CP 5711 PROFIBUS Connection Fails on S7-315-2DP: Troubleshooting

Problem Description

When using a Siemens CP 5711 USB adapter (order number 6GK1571-1AA00) to download or upload a STEP 7 project to an S7-300 CPU 315-2DP over the integrated PROFIBUS-DP interface (X2 / MPI/DP), the connection attempt rotates indefinitely in the STEP 7 "Establish Online Connection" dialog and never reports an online partner. The same CP 5711 connects to the same CPU without error when the project is set to the MPI/DP port's MPI mode, isolating the fault to the PROFIBUS path rather than the hardware or driver.

The root cause is almost always one of the following:

1. PROFIBUS is not activated in the offline project (no DP master system configured on the CPU's DP interface) — the most common cause.
2. The CP 5711 is configured to operate as the only bus master and the real DP master is also online.
3. The PROFIBUS bus parameters (baud rate, bus profile, highest station address) in STEP 7 disagree with the online network.
4. Termination or cable polarity is wrong, leaving the CP 5711 outside the segment.
5. USB driver, COM port assignment, or PG/PC interface selection is wrong.

CP 5711 Hardware and Firmware Reference

Reference data for the CP 5711 Operating Instructions (Siemens):

Prerequisites for a Healthy CP 5711 PROFIBUS Link

• CP 5711 driver installed; a new COM port is visible in Windows Device Manager under "Ports (COM & LPT)" — note the number (commonly COM8..COM20).
• PG/PC interface set to CP5711 (USB) -> PROFIBUS in Set PG/PC Interface.
• STEP 7 V5.5 SP2 or TIA Portal V13+ with CP 5711 HSP installed.
• PROFIBUS cable from CP 5711 D-sub to the S7-300 9-pin D-sub (X2). A standard PROFIBUS connector with PG socket (6ES7972-0BB12-0XA0 or the older 6ES7972-0BA12-0XA0) on the CPU side and a 9-pin male-to-male cable to the CP 5711 is required. Straight-through 1:1 wiring — pin 3 = B (red), pin 8 = A (green), shield on shell.
• Termination active only at the two physical ends of the PROFIBUS segment. The CP 5711 must therefore be a bus terminator (i.e. at one end) or installed as a drop line to a bus where the two ends are already terminated — not in the middle of a terminated segment.
• 24 V to the S7-300 PS 307, CPU in RUN or STOP (STOP is sufficient for download).

Diagnostic Procedure: CP 5711 → S7-315-2DP (PROFIBUS)

Work the steps in order. Each step is a gate — do not proceed to the next until the current one passes.

Step 1 — Verify CP 5711 enumeration on USB

1. Open Windows Device Manager → Ports (COM & LPT). The entry "CP5711 (USB)" or similar must appear with a COM number.
2. If the entry shows a yellow exclamation mark, re-install the driver from the STEP 7 DVD (folder CP5711\Driver) or the latest Siemens CP 5711 documentation package.
3. Unplug the CP 5711, plug it into a different USB port (avoid unpowered hubs), and observe the LED: solid green = ready, off = no USB power, red = driver / firmware fault.

Step 2 — Configure the PG/PC interface correctly

1. Start → SIMATIC → STEP 7 → Set PG/PC Interface (or Control Panel → PG/PC Interface).
2. Access point of the application: S7ONLINE (STEP 7) (or CP5711.PROFIBUS.1 if you created a dedicated access point).
3. Interface parameterization: CP5711 (USB) – PROFIBUS. Click Properties and confirm:
   • Station address: 0 (PG) or 1 — the CP 5711 is a master and must have a unique address on the segment.
   • Profile: DP.
   • Transmission speed: Auto (or fixed at 1.5 Mbps to start — auto-detect requires a clean, terminated segment).
4. Click Diagnostics in the CP 5711 Properties dialog. The window offers two tests: Test (driver / USB / CP internal) and Read (live PROFIBUS bus scan). Run Test first — it must report OK. If it fails, do not run Read; fix the driver / cable / port first.
5. Run Read. A list of all reachable PROFIBUS nodes is shown with their baud rate and station address. The S7-315-2DP (default address 2, default baud 1.5 Mbps) must appear. If it does not, the fault is at the bus level, not the project level — skip to Step 6.

Step 3 — Confirm PROFIBUS is active in the offline project

This is the single most common cause of an "it rotates forever" PROFIBUS connection with a CPU 315-2DP.

1. In SIMATIC Manager, open the S7 project → Hardware (HW Config).
2. Double-click the CPU 315-2DP → tab DP (or PROFIBUS interface). Confirm:

• Interface is set to PROFIBUS, not MPI.
• A DP master system is configured and at least the CPU itself is a DP master on it.
• An address exists in the master system (the CPU's own DP address, default 2).

