Resolving Siemens STEP 7 PROFIBUS Error 0x031B Synchronization on SIMATIC Stations

Engineers maintaining mixed SIMATIC S7-300 / S7-400 cells with a WinCC 2008 SP3 (WinCC V7.0 SP3) HMI and STEP 7 V5.5 engineering station frequently encounter a 0x031B synchronization error during the "Diagnose Hardware" sequence. The PG/PC reports that the bus is reachable, the LED test passes, the connector termination switch toggles through both states, and the cable is intact — yet the diagnostic interrupt still fires. This guide reconstructs the fault tree, validates the physical and logical layer assumptions, and walks through the corrective sequence that gets the bus back into cyclic DP operation without rewriting the project.

1. Problem Summary and Observable Symptoms

The fault presents as follows when the engineer runs the PG/PC online diagnostic against a PROFIBUS station:

• STEP 7 » "Diagnose Hardware" reports Error 0x031B: Synchronization error (problems with the PROFIBUS network).
• The same screen indicates the Hardware Test passed and the bus status LED on the CP flashes in the standard pattern.
• The PLC SF (group fault) LED may blink or be steady, with diagnostic buffer entries for PROFIBUS master system events.
• Communication with the affected station is intermittent: HMI tags time out, DP slaves go in and out of data exchange.
• The bus worked prior to a recent PC rebuild and reinstallation of WinCC 2008 SP3 and STEP 7 V5.5.

The pattern — a single station or the whole master system reporting 0x031B after a clean reinstall — is a textbook signature of logical layer desynchronization: the PG/PC and the DP master are no longer negotiating the same PROFIBUS parameters even though the wire is functional.

2. Decoding Error 0x031B

Siemens documents the 0x031B code within the PROFIBUS DP master diagnostic buffer (Event ID 0x031B). It belongs to the family of "synchronization" events that indicate the DP master has lost token/token-passing alignment with at least one DP slave, or that the parameter set sent to the slave was rejected. Per the SIMATIC NET PROFIBUS-DP Diagnostic Manual and the S7-300/S7-400 "System and Standard Functions" reference, the relevant diagnostic sources are:

0x031B is the lead event when a master cannot reach a defined cyclic state. The PLC records the event with a timestamp and (where the slave is identifiable) the affected DP slave's PROFIBUS address. The same code surfaces on the PG/PC side when the online view's "Diagnose Hardware" tool cannot establish a stable DP cycle.

3. Affected System Components

The 0x031B event typically involves one or more of the following components:

• Engineering PC — STEP 7 V5.5 + SPx, WinCC V7.0 SP3 (2008 SP3), SIMATIC NET PC software, CP 5611 / CP 5613 / CP 5621 / PC Adapter USB A2.
• PLC CPU — S7-300 CPU 31x-2 DP, CPU 31x-2 PN/DP, or S7-400 CPU 41x-3 DP with integrated PROFIBUS interface, or a dedicated CP 342-5 / CP 443-5.
• DP Slaves — ET 200M, ET 200S, ET 200pro, third-party DP devices, drives on PROFIBUS (e.g., Sinamics / Micromaster via PROFIdrive).
• Physical layer — PROFIBUS cable, 9-pin D-sub connectors, terminators (220 Ω between A and B at both bus ends), repeaters, OLM, OBT.

4. Root Cause Analysis

The 0x031B synchronization error has four root-cause families, ordered by frequency on rebuilt WinCC / STEP 7 stations:

4.1 PC-side configuration drift after reinstall

A fresh install of STEP 7 V5.5 and WinCC 2008 SP3 commonly leaves the PC interface (PG/PC interface) defaulted to PC internal, PLCSIM, or the wrong COM port. When the engineer starts "Diagnose Hardware" the PG sends a request at one baud rate and PROFIBUS profile while the bus is configured for another.

4.2 Bus parameter / profile mismatch

STEP 7 stores bus profiles (DP, Universal (DP/FMS), Standard, User-defined) for each PROFIBUS subnet. If the PG is set to Universal and the PLC's DP master to DP only, the master rejects the request, which manifests as a sync error. The same symptom appears when the highest station address (HSA) on the master is lower than the slave's PROFIBUS address — the master simply does not poll past HSA.

4.3 Termination and topology

PROFIBUS RS-485 requires active termination at both physical ends. The integrated terminator inside a Siemens 6ES7 972-0BA12 / 6ES7 972-0BB12 connector must be switched ON at the two end nodes and OFF at every node in between. After a PC swap, the engineer often reseats the bus cable but leaves a node's switch in the wrong position. With long cable runs (> 100 m at 1.5 Mbps, > 200 m at 500 kbps) reflections corrupt the token frame, and the master logs 0x031B.

