Profibus DP Troubleshooting: S7-300 CPU 314C-2DP and ET 200B

Problem Overview

When commissioning a SIMATIC S7-300 station with an integrated Profibus DP master (CPU 314C-2DP) and an ET 200B distributed I/O slave, two recurring faults appear at first power-up:

1. SF/BUSF LED lit on the master, with the diagnostic buffer reporting "module configured, but not available" for the ET 200B station.
2. "Transmission rate does not match the network transmission rate or wrong interrupt" error when a second slave is added to the same segment.

Both faults trace to the same class of physical-layer mistakes: incorrect Profibus termination switch positions, programming-cable termination left active, and inconsistent Profibus DP baud-rate configuration between master and slave GSD files. The remainder of this reference covers the underlying Profibus DP rules, the Siemens-specific parameters, the diagnostic workflow, and the verification sequence that brings the bus up cleanly.

Affected Hardware and Firmware

Confirm the firmware level of the CPU 314C-2DP before commissioning. The integrated DP master on the 314C-2DP requires STEP 7 V5.4+ or TIA Portal V13+ for full diagnostic support. Firmware versions earlier than V2.6 do not support the 12 Mbps profile, and you may see "Baud rate not supported" events if the slave GSD declares 12 Mbps.

Profibus DP Termination Fundamentals

Profibus DP is a multi-drop RS-485 bus. Each segment must be terminated at exactly two points — the physical start and the physical end of the segment — with a 220 Ω resistor (combined with the 390 Ω pull-up/pull-down network inside a Profibus connector). The termination serves three purposes:

• Establishes a defined idle bus voltage of nominally 5 V between A and B lines.
• Suppresses signal reflection on the segment ends.
• Defines the DC bias so that idle bits are correctly decoded as '1'.

Siemens Profibus connectors (e.g., 6ES7972-0BA12-0XA0, 6ES7972-0BB12-0XA0) expose a slide switch labelled ON / OFF. ON inserts the terminator. The same physical arrangement is duplicated on the ET 200B module: most ET 200B variants expose a small DIL switch bank or a slide switch next to the bus connector.

Root Cause: "Module Configured, but Not Available"

When the master CPU reports "Module configured, but not available" (German: "Baugruppe projektiert, aber nicht vorhanden", event ID 0xE401 / 0xE402 with diagnostic interrupt), the master has a Profibus configuration in HW Config / TIA Portal device configuration that includes the ET 200B at a specific Profibus address, but the master does not see any reply from that address during the parameterization telegram cycle. The top three causes, in order of frequency, are:

1. Bus termination missing or wrong — the most common cause. The slave never receives a valid frame, so it never replies, so the master never sees it.
2. Profibus address mismatch — the slave is set to address 3 (via DIL switches on the ET 200B) but the master is configured to expect address 5. The master polls address 5, gets no response, and raises the diagnostic event.
3. Shielding and grounding fault — the cable shield is not bonded at the cabinet entry, or the shield is continuous but not referenced to PE at both ends. Profibus requires the shield bonded to PE at every connector.

The CP 5611 programming interface is a frequent contributor when the PG/PC is left connected to the bus during commissioning. If the cable's integrated terminator (or the CP 5611 card's terminator DIP) is set to ON and the PG is connected in the middle of the segment, you have three terminators on the bus. The signal level collapses, no device can reliably frame bits, and the master reports the slave as missing. The official guidance for the CP 5611 is to leave its termination switch in the OFF position when it is used as a programming device on a live segment, and to switch it ON only when the CP 5611 is the physical end of the segment.

