CP 5611 A2 BF1 Error with S7-300 CPU 315-2PN/DP: Resolving "WRONG TRANSMISSION RATE or SHORT CIRCUIT"

Problem Overview

When a SIMATIC CP 5611 A2 PROFIBUS/MPI PCI card (6GK1561-1AA01) is connected to the integrated MPI/DP port of an S7-300 CPU 315-2PN/DP, the CPU's bus-fault LED BF1 illuminates solid and the STEP 7 "Set PG/PC Interface" dialog raises the message "WRONG TRANSMISSION RATE or SHORT CIRCUIT". Online access through the S7ONLINE access point fails, the "Accessible Nodes" list is empty, and HW Config download aborts. Communication with the same CP card and STEP 7 installation works on an S7-200 CPU on the same physical port, which isolates the issue to the bus medium, baud rate, or termination rather than the PCI card itself.

Typical reported symptom set:

• BF1 LED on the CPU 315-2PN/DP remains steady-on after the cable is attached.
• STEP 7 SIMATIC Manager → Options → Set PG/PC Interface → Diagnostics on CP5611.A2 reports "WRONG TRANSMISSION RATE or SHORT CIRCUIT".
• The same CP 5611 A2 communicates normally with a SIMATIC S7-200 (MPI/PPI port).
• Download to the CPU succeeds when the laptop is connected through a USB PC Adapter (USB-MPI, 6ES7972-0CB20-0XA0) at 187.5 kbps.
• Replacing the connecting cable eliminates the fault.

Affected Hardware and Part Numbers

The following Siemens catalog numbers are referenced throughout this guide. Verify the exact MLFB on the device label before ordering replacements.

Root Cause Analysis: Cable Baud-Rate Limitation

Siemens publishes hard maximum baud-rate specifications for the two cable families. The gray RS 485 MPI cable (6XV1 750-1A..) was specified in the SIMATIC NET cabling manual for operation up to 187.5 kbps. Above that rate, signal integrity cannot be guaranteed and the CP 5611 A2 firmware reports the bus as faulty, even though the LED on the connector may not show a hard short.

The CP 5611 A2's PROFIBUS firmware performs a bus check on connect. If the segment does not return a clean echo, the master concludes either a short circuit or a baud-rate mismatch (the receiver cannot decode frames at the assumed bit time) and surfaces the "WRONG TRANSMISSION RATE or SHORT CIRCUIT" message. Two situations are commonly mistaken for one another:

1. The PLC has been configured (or the project was re-loaded) with a PROFIBUS master/slave profile that runs at 1.5 Mbps, 3 Mbps, 6 Mbps, or 12 Mbps. The gray cable cannot sustain these rates.
2. The PLC's MPI/DP port has been switched to a non-default PROFIBUS profile (e.g., for an ET 200S slave or a CP 342-5) so that the active baud rate is no longer 187.5 kbps.

That an S7-200 on the same CP 5611 A2 works is consistent with the diagnosis: the S7-200's PPI/MPI port auto-negotiates a low baud rate, and many S7-200 firmware versions remain at 9.6 kbps or 19.2 kbps by default. The gray cable handles those rates without difficulty. The S7-300 CPU 315-2PN/DP defaults to 187.5 kbps on MPI, but STEP 7 project configurations or earlier downloads commonly raise that to 1.5 Mbps PROFIBUS, which exceeds the cable.

MPI Cable vs PROFIBUS Cable Comparison

Siemens published a SIMATIC NET PROFIBUS Wiring Guidelines entry that explicitly retires the gray MPI cable for new installations. Any new point-to-point PC-to-PLC link that must run at more than 187.5 kbps requires the violet PROFIBUS cable with a 150 Ω characteristic impedance, and a PROFIBUS D-sub connector with the integrated 390 Ω / 220 Ω / 390 Ω termination network on the segment ends.

CP 5611 A2 Configuration in PG/PC Interface

The CP 5611 A2 reports the fault only after the PG/PC interface assignment, the bus profile, and the slot PCI resources are correct. Verify the following before condemning the cable.

