Resolving PROFIBUS Dropouts by Reducing Bus Speed to 187.5 kbps

Sporadic PROFIBUS-DP communication losses on Siemens S7-400 installations that mix ET 200 distributed I/O, OP170B/OP panel HMIs (Master Class 2), and Micromaster 4 / SINAMICS drives (slaves) are almost always caused by violations of the physical-layer constraints: segment length, stub length, node count per segment, or signal-quality degradation. Lowering the bus transmission rate from 1.5 Mbps to 187.5 kbps is a well-proven mitigation because it extends the maximum trunk length from 200 m to 1,000 m, increases the allowable stub length, and reduces the eye-diagram closure from reflections and EMC pickup. This article documents the full procedure for changing the PROFIBUS baud rate, the rule of thumb for repeater placement, and the diagnostic checks required to confirm the fix.

1. PROFIBUS-DP Baud Rates and Maximum Segment Length

PROFIBUS-DP (IEC 61158/61784) supports the following transmission rates on a bus segment terminated with standard 9-pin D-sub connectors and PROFIBUS cable (Siemens 6XV1 830-0EH10 or equivalent, characteristic impedance 150 Ω ±15 Ω, loop resistance 110 Ω/km, 24 AWG). The maximum segment length in the table below is the trunk (between two repeaters or between repeater and terminating resistor) - not the total installed cable length including stubs.

Source: PROFIBUS & PROFINET International (PI) Installation Guide, IEC 61158-2 physical layer specification. With 50 devices across 300-400 m, reducing the rate to 187.5 kbps gives a 5× safety margin on trunk length and a 20× improvement in allowable stub length.

2. PROFIBUS-DP Node Counting Rules

Two distinct node limits apply on every PROFIBUS segment:

• 32 nodes per segment. Every repeater, master, slave, and diagnostic repeater counts as one node. Each segment is the bus section between two repeaters, or between a repeater and the bus terminator.
• 126 total nodes per PROFIBUS-DP network. Reached by cascading up to four repeaters in series (4 × 32 - master counting rules; practical maximum is 126 including all masters and slaves).

With 50 devices in the user's plant, the network needs at least two segments - the single repeater mentioned in the source may already be mis-located (after node 33) or may not be present at the correct position for a 400 m trunk.

3. Root-Cause Analysis: Why Devices Sporadically Drop

The classic PROFIBUS-DP symptom pattern - "device goes offline for 1-3 token rotations, then comes back without operator action" - is almost always one of the following, in order of frequency:

4. Where to Change the PROFIBUS Baud Rate

On a PROFIBUS-DP network, three classes of device exist:

• Master Class 1 (DPM1) - typically the S7-400 CPU (e.g., 6ES7 414-3EM06-0AB0). The DPM1 holds the configured bus profile and pushes it to all slaves during parameterization.
• Master Class 2 (DPM2) - the HMI (OP170B, MP 277, TP 177, etc.) used by ProTool / WinCC flexible / TIA Portal for diagnostic and operator input.
• Slaves - ET 200M, ET 200S, ET 200pro, Micromaster 4 (6SE64), SINAMICS G110/G120, etc.

The baud rate is set once in the Master Class 1 (S7-400) hardware configuration. The DPM1 transmits the rate during the parameterization phase of every slave at start-up. The HMI panels (DPM2) and slaves do not need their own baud-rate setting - they auto-detect from the master's parameterization telegram. This is a key point from the field report: only change the rate at the PLC.

4.1 STEP 7 / TIA Portal - Master Class 1 Configuration

In STEP 7 V5.5 or earlier, open the S7-400 project in HW Config, select the CPU's DP interface, open "Properties - PROFIBUS Interface DP", and set "Network settings - Transmission rate" to 187.5 kbps. Save and compile (Station > Save and Compile), then download to the CPU. The CPU must be in STOP for the HW Config download.

In TIA Portal V13+ (when using S7-400H or S7-410 only), the procedure is similar: project tree > CPU > Properties > PROFIBUS interface > Transmission rate drop-down.