Root Cause: Transmission Rate Mismatch

After fixing termination, adding a second slave often surfaces a new error: "Transmission rate does not match the network transmission rate or wrong interrupt" (German: "Übertragungsgeschwindigkeit passt nicht zur Netzwerk-Übertragungsgeschwindigkeit oder falscher Interrupt"). Profibus DP only operates when all nodes on a segment agree on a single baud rate — there is no auto-baud negotiation beyond the master's "Find baud rate" service. The error is reported when:

• The newly added slave is set to a fixed baud rate (via DIL switch) that differs from the master's profile. For example, the master is configured for 1.5 Mbps in HW Config but the new ET 200B is set to its default auto-detect position, which on older firmware defaults to 9.6 kbps until a successful parameterization telegram has been received.
• The new slave's GSD file declares a maximum baud rate (e.g., 12 Mbps) that the master's firmware revision does not support. CPU 314C-2DP firmware earlier than V2.6 cannot run at 12 Mbps.
• The slave is a "DP-V1" device and the master has "DP-V1" disabled in the master's Profibus properties. The first parameterization handshake fails and the master falls back to 9.6 kbps while the slave holds at the configured rate.

CP 5611 Programming Interface Configuration

The CP 5611 is a PCI Profibus interface that integrates into STEP 7 and TIA Portal as the PG/PC interface. It has a small DIP switch block on the rear bracket that controls whether the card itself terminates the bus. The correct configuration depends on the role of the PG/PC in the segment:

For commissioning, the recommended workflow is to keep the PG/PC on the master's PG socket (the second D-sub on the bus connector at the CPU 314C-2DP end) with the master's terminator active and the PG cable's terminator set to OFF. The CP 5611's onboard terminator is irrelevant when the PG is connected through the master because the bus connector on the master side is a passive tap, not a bus end.

To browse the live bus from the PG, use the online access function in TIA Portal. Double-click "Update accessible devices" below the CP 5611 entry in the project tree to enumerate all reachable Profibus nodes. The TIA Portal documentation describes this in the "Displaying accessible devices" online help article.

Diagnostic LED Interpretation

Both the CPU 314C-2DP and the ET 200B expose status LEDs that pinpoint the fault class. Reading them before opening STEP 7 / TIA Portal saves time.

Step-by-Step Resolution Procedure

Follow this sequence in order. Do not skip to TIA Portal / STEP 7 configuration changes until the physical layer is confirmed clean — software changes will not recover a mis-terminated bus.

1. De-energize the segment. Power down the CPU 314C-2DP, the ET 200B, and any repeaters on the bus.
2. Identify the physical ends of the segment. Trace the purple Profibus cable from the master's X2 connector to the last device. The end nodes are the master (CPU 314C-2DP) and the last ET 200B (or any other DP device) on the chain.
3. Set termination ON at exactly those two points.
   • CPU 314C-2DP: slide the bus connector's switch to ON on the connector that sits on the master's X2 socket. There is no onboard terminator on the CPU itself.
   • Last ET 200B: slide its bus connector switch to ON. If the ET 200B is a B-16DI variant, the terminator is integrated — set the DIL switch on the module front to ON for the last station, OFF for every station in between.
4. Verify the in-between nodes are OFF. Every ET 200B, repeater, or other DP device between the two physical ends must have its terminator switch set to OFF. A middle terminator collapses signal amplitude and causes intermittent faults that look like baud rate errors.
5. Set the CP 5611 termination to OFF. If the PG/PC is connected via the master's PG socket, both the CP 5611's onboard terminator and the programming cable's terminator must be OFF.
6. Re-apply power and observe LEDs. The master should show RUN (green), no SF, no BUSF. The ET 200B should show its ON (green) and no BF.
7. Verify the Profibus address of every slave. On the ET 200B, the Profibus address is set by a rotary or DIL switch on the module front. Cross-check against the HW Config / TIA device configuration. A common commissioning error is leaving the default address (3) when the configuration expects address 4.
8. Open TIA Portal / STEP 7 and use "Update accessible devices" on the CP 5611 interface. All configured slaves should appear in the online accessible nodes window. Reference: TIA Portal – Displaying accessible devices.
9. Resolve any residual baud rate error. If the second slave raises "transmission rate does not match", open HW Config (STEP 7) or the device configuration (TIA) on the master, select the Profibus subnet, and check the "Network settings" tab. Match this rate on the slave's DIL switch if it is set to a fixed rate, or leave the slave in auto-detect (recommended for ET 200B).
10. Download the hardware configuration to the CPU 314C-2DP and observe the BUSF LED — it should extinguish within two to three parameterization cycles (typically 1–2 seconds at 1.5 Mbps).