1. Open Control Panel → Set PG/PC Interface (or in STEP 7: Options → Set PG/PC Interface).
2. Select S7ONLINE (STEP 7) → CP5611.A2 (PROFIBUS - Active) as the access point. If the entry shows a yellow triangle, the SIMATIC NET driver is not installed or the card is not recognized; reinstall the SIMATIC NET PC software package and verify in Device Manager.
3. Click Properties → PROFIBUS:
   • Transmission rate: set to 187.5 kbps for first connection attempt, or use Auto if the S7-300 baud rate is unknown.
   • Highest station address (HSA): 126 default; do not reduce below the configured nodes.
   • Profile: DP (standard) for PROFIBUS or MPI standard. Avoid "User-defined" unless the bus parameters were intentionally created.
4. Click Test. A successful test shows all "OK". A failure shows either "Bus fault" or the same "WRONG TRANSMISSION RATE or SHORT CIRCUIT" message.
5. If the test passes at 187.5 kbps but fails at higher rates, the cable or termination is the fault. Proceed to the resolution procedure below.

The CP5611 (AUTO) option is particularly useful when the baud rate on the PLC is not known. AUTO runs the entire PROFIBUS baud range (9.6 kbps through 12 Mbps) and locks on the first reply. It requires a healthy cable; on a degraded gray cable it will time out at every step and eventually present the same WRONG TRANSMISSION RATE error.

S7-300 CPU 315-2PN/DP MPI/DP Port Configuration

The integrated MPI/DP port (X1) on a CPU 315-2EH14 can be either an MPI node or a PROFIBUS DP master or slave, depending on the loaded hardware configuration. Confirm the active profile before changing cable:

1. In STEP 7 SIMATIC Manager, open the project that was last downloaded to the CPU.
2. Open HW Config, double-click the CPU 315-2PN/DP, select the Interface tab → MPI/DP Properties.
3. Read the Transmission rate and the Interface type (MPI / PROFIBUS).
4. If the interface is set to PROFIBUS, note the master/slave role and the configured bus parameters. A CPU running as a PROFIBUS master typically runs at 1.5 Mbps; a CPU running as MPI typically runs at 187.5 kbps.
5. Cross-check against the CPU diagnostic buffer (PLC → Diagnostic/Setting → Diagnostic Buffer). Look for entries such as "Bus fault", "Station failure", or "Parameter assignment error". A BF1 LED combined with no parameter-assignment fault and a station-failure entry on address 0 (the CP 5611 A2 as master) points to a physical-layer issue.

If the original STEP 7 project is not available, use the USB PC Adapter to read the CPU type and firmware version, and to upload the project from the PLC. The USB-MPI adapter reliably handles the gray cable because it auto-bauds and does not enforce strict RS 485 edge timing.

PROFIBUS Termination Requirements

A PROFIBUS DP segment requires a bus terminator at the first and last physical node, never in the middle. The terminator is a 220 Ω resistor between the A and B lines in series with a 390 Ω pull-up on B (to +5 V) and a 390 Ω pull-down on A (to GND), forming the canonical 390-220-390 bias network that biases the idle bus to a defined logic level.

For a point-to-point PC-to-CPU link the rules are:

• The PROFIBUS connector at the PC (CP 5611 A2) must be set to ON (terminating resistor engaged) and no other node connected.
• The PROFIBUS connector at the S7-300 must be set to ON as well; it is the last node on the segment.
• If a third node (HMI panel, ET 200 station) is in the middle, its terminator must be OFF.

With a gray MPI cable, a D-sub connector is often used without an integrated switchable terminator. The lack of proper bias resistors on both ends of the segment is itself a failure mode at PROFIBUS speeds. The violet PROFIBUS cable and a 6ES7972-0BA12 PROFIBUS connector provide the proper bias and switchable termination by sliding a switch on the connector body.

Termination verification procedure:

1. Power down both nodes.
2. Measure resistance between pins 3 (B) and 8 (A) of the D-sub at each end of the segment with the termination switch ON. Expected reading: ~220 Ω (a single terminator engaged) or ~110 Ω (two terminators, one at each end).
3. Measure resistance from pin 3 to +5 V and pin 8 to GND. Expected: ~390 Ω each, only when the segment power is off and the connector is the only source of bias on the segment.
4. Any reading close to 0 Ω between A and B indicates a wiring short. Any reading open when the switch is ON indicates a damaged connector or an open terminator resistor.

Step-by-Step Resolution Procedure

Follow the sequence in order. Each step is independently verifiable; do not skip ahead.