For TIA Portal support of S7-400 see the Siemens S7-400 function manual: S7-400 Automation System System Manual.

4.2 OP170B / OP 73 / TP 177 (Master Class 2) - Do You Need to Change the Rate?

On a PROFIBUS-DP HMI, the transmission rate is set in two places:

1. HMI project (ProTool / ProTool/Pro, WinCC flexible, or TIA Portal) - the "Connections" editor for the PROFIBUS subnet.
2. OP panel Control Panel (start-up phase, before WinCE loads the project) - this is the fallback rate used if the project is missing or corrupted.

Strictly, the OP panel does not need a separate project change because the panel accepts the master's parameterization. However, two practical points apply:

• If the panel is configured as a DP slave (older ProTool projects often set OP170B as a slave for status data), then it must be in the HW Config DP slave list with the same 187.5 kbps set, otherwise it will be flagged as "Configuration error" and not start.
• If the panel is used as a Master Class 2 only (standard for OP170B), the project connection's "Baud rate" field can be left at "Automatic".

To change the rate in the OP panel Control Panel on an OP170B (under Windows CE 3.0): Start > Settings > Control Panel > PROFIBUS > Baud rate > 187.5. Restart the panel.

4.3 ET 200M / ET 200S (Slaves)

No action required at the slave. The S7-400 master downloads the rate via the parameterization telegram. The ET 200M IM 153-1 (6ES7 153-1AA03-0XB0) and IM 153-2 (6ES7 153-2BA10-0XB0) accept any rate between 9.6 kbps and 12 Mbps automatically. The bus terminator must still be ON at the two physical ends of the segment only.

4.4 Micromaster 4 / SINAMICS Drives (Slaves)

Micromaster 4 (6SE64 series) and SINAMICS G110/G120 with PROFIBUS option module (6SE6400-1PB00-0AA0 for MM4) accept the master's rate automatically. The drive's own PPO type (1, 2, 3, 4, 5) and PROFIBUS address (P0918 / P2050) are independent of the baud rate. No change required to the drive parameters.

5. Step-by-Step Procedure to Reduce the Bus Rate to 187.5 kbps

1. Document the current network. Use a PROFIBUS tester (HMS Anybus ProfiHub B2, Softing PROFINET/PROFIBUS Diagnostic Tester, or Procentec ProfiHub) to capture the bus health report. The HMS Networks PROFIBUS Diagnostics whitepaper explains the procedure: Anybus PROFIBUS Diagnostics eBook (PDF).
2. Verify the repeater position. Physically trace the bus. Confirm that each segment contains ≤ 32 nodes, including the repeater. The repeater is itself a node on both sides.
3. Back up the STEP 7 project. Archive the S7-400 station and all OP panel projects.
4. Stop the CPU and set the mode selector to STOP. Wait for the "STOP" LED steady-on.
5. Open HW Config in STEP 7. Right-click the S7-400 CPU > Properties > DP Interface > PROFIBUS Properties > Transmission rate = 187.5 kbps.
6. Compile and download the HW Config to the CPU. The download is automatic if Ethernet/MPI routing is configured; otherwise connect the Siemens PC adapter (USB to MPI/DP, 6GK1 571-0BA00-0AA0) and download to MPI/DP.
7. Update the OP panel Control Panel (only if the OP170B fails to come online after CPU restart): Start > Settings > Control Panel > PROFIBUS > 187.5. This is the belt-and-suspenders backup step.
8. Reboot all slaves (or cycle segment power if a ProfiHub diagnostic repeater with diagnostic address is installed). Each slave's parameterization telegram will now contain the 187.5 kbps profile.
9. Run the CPU back to RUN.

6. Repeater Placement and Topology Validation

PROFIBUS requires a repeater when either of these conditions is met:

• Trunk length exceeds the rate-dependent limit (e.g., 400 m at 1.5 Mbps).
• A segment reaches 32 nodes.