Verification and Commissioning

After the physical layer is clean and the configuration is downloaded, perform the following verification checks before placing the system in production:

• Diagnostic buffer: In TIA Portal / STEP 7, open Online & Diagnostics → Diagnostic buffer on the CPU. The buffer should not contain any new Profibus entries after a 5-minute run. If BUSF reappears, the bus is flapping — usually a loose connector, a missing shield bond, or a marginal terminator contact.
• Bus statistics (master-side): From the CPU's online diagnostics, read the Profibus statistics counters. The "Frame repeats", "Frame errors", and "Slave diagnostics" counters should be stable over time. Repeated frame errors on a specific slave indicate a marginal signal — typically a terminator problem on a neighbor device, not on the failing slave.
• ET 200B I/O sweep: Force each input from the field side and verify it in the CPU's input process image (e.g., IW 0 for the first word of the ET 200B-16DI). Toggle each output from the PLC program and verify field-side voltage. A clean bus shows no input chatter; a bus with a marginal terminator shows random input bits flipping at the baud rate.
• Disconnect the PG and re-power. Once commissioning is complete, remove the PG cable. A common production failure is leaving the PG cable connected with its terminator ON — the bus works during commissioning and fails intermittently in production because the operator's PG cable is added to the segment.

Edge Cases and Field-Proven Caveats

Several non-obvious situations surface repeatedly on Profibus DP commissioning of S7-300 + ET 200B stations:

• Segment length vs. baud rate. The maximum segment length on Profibus DP with type-A cable (6XV1830-0EH10) is 1200 m at 9.6 kbps, 200 m at 1.5 Mbps, and 100 m at 12 Mbps. Exceeding the length budget does not surface as a hard error — it surfaces as a slow degradation in frame error rate that is invisible until the cell is in production.
• Stubs. Profibus DP supports short stubs off the main segment up to 6.6 m at 1.5 Mbps. Long stubs (1 m or more at 12 Mbps) act as unterminated transmission lines and cause intermittent framing errors. Avoid daisy-chaining off the side of a connector instead of running a continuous cable.
• Shield bonding. Profibus cable shields must be bonded to PE at every connector, at the cabinet entry, and at the field device. A shield that is continuous but not bonded at one end acts as an antenna and injects noise into the bus, which can present as "transmission rate mismatch" because the master's parameterization checksums intermittently fail.
• Repeaters count as a bus segment. A Profibus repeater (e.g., 6GK1711-1PE12-0AA0) regenerates the bus and starts a new segment. The terminator ON / OFF rules apply separately to each side of the repeater. A repeater is the only device that should have two terminators active — one on each segment it faces.
• GSD version drift. When you update the master's HW Config library or the TIA Portal hardware catalog, the slave GSD file is sometimes updated to a newer revision. Newer GSD files can change the default DP-V0/V1 setting. If a previously working slave suddenly reports "not available" after a software update, check the master's Profibus interface properties → "Operating mode" → "DP-V0/V1" toggle.
• CP 5611 versus CP 5621. The CP 5621 is the modern PCI Express successor to the CP 5611. The DIP switch convention is identical, but the CP 5621 ships with termination OFF by default. The CP 5611 ships with the DIP switch position undefined — verify the as-delivered state with a multimeter across pins 3 and 8 of the D-sub before commissioning.

Quick Reference: Termination and Baud Rate Matrix

Frequently Asked Questions