S7-300 PROFIBUS PC Adapter: Resolving LAN Cable Connection Errors

Problem Details

An S7-300 system built around a CPU 315-2DP (typical catalog number 6ES7 315-2AH14-0AB0) must communicate with a PG/PC for programming, diagnostics, and online test work. The reported configuration attempts to bridge roughly 20 m or more between the PLC and the engineering station using:

• Two PROFIBUS bus connectors, 6ES7 972-0BB52-0XA0 (90° cable outlet, with PG socket, without programming device diagnostic LEDs).
• A PC Adapter, 6ES7 972-0CB20-0XA0 (USB PC Adapter, RS-232 on the PLC side; predecessor of the A2 generation).
• A standard Cat 5 / Cat 6 LAN patch cable cut and terminated directly into the PROFIBUS D-sub connectors, in place of a proper PROFIBUS DP cable.

Symptoms observed in this scenario include:

• Online > Accessible Nodes returns no partner or returns an MPI/PROFIBUS address mismatch.
• STEP 7 / TIA Portal reports "The interface is not accessible" or "No connection to the target module".
• Frequent bus faults (SF / BF on the CPU, BF flashing on the PC Adapter LED bar) when the cable length exceeds a few metres.
• Repeating "Error establishing connection to the target module" even though the Windows USB driver enumerates the PC Adapter.

The root cause is not the PC Adapter, the bus connector, or the CPU — it is the substitution of Ethernet twisted-pair cable for PROFIBUS DP cable across a 20 m run.

Root Cause Analysis

PROFIBUS DP and Ethernet 100BASE-TX share the same physical medium — a 100 Ω differential pair on RJ45 — but they are not electrically compatible at the segment level. Three physical-layer differences drive the failure.

1. Cable Construction Mismatch

PROFIBUS DP cable (violet, type A) per IEC 61158-2 / EN 50170 specifies a specific impedance, capacitance, and a tinned-copper braided shield with at least 60 % coverage. The reference cable type is:

• Impedance: 135–165 Ω at 3–20 MHz (typical 150 Ω).
• Capacitance: ≤ 30 pF/m.
• Loop resistance: ≤ 110 Ω/km.
• Shield: tinned copper braid, ≥ 60 % optical coverage.
• Conductor: solid 0.64 mm (22 AWG), stranded 0.34 mm² typical.

A Cat 5/6 LAN patch cable is specified at 100 Ω ± 15 Ω at 100 MHz. The impedance mismatch distorts the RS-485 edge transitions used by PROFIBUS, increasing jitter and ISI until the eye diagram collapses. The LAN cable also uses an aluminium/Mylar foil + drain wire shield (FTP/UTP) rather than a low-resistance braid, so high-frequency common-mode noise couples directly into the bus.

2. Shield Bonding and Ground Reference

Each 6ES7 972-0BB52-0XA0 bus connector exposes a shield clamp that must bite into the PROFIBUS cable braid and route the drain to the D-sub metal shell. Cutting an RJ45 and soldering the four used pairs into the connector bypasses the shield clamp entirely. The shield then either floats or makes contact through the drain wire only, defeating the EMC design that lets PROFIBUS survive in 24 V cabinet environments.

3. PC Adapter Power Path

The PC Adapter 6ES7 972-0CB20-0XA0 (and its successor 6ES7 972-0CB20-0XA0-class devices) draws its 24 V supply from pin 2 of the MPI/DP D-sub on the CPU side through the bus cable. When the cable is hand-wired from a LAN patch lead, the power pin may be left open or connected to the wrong conductor. The PC Adapter then boots but the RS-485 driver never enables, producing the "interface is initialising indefinitely" condition.

Specifying the Correct Cable and Limits

Use only PROFIBUS cable type A. Siemens sells it as 6XV1 830-0EH10 (standard), 6XV1 830-0EH10-0BB0 (trailing), and 6XV1 830-3FH10-0BB0 (festoon). At a minimum, any cable carrying 9.6 kbit/s to 12 Mbit/s PROFIBUS traffic must meet:

For the 20 m run described in the source, the constraint is not distance — it is the cable itself. With proper PROFIBUS DP cable, the segment runs cleanly at any baud rate the CPU 315-2DP supports (up to 12 Mbit/s).

Solution Path A: Stay on PROFIBUS, Replace the Cable

If the application must remain on PROFIBUS DP or MPI, this is the minimum-change path.

Parts List

Termination

Enable the terminating resistor in the bus connector at each end of the segment. On the 6ES7 972-0BB52-0XA0, this is the slide switch on the connector body — ON at the two physical ends, OFF on every connector in between. Never insert a terminator in the middle of a segment.