4.4 CP firmware and driver mismatch

CP 5611 (6GK1561-1AA01), CP 5621 (6GK1562-1AA00), and CP 5613 (6GK1561-3AA01) each require a specific SIMATIC NET device driver version. A WinCC 2008 SP3 install may pull in an older S7DOS / S7Epan driver. Mismatched driver/firmware pairs cause the CP to come up, pass self-test, but fail parameter negotiation — again logged as 0x031B.

5. Pre-Diagnostic Checklist

Before opening STEP 7, capture the following baseline. The data is invaluable for comparing against the working state prior to the rebuild.

6. Diagnostic Procedure

Execute the following steps in order. Each step produces an unambiguous go/no-go result. Do not skip steps; 0x031B is multi-causal and a partial fix will look like a clean fix for a few hours before the bus drops again.

1. Open the PG/PC interface applet (Start » SIMATIC » PG/PC Interface). Confirm the assigned access point is S7ONLINE and the interface parameter is the physical PROFIBUS CP, not PLCSIM or Softnet.
2. Run "Diagnostics" on the CP. From the applet, click "Diagnostics" on the CP. A clean CP reports "No diagnostic events" and lists the bus topology. Any error here is a CP/driver/firmware issue, not a PLC issue.
3. Verify bus profile and baud rate against the PLC's PROFIBUS subnet (HW Config » DP master » Properties » PROFIBUS » Profile / Transmission Rate). Mismatch = guaranteed 0x031B.
4. Confirm HSA. STEP 7 defaults HSA to 126. If the project specifies a lower HSA, ensure the PG's HSA is the same or higher.
5. Open the PLC's diagnostic buffer (STEP 7 » PLC » Diagnostic Buffer). Look for paired entries: a 0x031B Synchronization error followed by a 0x031C Station failure for the affected PROFIBUS address. The second entry tells you which slave.
6. Physically inspect the bus. Walk the segment, confirm terminator switches at both end nodes, count total nodes against the max of 32 per segment (127 with repeaters).
7. Measure the bus with a PROFIBUS tester (e.g., Softing BC-700-PB, Procentec ProfiHub, or Siemens BT 200). Acceptable signal quality: ≥ 3 V peak-to-peak, ≤ 1.5 V asymmetry, reflection margin > 20%.

7. Step-by-Step Resolution

The corrective sequence is structured to fail fast. Each step is verified before moving on.

7.1 Step 1 — Reset the PG/PC interface to the physical CP

1. Close all STEP 7 / WinCC instances.
2. Open Set PG/PC Interface.
3. Select the access point S7ONLINE → CP 5611 (PROFIBUS) (or CP 5621 / CP 5613 / PC Adapter, as installed).
4. Click Properties » confirm Transmission rate and Profile match the PLC's DP master.
5. Click OK » close the applet. Restart STEP 7.

7.2 Step 2 — Reconcile the PROFIBUS profile in HW Config

1. Open the STEP 7 project, double-click the DP master (CPU 31x-2 DP or CP).
2. Properties » PROFIBUS interface DP » Properties button.
3. Set Profile to DP unless the segment contains FMS devices, in which case use Universal.
4. Set the Transmission Rate consistent with cable length:

   Baud rate	Max segment length
   9.6 kbps	1200 m
   19.2 kbps	1200 m
   45.45 kbps	1200 m
   93.75 kbps	1200 m
   187.5 kbps	1000 m
   500 kbps	400 m
   1.5 Mbps	200 m
   3 Mbps	100 m
   6 Mbps	100 m
   12 Mbps	100 m

5. Click OK, save and recompile (Station » Save and Compile).

7.3 Step 3 — Re-seat and re-terminate the bus

1. Power the entire PROFIBUS segment to a known state.
2. Identify the two physical end nodes. Set their connector terminator switches to ON (a Siemens 6ES7 972-0BA12 connector has a slide switch visible from the top; ON connects a 220 Ω resistor + 390 Ω pull-up/pull-down between A, B, and 5 V).
3. Set every other node's switch to OFF.
4. For long runs, attach a repeater (e.g., Siemens 6GK1 500-0AB10) at segment boundaries. Each repeater creates a new segment that must be terminated independently.

7.4 Step 4 — Verify the CP firmware and SIMATIC NET driver

1. Open Device Manager » SIMATIC NET » your CP » Properties » Firmware. Note the version.
2. Cross-reference with the compatibility matrix in the SIMATIC NET PROFIBUS Diagnostic Manual and the CP's firmware readme. CP 5611 firmware 8.x or higher is recommended for STEP 7 V5.5 SP3 and later.
3. If the firmware is outdated, flash it from the SIMATIC NET » Commissioning tool using the latest approved firmware image (order number on the Siemens support portal).
4. Confirm the SIMATIC NET software version matches the WinCC / STEP 7 release. WinCC V7.0 SP3 is qualified for SIMATIC NET V8.1 / V8.2.