For the user's 50-node, 300-400 m plant at 187.5 kbps, the topology should be:

If the single repeater is positioned after node 33 on a 200 m trunk, the segment count is fine but the trunk length may be the bottleneck. Dropping the rate to 187.5 kbps recovers 5× trunk-length headroom without rewiring.

6.1 Recommended Diagnostic Repeater (ProfiHub)

For noisy industrial environments, replace the standard repeater with a diagnostic repeater such as the HMS Anybus ProfiHub B2 (6GK5 173-1LB00-1AA0) or Procentec ProfiHub B1. The diagnostic repeater offers:

• Per-port signal-quality bar graph (RSSI-like indicator, 0-7 LEDs).
• Web-based diagnostic page with error counters per segment.
• Isolated segments so a single bad node does not bring down the entire bus.

7. PROFIBUS Cable and Connector Best Practices

After changing the baud rate, the cable infrastructure often becomes the new bottleneck because older 300-400 m runs may have:

• Twist-rate violations (PROFIBUS cable must be twisted 30+ turns per meter, 24 AWG).
• Cross-sections below 0.34 mm² (24 AWG) on long runs.
• Stubs > 0.3 m at 1.5 Mbps - acceptable at 187.5 kbps but still discouraged.
• Shield bonding only at one end (must be bonded at both ends, 360° gland, low-impedance).

Strip the PROFIBUS cable with the HMS Anybus Stripper Tool or equivalent (the HMS Networks PROFIBUS Diagnostics eBook documents the procedure in detail - see the whitepaper link in §5). Maintain a minimum 200 mm separation from VFD motor output cables; cross at 90° if crossing is unavoidable.

8. Verification Steps After the Rate Change

1. CPU in RUN with no SF LED. The S7-400 CPU's SF (red) and BF (bus fault) LEDs must be off. The BF LED is on the DP interface port of the CPU (or on the IF 964-DP interface module).
2. All ET 200 slaves show "OK" on HW Config diagnostic view (DP Slave > Online > Diagnose).
3. All Micromaster / SINAMICS drives show bus healthy. Drive parameter r2054[0] (DP state, MM4) or r2090 (SINAMICS) should display "2" (RUN).
4. OP panel connects. OP170B shows live process values; no "Connection to PLC interrupted" event.
5. Run a long-term log. Use the diagnostic repeater's web UI or a tool like Procentec ProfiTrace to capture 24-72 hours of bus traffic. Count error frames, retries, and station failures. Pre-fix baseline: 50+ retries/hour is typical for a sick bus. Post-fix target: < 2 retries/hour.

9. Field-Proven Caveats

• Never set the rate on a running bus. The change in HW Config must be downloaded with the CPU in STOP. Online edits to the PROFIBUS rate require a CPU restart and brief production stoppage.
• OP170B firmware matters. OP170B units with Windows CE 3.0 firmware below V6.0 may not auto-detect 187.5 kbps cleanly. Update the OP firmware to the latest available for the panel. The Siemens ProTool help specifies the supported baud rates per firmware revision.
• Micromaster 4 firmware 1.18+ is required for reliable 187.5 kbps parameterization. Earlier firmware versions default to 1.5 Mbps and may stall at lower rates. Check the drive nameplate firmware (P0018) and update via DriveMonitor if needed.
• PROFIBUS-PA devices on the same bus are always 31.25 kbps and require a segment coupler (PA coupler, 6GK1 545-0AA00) - they cannot be mixed with DP at 187.5 kbps. The user's plant description (Micromaster 4, ET 200, OP) is pure DP, so this is not an issue here.
• Spare cable length. If the existing trunk is exactly 400 m with 50 m coiled at the end, the unspooled loop can act as an antenna. Always terminate leftover cable in a wall-mount patch panel or remove it.

10. Troubleshooting Matrix

11. Summary of Constants Used in This Article

References: IEC 61158-2 (PROFIBUS physical layer), IEC 61784 (PROFIBUS communication profile), Siemens S7-400 System Manual (A5E00752850-01), PROFIBUS & PROFINET International (PI) Installation Guideline (Version 2.4, 2016).