If the interface shows MPI, change it to PROFIBUS, give it a station address (e.g. 2), and save / compile the project. Re-download to the CPU (use the working MPI port for this one download, since PROFIBUS still cannot connect).

Step 4 — Re-attempt "Establish Online Connection" over PROFIBUS

1. In SIMATIC Manager, select the S7 station → PLC → Establish Online Connection.
2. In the dialog choose PG/PC interface: CP5711 (USB) – PROFIBUS, target station address = CPU's DP address (default 2), target slot = 2 (for the CPU rack slot 2).
3. Click OK. The "rotating wheel" should clear within 5–10 s and the online view should open.

If it still rotates, the offline project's DP master system parameters (HSA, baud rate, Tslot, watchdog) do not match the live bus. The PG/PC interface's Read test in Step 2 only reports active devices; it does not flag parameter mismatches. Move to Step 5.

Step 5 — Match bus parameters between PG and CPU

1. Open the DP master system → Properties → PROFIBUS in HW Config. Note the baud rate, the highest station address (HSA, default 126), and the bus profile.
2. Open the CP 5711 PG/PC properties (Step 2) and force the same baud rate and HSA. Auto detection is fragile on segments with mixed devices or reflection problems — pin both sides to a fixed 1.5 Mbps for diagnosis.
3. Save, exit, retry the online connection.

Step 6 — Physical layer checks (cable, termination, addresses)

1. CP 5711 segment position: confirm it is a terminator or on a drop line. End termination must be ON only at the two ends of the segment (resistor in the PROFIBUS connector visible between pins 3 and 8). Disable termination on every intermediate node.
2. Cable polarity: A (green) → pin 8, B (red) → pin 3. The CP 5711 end is a standard 9-pin male; the S7-300 end is the PROFIBUS connector with PG socket. Reverse polarity yields activity on the bus (PG/PC Read still shows devices) but no master handshake — easy to mistake for a project fault.
3. Station address uniqueness: every device on the segment must have a unique address. The CPU default is 2, the CP 5711 PG address is set in Step 2. If two devices share 2, the CP 5711 Read test still passes but Establish Online Connection rotates forever.
4. Disconnect all other DP slaves and try a CPU-only segment. If the connection works, a downstream slave is duplicating an address or holding the bus.

Step 7 — "PG/PC is the only master on the bus" option

In some STEP 7 / TIA Portal builds, the CP 5711 PG/PC parameter dialog has a checkbox PG/PC is the only master on the bus. If the S7-315-2DP's DP master system has another PROFIBUS master (e.g. an HMI, a PLC-CPU acting as master, or a CP 343-1 CP on the same bus), and this checkbox is enabled, the CP 5711 will refuse to take the token and the online connection dialog rotates indefinitely.

Workaround: deselect PG/PC is the only master on the bus, especially when the CPU is itself the only DP master. If there really is a second master on the same segment, the bus topology must be split or the CP 5711 used only as a passive diagnostic observer (read-only).

Verification

Successful commissioning is demonstrated by the following three checks:

1. CP 5711 LED: solid green during the active PROFIBUS session, returning to solid green when disconnected.
2. SIMATIC Manager → PLC → Establish Online Connection: connects within 5 s, online view shows the CPU as reachable, online/offline comparison becomes available.
3. PG/PC interface Diagnostics → Read: S7-300 CPU 315-2DP is visible with its configured DP station address and the bus baud rate is reported correctly.

Quick-Reference Troubleshooting Matrix

Field-Notes and Caveats

• A connection that partially works — i.e. the CP 5711 Read test shows devices but Establish Online Connection hangs — is almost always a bus parameter mismatch, not a hardware fault. Pin both ends to a fixed baud rate (1.5 Mbps is the most robust on short PG/PC segments).
• The CP 5711 draws its power from the USB port. On underpowered hubs or long USB cables, enumeration succeeds but the PROFIBUS transceiver can lose sync, producing intermittent online connections. Use a direct port or a powered hub.
• The PROFIBUS connector on the S7-300 side must have its DP termination ON if the CPU is the end of the segment. The internal S7-300 bus terminator inside the CPU does not exist; termination lives in the connector.
• If the project has not had the CPU's DP interface enabled, downloading over MPI with PROFIBUS still unconfigured is the only way to recover — you cannot use the failing PROFIBUS path to deliver the fix.
• STEP 7 V5.5 SP2 and later recognise the CP 5711 natively. Earlier service packs may list it as "unknown" in the PG/PC interface; install SP2 or later before troubleshooting further.
• TIA Portal users must install the CP 5711 Hardware Support Package (HSP) and select PC Interface → CP5711 PROFIBUS in the project tree. The HSP is bundled with TIA Portal updates or available from the Siemens support portal.

FAQ