Wiring at the Connector

1. Strip 25 mm of the violet jacket.
2. Fold the braid back over the jacket; do not cut it off.
3. Insert the cable into the connector body so the braid is clamped by the shield clamp.
4. Strip the red and green core insulation by 6 mm.
5. Insert into the D-sub terminals: Pin 3 = B-line (red), Pin 8 = A-line (green).
6. Tighten the strain relief and verify the connector shell makes 360° contact with the braid.

Solution Path B: Move to Ethernet with a CP 343-1

If the 20 m run will grow, if the PC needs a permanent connection, or if the panel PC has no PROFIBUS card, switching the S7-300 onto Industrial Ethernet is the field-proven choice. This also removes the PC Adapter from the equation.

Required Hardware

Commissioning Steps

1. Insert the CP into a free slot of the S7-300 rack. Slot 4–11 are typical; consult the CPU hardware catalog for the exact PIC/Slot rule.
2. Configure the CP in STEP 7 HW Config (or TIA Portal device configuration). Assign an IP address (e.g., 192.168.0.10) and subnet mask (255.255.255.0).
3. Enable the S7 connection in the CP properties — only the S7 connection is needed for PG/PC programming over Ethernet.
4. Set the PC Ethernet adapter to the same subnet (e.g., 192.168.0.20).
5. In STEP 7, select TCP/IP > Network card as the PG/PC interface and create an S7 connection to the CPU.

Solution Path C: Keep the PC Adapter, Power It Externally

If the PC Adapter must remain and the LAN cable has already been installed, the PC Adapter can be powered from a separate 24 V source. This does not fix the cable impedance mismatch, but it lets the operator confirm whether the PC Adapter itself is functional before any rework.

1. Provide a regulated 24 V DC (± 5 %) supply at the PC Adapter's MPI connector (pin 2 = 24 V, pin 7 = 0 V on the PC Adapter's MPI D-sub, per the 6ES7 972-0CB20-0XA0 manual).
2. Limit cable length to under 5 m while diagnosing.
3. If the bus remains unstable, the cable itself must be replaced — there is no software workaround.

When to Upgrade the Adapter

The 6ES7 972-0CB20-0XA0 is the older USB A2 generation. The current USB version is the PC Adapter USB A2 with catalog number 6ES7 972-0CB20-0XA0 (variant with USB 2.0 isolation), and the higher-tier CP 5711 (6GK1 571-1AA00) provides USB-isolated MPI/PROFIBUS at 12 Mbit/s plus PROFINET. The CP 5711 is the recommended replacement when the bus will be used for production diagnostics rather than commissioning only.

Configuration of the CPU 315-2DP Interface

Before re-trying any of the three paths, verify the DP interface parameters on the CPU itself. From STEP 7 HW Config > CPU 315-2DP > Properties > MPI/DP interface:

Verification Procedure

After applying Solution Path A or B, run the following verification sequence. Each step gates the next.

1. Bus physical check. With power off, measure between pins 3 and 8 at each end. With termination ON at both ends, you should read 220 Ω ± 10 % (two 220 Ω terminators in parallel, equivalent 110 Ω across the bus). With termination ON at only one end, read 220 Ω. With termination OFF everywhere, read high impedance (kΩ range).
2. Shield continuity. Measure resistance between the D-sub shell at one end and the shell at the other end. Must be < 1 Ω. Then measure shell-to-PE at the cabinet — must also be < 1 Ω.
3. No shield loops. Confirm the shield is bonded to PE at exactly one point per cabinet, never at both ends of a long external run.
4. STEP 7 online test. Open Online > Accessible Nodes. The CPU 315-2DP must appear with its MPI/DP address and a green status dot. If the device shows with a yellow warning triangle, the baud-rate auto-detect has not converged — drop the baud rate and retry.
5. Diagnostic buffer. From the online CPU, open PLC > Diagnostic Buffer. The buffer should be free of "DP bus fault" and "station failure" entries after five minutes of operation.
6. Loopback test (PROFIBUS only). Temporarily insert a PROFIBUS DP terminator at the far end of the run, leave the near end unterminated, and check that the CPU 315-2DP BF LED goes dark. If it stays lit, the wiring is reversed on one end (Pin 3 ↔ Pin 8 swap).

Troubleshooting Matrix

Long-Distance and Future-Proof Considerations

For distances beyond 100 m on PROFIBUS, insert a PROFIBUS Repeater (6GK1 500-0AB10) or an OLM (Optical Link Module) pair for fibre conversion. For distances beyond 100 m on Ethernet, simply use fibre media converters — S7-300 with a CP 343-1 Advanced and an SCALANCE XC-200 switch gives 100 m copper plus up to 80 km on single-mode fibre with the right SFP.

Where the S7-300 must coexist with newer S7-1500 or ET 200SP stations, plan to migrate the CPU 315-2DP onto PROFINET — the DP interface on the 315-2DP is PROFIBUS only, and the migration path is to add a CP 343-1 and replace DP slaves with PROFINET devices over time.