7.5 Step 5 — Clear the diagnostic buffer and re-test

1. Connect online to the PLC.
2. PLC » Diagnostic Buffer » right-click » Clear buffer. (Note: this only clears display; the events remain in flash, but it lets you observe fresh entries.)
3. PLC » CPU Messages » set Update to Automatic.
4. Run STEP 7 » Diagnose Hardware again.
5. Confirm the "Error 0x031B" message no longer appears and that the diagnostic window shows "No error" for the master and each slave.

8. Bus Termination and Physical Layer Verification

Physical layer faults are the most common hidden cause of synchronization errors that survive a clean software install. Use this checklist before re-engineering the logical layer:

• Topology: Pure bus. Spurs are limited to ≤ 1.5 m total per branch and may not exceed 6 Mbaud · m of accumulated stub length at higher speeds.
• Cable: PROFIBUS cable type A (Siemens 6XV1 830-0EH10, 6XV1 830-3EH10) — characteristic impedance 135 Ω to 165 Ω, 30 pF/m loop capacitance. Avoid Type B (PVC) for new installs.
• Shield: Clamp 360° at each connector backshell and at cabinet entries. Lift shield at one end only when using surge protection; otherwise bond at both ends.
• Termination resistors: 220 Ω between A and B, 390 Ω from B to VP (5 V), 390 Ω from A to DGND, in each end-node connector.
• Shielding of VFD cables: keep PROFIBUS ≥ 200 mm from VFD power cables; cross at 90°.

A quick oscilloscope verification on the A/B pair should show a roughly square wave of 5 V nominal at 1.5 Mbps with 2.5 V common mode on A and 0.5 V on B (idle). Any rounded edges, ringing, or DC offset drift indicates termination or cable damage.

9. Baud Rate and Profile Configuration Reference

The following table is the de-facto decision matrix when the PG and the PLC disagree on PROFIBUS parameters. Use it as a field-tested shortlist before invoking the full SET PG/PC interface tool.

10. CP Firmware, Driver, and PC Interface Verification

The engineering PC stack must be in lockstep with the installed SIMATIC NET version. The following compatibility table is taken from the Siemens Industry Online Support release notes for STEP 7 V5.5 SP3 and WinCC V7.0 SP3. Verify the installed versions match the matrix:

If a downgrade or partial uninstall has left an older S7DOS or S7Epan driver in place, the CP will enumerate and pass self-test, but the user-mode bus driver will return a sync error on the first parameter handshake. Reinstall SIMATIC NET from the latest approved version. The official install is documented in the SIMATIC NET Installation Guide on the Siemens support portal.

11. Verification and Commissioning

Once the corrective sequence is complete, validate the fix with a deterministic procedure that an independent engineer can repeat at acceptance:

1. Power cycle the segment.
2. Open STEP 7 » PLC » Monitor/Modify. Confirm a DP slave is reachable from the master by reading a known process value (e.g., a digital input at I0.0 of an ET 200S).
3. Open WinCC Explorer » Tag Management » force a tag update and confirm the quality code is "Good" (0xC0), not "Uncertain" or "Bad".
4. From the diagnostic buffer, run the project for ≥ 30 minutes and verify no new 0x031B events are appended.
5. Re-run the Diagnose Hardware tool. The result must read "No error" on both the master and each slave.
6. Disconnect and reconnect the bus connector at the master to simulate a hot-attach. The bus must re-acquire within ≤ 5 s and the diagnostic buffer must not log 0x031B.

12. Preventive Maintenance

• Lock the PROFIBUS terminator switches with a dab of hot glue or use locking connectors (e.g., 6ES7 972-0BA52) in vibration-prone cabinets.
• Stamp the CP and the PLC DP master with the firmware version once commissioned, so a future field engineer has a known-good baseline.
• Maintain a backup of the SIMATIC NET configuration via Start » SIMATIC » SIMATIC NET » Commissioning » Save Configuration.
• Schedule a yearly PROFIBUS cable and termination inspection; record signal quality trends for predictive maintenance.
• Keep the PG/PC interface assignment constant in S7ONLINE; disable auto-detect of CP for the engineering PC.

13. Field Troubleshooting Matrix

14. Related Diagnostic Codes to Expect in the Same Buffer

After clearing 0x031B you may see complementary events. Knowing their meaning helps you avoid chasing phantom faults:

15. Frequently Asked Questions