Programming Siemens CPU 315-2DP with a Damaged MPI Port

1. Problem Overview

The Siemens SIMATIC S7-300 CPU 315-2DP (order number 6ES7 315-2AG10-0AB0 and related variants) provides two physical communication interfaces on the front panel: an MPI/DP combined port on X1 and a dedicated PROFIBUS-DP port on X2. When the X1 interface fails due to mechanical damage, ESD, over-voltage on the connector, bent pins, or a burnt transceiver, the engineer loses the most common path for online programming. The PLC is otherwise fully functional, the program in work memory continues to execute, and the PROFIBUS-DP slave/master on X2 typically keeps running distributed I/O — yet STEP 7 cannot reach the station through the familiar 187.5 kbit/s MPI bus.

This reference documents the field-proven alternatives for uploading the project archive and downloading a corrected program when the MPI port on a CPU 315-2DP is no longer usable. The three primary recovery paths are: (a) the surviving PROFIBUS-DP port X2, (b) a CP 343-1 Ethernet communications processor if one is mounted in the rack, and (c) physical removal and re-insertion of the Micro Memory Card (MMC) between the CPU and a PG/PC equipped with an external MMC reader.

2. Affected Hardware and Interface Map

The CPU 315-2DP family spans several firmware releases; the interface layout is identical for all of them:

When X1 is unusable, X2 remains fully available for PG/OP communication as long as it is not permanently bound to a slave line that is already carrying cyclic traffic. Programming on the same physical wire as DP slaves is supported in STEP 7 — the PG is treated as an additional DP master that opens an acyclic connection to the CPU.

3. Path A — Use the Surviving PROFIBUS-DP Port (X2)

3.1 Why this works

The CPU 315-2DP can be addressed as a DP slave or as a programming node on its second interface. STEP 7, COM PROFIBUS, and the PC Adapter USB drivers all treat a 9-pin Sub-D PROFIBUS connector on either port as a valid PG attachment point, provided the bus is terminated and the PG can be added as a class-2 master. The diagnostic buffer entry you will see after a successful online attach over X2 is "Online: PG connection established via PROFIBUS interface X2".

3.2 Hardware required

• PC Adapter USB A2 (6ES7 972-0CB20-0XA0) — the standard USB-to-MPI/PROFIBUS converter used by service engineers. Older 6ES7 972-0CA10-0XA0 (PC Adapter USB) is electrically compatible but the newer A2 revision is recommended for noise immunity on long cables.
• PROFIBUS DP cable (6XV1 830-0EH10, purple, dual-shielded) cut to length, or a pre-terminated PG cable.
• PROFIBUS connector with PG socket (6GK1 500-0EA02 — 90° cable outlet, or 6GK1 500-0EB02 — 35° outlet). The PG socket is mandatory: it lets you tap the bus from the side of the connector while the trunk continues downstream to the slaves.
• Two PROFIBUS terminators (one at each end of the segment, switches ON in the connector).

3.3 Cabling topology

The PC Adapter USB sits between the laptop and the PG socket of the first DP connector; the trunk continues to the existing slaves with termination active on the last connector only.

3.4 STEP 7 configuration — Set PG/PC Interface

1. Close any open STEP 7 project that uses the MPI target.
2. Open Start → SIMATIC → STEP 7 → Set PG/PC Interface.
3. In the Interface Parameter Assignment Used dropdown, select PC Adapter (PROFIBUS) — not "PC Adapter Auto" and not "PC Adapter (MPI)". The Auto mode will default to MPI and fail against the X2 port.
4. Click Properties and set: Transmission rate = 1.5 Mbit/s (or 12 Mbit/s, 6 Mbit/s, 3 Mbit/s, 500 kbit/s, 187.5 kbit/s, 45.45 kbit/s) to match the DP line in the field. Mismatched baud rates are the single most common reason for "Bus fault — no response from station".
5. Confirm Highest station address = 126 and PG/PC is the only master on bus = ON when no other class-2 master is present.
6. Click OK, then Diagnostics to confirm the adapter LED is green.

3.5 Online attach

1. In SIMATIC Manager, choose PLC → Accessible Nodes.
2. The CPU appears with its DP address. For a CPU 315-2DP the default is 2; verify in the project hardware configuration if the bus address was changed.
3. Double-click the node → PLC → Upload Station to PG to retrieve the project archive from the PLC.
4. For downloading, open the project, place the CPU in STOP (or RUN-P if the change is non-critical), then PLC → Download.

4. Path B — Use a CP 343-1 Ethernet Processor

4.1 Prerequisites

If the S7-300 rack has a CP 343-1 (6GK7 343-1EX30-0XE0 for the basic version, 6GK7 343-1EX40-0XE0 for advanced IT functions) or a CP 343-1 Lean (6GK7 343-1CX00-0XE0) installed in slot 4 to 11, the PG can reach the station over TCP/IP using the ISO-on-TCP (RFC1006) transport. The CP 343-1 is a true alternative to MPI programming and is preferred on large plants where the PG is already on the office LAN.