1. Verify the PLC is healthy. Connect the USB PC Adapter (6ES7972-0CB20-0XA0) to the MPI/DP port of the CPU 315-2PN/DP. Open STEP 7, set PG/PC interface to PC Adapter (MPI) → Auto, run PLC → Accessible Nodes. The CPU should appear with its MPI address (default 2) and firmware version. If this fails, the fault is the CPU, not the cable.
2. Verify the CP 5611 A2 is healthy. Move the CP 5611 A2 to the S7-200 MPI/PPI port and run the test from step 1 with the same laptop. If it works, the PCI card and driver are confirmed good.
3. Replace the gray MPI cable with a violet PROFIBUS cable. Use a pre-assembled PROFIBUS cable (6XV1830-0AN.. for short lengths, 6XV1830-0BH.. for up to 50 m, or a custom cut using 6XV1830-0EH10) terminated with 6ES7972-0BA12-0XA0 PROFIBUS connectors on both ends.
4. Set termination at both ends to ON. Slide the termination switch on each PROFIBUS connector to ON. There is no powered device between PC and CPU in a point-to-point link, so both ends of the segment are the only devices and both must be terminated.
5. Configure PG/PC Interface for the corrected cable. Set S7ONLINE → CP5611.A2 (PROFIBUS - Active), transmission rate 187.5 kbps (or Auto for first contact). Click Test. Expect a green OK.
6. Establish online. In SIMATIC Manager select PLC → Accessible Nodes. The CPU 315-2PN/DP must appear with its PROFIBUS address. The BF1 LED must extinguish once the connection is up.
7. Download the project if needed. PLC → Download the HW Config and the user program. After download, the configured PROFIBUS baud rate takes effect; re-verify the cable at that rate (it will, by construction, because the violet cable is rated to 12 Mbps).

Verification and Diagnostics

After applying the resolution, run the following checks. A pass on all four is required to confirm the fix is permanent.

Troubleshooting Matrix

Field Commissioning Checklist

Use this list when wiring a new PC-to-S7-300 segment with a CP 5611 A2:

• Use only 6XV1830-0EH10 cable (violet, 150 Ω). Cut to length and strip back exactly the dimension shown in the connector data sheet (typically 25 mm of jacket, 5 mm of shield drain, 11 mm of conductor).
• Use 6ES7972-0BA12-0XA0 (90° cable outlet) or 6ES7972-0BB12-0XA0 (35° outlet) PROFIBUS connectors. Older 6ES7972-0BA10 variants require an external 5 V termination source; avoid them on a CP 5611 A2 segment.
• Verify the CP 5611 A2 PCI card sits firmly in a 32-bit, 33 MHz PCI slot. PCI-X and PCIe slots require the CP 5611 A3 (6GK1561-1AA02) or CP 5621 (6GK1562-1AA00); see SIMATIC NET CP 5611 A2 release notes for compatibility matrix.
• Set S7ONLINE access point to CP5611.A2 (PROFIBUS - Active). Do not use CP5611 (MPI) on a segment that is configured as PROFIBUS.
• Set the PROFIBUS transmission rate to match the CPU configuration. If unknown, use 187.5 kbps for first contact then Auto for re-scan.
• Terminate the segment at both ends. No terminator in the middle.
• Document the cable route, segment length, connector type, and station addresses on the cabinet drawing.

Related Diagnostics and Edge Cases

Several related conditions can re-introduce the same BF1 / WRONG TRANSMISSION RATE pattern after the cable is corrected. The following are the most common ones encountered on retrofits:

• PROFIBUS address conflict. If the CP 5611 A2 is set to PROFIBUS address 2 and the CPU is also at address 2, the master sees an echo from itself and aborts. Assign the CP 5611 A2 to address 0 (default master) or 1, and the CPU to a unique address on the segment.
• Duplicate segment powered from two terminators. Some PROFINET/PROFIBUS gateways supply their own bus termination. If the gateway is in the middle of a segment, its internal terminator must be disabled (most have a switch or a DIL switch labeled "Term on/off").
• Firmware mismatch on the CP 5611 A2. The card's firmware (loaded at SIMATIC NET install) must match the SIMATIC NET PC software version. A firmware-version warning in Set PG/PC Interface → Diagnostics should be cleared by reflashing the firmware from the current SIMATIC NET DVD.
• STEP 7 version vs. SIMATIC NET version. STEP 7 V5.5 SP4 and SIMATIC NET V12 (or later) is the supported pairing for Windows 7/10. Mixed versions can cause the S7ONLINE access point to be missing entirely; reinstall in the correct order: SIMATIC NET first, then STEP 7.
• PROFIBUS diagnostic repeater in series. If a 6ES7972-4AA02-0AA0 PROFIBUS diagnostic repeater is in the line, set its segment speed manually to match the master. A repeater left in "Auto" can lock to 45.45 kbps on power-up and refuse to up-shift.

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