repeat for the IM 153-1.
4. If the ranges overlap, change the DP/DP Coupler's start address to a free area or enable Automatic address assignment for the coupler only.

Address overlap does not directly produce "Slave not available" (it produces "I/O access error" or OB 122 instead), but in some firmware revisions the OB 86 fault handling combined with overlapping diagnostics can produce a cascade that drops the slave from the cyclic exchange list. Treat this cause as the last one to verify, after addressing, bus parameters, and termination have been ruled out.

9. Step-by-Step Resolution Procedure

1. Capture the diagnostic buffer from the CPU 318-2 DP and save the text export for the maintenance log.
2. Roll back the HW Config to the last known good configuration (the backup file). Confirm the network is clean: all IM 153-1 BF LEDs are off and all 19 slaves show green check marks in the online view.
3. Open the current HW Config and locate the DP/DP Coupler object in the DP slave catalogue (PROFIBUS DP > Other Field Devices > Gateway > DP/DP Coupler).
4. Record the coupler's PROFIBUS address from HW Config. Compare with the addresses of the 19 existing slaves. If there is a duplicate, change the coupler's address to an unused value (1 to 99 for DIP switches, 1 to 125 for the configured value).
5. Set the DIP switches on the physical coupler to match the address configured in HW Config. Both ports if the coupler is on the same segment, or only the master-side port if the coupler sits at the boundary between two masters.
6. Verify the bus parameters in Master System > Object Properties > Bus Parameters. Switch to "User-defined" and confirm the values do not exceed the limits of any slave, including the coupler. Pay particular attention to T_slot, max T_SDR, and T_TR.
7. Check power and termination on the physical bus. Confirm 24 V at the coupler and at the affected IM 153-1. Confirm termination is ON at both ends and OFF in the middle.
8. Check the I/O address assignment of the coupler. Move the start addresses out of the IM 153-1's range if necessary.
9. Download the corrected HW Config to the CPU 318-2 DP only. Do not download to the slaves individually; the master will re-parameterize them on the next cycle.
10. Monitor the diagnostic buffer and the BF LED on the affected IM 153-1 for at least two full PROFIBUS cycles.
11. If the BF LED clears, save the HW Config as the new master backup with a timestamp and operator initials.
12. If the BF LED remains on, repeat from step 4. Address conflict is the most common, but timing parameters are the second most common cause of a persistent fault. Verify the bus profile again, and consider dropping the baud rate as a diagnostic step before further hardware changes.

10. Verification and Commissioning Checks

10.1 PROFIBUS Diagnostics via HW Config

After the network is clean, capture the bus statistics from the online diagnostics to confirm the segment is stable:

1. Go online with the CPU 318-2 DP.
2. Right-click the DP master system line and select Diagnostics > PROFIBUS Diagnostics.
3. Open the "Bus Statistics" tab. Confirm:

• Frame errors: 0
• Retry counter: stable (no upward trend)
• Slave diagnostic events: empty
• Token rotation time: stable and below T_TR x G

1. Capture a screenshot for the commissioning folder. Run the capture for at least 10 minutes of clean operation to catch intermittent retries.

10.2 Re-initializing the Slave via SFC 12

If the BF LED clears on the diagnostic buffer but the IM 153-1 does not return to cyclic data exchange (a rare condition after a download), call SFC 12 (D_ACT_DP) from the S7 program to deactivate and reactivate the slave:

CALL "D_ACT_DP"
   REQ       := TRUE                  // activate request
   MODE      := B#16#2                // 2 = deactivate then activate
   LADDR     := W#16#0C               // logical base address of the IM 153-1
   RET_VAL   := MW 100                // return value
   BUSY      := M 110.0               // busy flag

For an SFC 12 call, LADDR is the diagnostic address (default PIB 1000 in the example above). The call forces the master to re-parameterize the slave from scratch, clearing any residual state that a configuration download alone does not reset.

11. Preventive Best Practices

• Document the address map: maintain a spreadsheet of every PROFIBUS slave's address, MLFB, firmware, and the date the device was added. Update the spreadsheet every time a device is added or moved.
• Reserve an address range for the DP/DP Coupler: assign the coupler the highest free address in the segment (for example, 32 or 64) and reserve that range in the documentation to avoid accidental reuse.
• Use the standard profile: keep the bus profile at "DP" rather than "User-defined" unless the segment requires special values. The standard profile selects the slowest slave's values automatically, which is usually the correct conservative choice.
• Plan the topology before adding devices: if a new device is inserted in the middle of a previously terminated segment, plan the termination moves and update the cabling diagram before powering up.
• Save a backup of HW Config: STEP 7 archives every download with a timestamp. Keep at least the last three good configurations on a shared engineering drive with a clear naming convention (ProjectName_HWConfig_YYYYMMDD_HHMM.zip).
• Run the PROFIBUS diagnostics during commissioning: after every change, check the bus statistics for at least 10 minutes of operation to catch intermittent retries before they become hard faults.
• Keep the GSD files up to date: when a new variant of the DP/DP Coupler is installed, install the latest GSD file (SIEM819D.GSD for the current 6ES7158-0AD01-0XA0) to pick up any timing or diagnostic improvements.
• Use repeaters for long segments: if the segment approaches the maximum cable length for the chosen baud rate, install a PROFIBUS repeater (6ES7972-0AA01-0XA0) to extend the segment without reducing the baud rate.

12. Frequently Asked Questions