4.2 STEP 7 configuration

1. Open Set PG/PC Interface and select TCP/IP → [your network adapter].
2. In the project, set the CP 343-1 IP address, subnet mask, and router in HW Config → CP343-1 → Properties → Ethernet Interface. Default address of an unconfigured CP is 0.0.0.0; STEP 7 will prompt for an initial IP assignment via MAC on first contact.
3. From SIMATIC Manager, use PLC → Accessible Nodes over the Ethernet adapter. If the project was built with the original IP, the CPU appears immediately. If the IP was forgotten, use PLC → Ethernet node → Browse (broadcast search on local subnet).

5. Path C — MMC Transfer

All CPU 31x-2 from firmware V2.0 onwards use a Micro Memory Card (MMC) for program and firmware storage. The MMC can be removed from the CPU while power is removed, read by a PG/PC equipped with a SIMATIC USB MMC reader (6ES7 764-0AA00-0AA0 or third-party), and written to offline using STEP 7. This path requires the CPU to be powered down for the duration of the card swap, which is acceptable during a planned outage.

5.1 Procedure

1. Power down the S7-300 rack.
2. Open the protective cover on the CPU front and eject the MMC by pressing the spring-loaded ejector.
3. Insert the MMC into the USB MMC reader connected to the PG.
4. In STEP 7, choose File → Memory Card → S7-Memory Card → Open to browse the card content. Project blocks appear in the offline container.
5. Save the project archive (File → Archive) before any modification.
6. Edit blocks, then choose PLC → Download to Memory Card. Verify the target card has at least the size indicated in the project (e.g. 4 MB MMC 6ES7 953-8LM20-0AA0).
7. Reinsert the MMC into the CPU, power up. The CPU will perform a memory reset and copy the contents of the MMC into work memory during startup.

6. Comparing the Three Paths

7. CP 5612 A2 vs. PC Adapter USB

The original question often comes from engineers deciding between two PC-side adapters:

• CP 5612 A2 (6GK1 561-2AA00) is a PCI card that gives a native PROFIBUS interface on the PG. It is faster (12 Mbit/s sustained) and is the preferred choice in commissioning racks and lab environments where the laptop has a spare PCI or PCIe slot. Driver support ends with STEP 7 V5.x; TIA Portal can still use it through the S7DOS / S7EPI driver.
• PC Adapter USB A2 (6ES7 972-0CB20-0XA0) is portable, uses the standard 6ES7 972-0CB20 USB cable family, and supports both MPI and PROFIBUS through a single hardware. The trade-off is throughput limited to 1.5 Mbit/s on PROFIBUS.

For the specific case of an emergency upload through the surviving DP port, either adapter works; the configuration in Set PG/PC Interface changes accordingly:

8. Field-Proven Tips

1. Disable any active PROFIBUS diagnostic repeater during the first attempt, so it does not mask echo problems on a single-segment line.
2. Set the PG PROFIBUS address to 0 (master) and the CPU X2 address to its configured value (often 2). Address clashes on the same segment will prevent token passing.
3. If "Accessible Nodes" returns nothing after 30 s, lower the baud rate (1.5 Mbit/s → 187.5 kbit/s). Many installed DP lines still run at the historical 1.5 Mbit/s because of older slaves.
4. Check the diagnostic buffer with a working PG of the same model — entry "Interface X1 fault, hardware defect" confirms a physical port failure and validates the alternate-path approach.
5. When uploading over PROFIBUS, the password-protected block protection remains in force; ask the customer for the project password before attempting online attach, or the upload will stop at the first CPU-relevant block.

9. Verification Checklist

• Accessible Nodes lists the CPU 315-2DP with the correct DP address.
• Online → Module Information → Diagnostic Buffer shows the most recent PG attach event.
• Upload Station to PG produces a project archive that opens without "unknown module type" warnings.
• Download completes, CPU transitions to RUN, output wiring responds to the new logic.
• CP 343-1 (if present) remains reachable over its configured IP — confirming the X1 damage is isolated and not a power-supply issue.

10. Long-Term Remediation

If the damaged MPI port is part of a recurring failure (loose connector, vibration environment, ESD-prone cabinet), a permanent hardware replacement is the correct answer:

• Replace the CPU with a refurbished 6ES7 315-2AG10-0AB0 or current equivalent 6ES7 315-2AH14-0AB0. Both retain identical X1/X2 pinouts.
• Add a CP 343-1 if not present; this gives a redundant programming path that survives another X1 failure.
• Install a PROFIBUS connector with strain relief and PG socket (6GK1 500-0EA02) to prevent re-occurrence of the mechanical damage.
• Apply a service-proven label next to X1 stating "Do not plug/unplug under power" to reduce future connector wear.

11. Troubleshooting Matrix

12. References Inside the SIMATIC Documentation

Engineers preparing the recovery should keep the following primary documentation on hand:

• S7-300 CPU 31xC and CPU 31x Manual (Edition 11/2018) — interface pinouts and diagnostic buffer for CPU 315-2DP.
• SIMATIC PC Adapter USB A2 Operating Instructions — installation of the USB driver, Set PG/PC Interface walkthrough.
• SIMATIC NET CP 343-1 Manual — Ethernet configuration and accessible nodes over TCP/IP.
• STEP 7 V5.5 Programming and Operating Manual — Accessible Nodes, Upload Station to PG, Download to Memory Card procedures.