S7-300 CPU 315-2DP Upload/Download Troubleshooting via MPI

Problem Overview

The Siemens SIMATIC S7-300 CPU 315-2DP is a workhorse PLC with two integrated interfaces: a combined MPI/PROFIBUS DP port (X1) and a dedicated PROFIBUS DP port (X2). When a field engineer connects a programming device (PG) or PC and STEP 7 reports "Upload/Download not done," or the "Accessible Nodes" window remains empty, the root cause almost always falls into one of four layers:

1. Physical layer — cable, connector, or PC adapter hardware failure.
2. Interface layer — PG/PC interface assignment, MPI address, or transmission rate mismatch.
3. CPU port layer — damaged MPI transceiver on the CPU front connector.
4. Host layer — file system corruption, low disk space, or driver/STEP 7 issue on the PG.

This reference walks the engineer through a deterministic isolation procedure for the CPU 315-2DP (order numbers 6ES7315-2AF03-0AB0, 6ES7315-2AF04-0AB0, 6ES7315-2AH14-0AB0) using the PC Adapter USB family, the original MPI cable approach, and STEP 7 V5.x / TIA Portal diagnostics.

CPU 315-2DP Hardware Identification

Before troubleshooting, confirm the exact variant. The MPI behavior and diagnostic buffer content differ between firmware releases.

The CPU front panel exposes two 9-pin Sub-D female connectors. By Siemens convention, X1 is the combined MPI/PROFIBUS DP master, and X2 is PROFIBUS DP only. Both have a terminating resistor switch and node-address rotary selectors next to the connector.

LED Diagnostics on the CPU 315-2DP

When the CPU is in STOP (typical state for upload-only situations), read the LED status from left to right across the front panel:

If SF is lit but the MPI link lights correctly, the upload/download path is healthy — the fault is in the user program or hardware configuration, not the cable. Record the diagnostic buffer (PLC → Diagnostic/Setting → Diagnostic Buffer in STEP 7) and search the Siemens SIMATIC Customer Support Knowledge Base by event ID before assuming a port failure.

PC Adapter LED Diagnostics

The Siemens PC Adapter USB (6ES7972-0CB20-0XA0) and PC Adapter USB A2 (6ES7972-0CB35-0XA0) each have three status LEDs that provide a fast health check without launching STEP 7:

Use this sequence:

1. Plug the adapter into the PC, install SIMATIC S7DOS USB driver from the STEP 7 / TIA Portal DVD or the Siemens S7DOS download page. The Power and USB LEDs should illuminate.
2. Connect the MPI cable to the CPU X1. The MPI/DP LED should light within one second.
3. If the MPI/DP LED stays dark, the X1 transceiver, the cable, or the PROFIBUS connector is the problem.
4. Swap the connector to X2 with the terminator OFF. If MPI/DP lights up on X2 but not X1, the X1 driver on the CPU is damaged — the CPU module must be replaced.

Cable and PROFIBUS Connector Verification

Field experience shows that ~70% of upload/download "not done" reports on the S7-300 family are caused by the PROFIBUS connector or the cable, not by STEP 7. Verify each link in the chain:

• Inspect the 9-pin Sub-D connector pins for bent or recessed pins. A 6-pin PROFIBUS connector (Siemens 6GK1500-0FC10) is the minimum: pins 3 (B-line), 8 (A-line), 5 (shield/ground), 6 (VP, +5 V terminated), 4 (RTS), 2 (GND).
• Measure between pin 3 and pin 8 with a multimeter. A healthy cable reads < 110 Ω (typically 95 Ω) looped through the terminators. Open circuit means a broken conductor.
• Check the 24 V terminator supply on pin 6 — it must read 5 V ± 5%. The CPU 315-2DP supplies terminator voltage only if its X1 switch is ON and the PLC is powered.
• Confirm shield continuity from connector to cabinet ground; a floating shield allows CM noise that can drop the link.

PG/PC Interface Configuration

The Set PG/PC Interface control panel is the single most common source of upload failures. The wrong interface point, wrong transmission rate, or wrong PC Adapter assignment produces exactly the symptoms described — STEP 7 sits for 10–30 s, then reports the node is not accessible.

1. Open Start → Control Panel → Set PG/PC Interface.
2. For PC Adapter USB A2, select PC Adapter (Auto) or PC Adapter (MPI). Avoid PC Adapter (PPI) — PPI is a S7-200 protocol, not valid for the S7-300.
3. Click Properties and set the Transmission Rate to 187.5 kbps (default) or the value configured in HW Config → Properties → MPI Interface of the CPU.
4. Set the Highest Station Address to 31 for a single-node download, or 126 for multi-node MPI.
5. Set the PC Adapter MPI Address to 0 (default, master-capable).
6. Confirm the COM/USB port matches the physical connection: USB for PC Adapter USB A2, COMx for legacy RS-232 MPI.

If the PC is running Windows 10 build 1903 or newer with the Microsoft USB Serial converter, the legacy PC Adapter (MPI) entry may fail even though Power and USB LEDs are on. Install the Siemens S7DOS64 driver and select PC Adapter (Auto) instead — it auto-detects both USB and serial variants.

MPI Address and Baud Rate

The default MPI address of the CPU 315-2DP is 2. STEP 7 broadcasts to addresses 1–31 by default. If the address was changed in HW Config or by a previous programmer, STEP 7's "Accessible Nodes" may return nothing.

To find a CPU with an unknown address, choose PLC → Edit Ethernet Node does not apply for MPI — use PLC → Show Accessible Nodes or scan all 31 addresses by reducing Highest Station Address in the PG/PC interface and watching the LED activity on the PC Adapter. If the CPU is isolated (terminators ON at both ends, no other nodes), the master PG will find it on address 2 in approximately 4 seconds.

STEP 7 / TIA Portal Software Issues

Upload failures sometimes surface as STEP 7 errors rather than a true communication timeout:

The CPU 315-2DP order number -2AF03 (V1.x) is not supported in TIA Portal V16+. To upload the program from a V1.x CPU you must use STEP 7 V5.5 SP4 or V5.6. The newer -2AH14 (V3.x) is fully supported by TIA Portal V15.1 onward. Confirm the firmware version from the front label before changing the toolchain.

Host-Side File System and Storage Errors

When "upload/download not done" is accompanied by a Windows file dialog, the problem has moved off the PLC bus and onto the PG's storage stack. Two Microsoft-documented failure modes match the symptom exactly:

1. Disk full — STEP 7 stores .s7p project archives in %USERPROFILE%\Documents\Siemens\Automation. If the drive has < 200 MB free, the upload truncates and reports failure. Free up space, as described in the Google Chrome disk-space guidance for general download errors and the corresponding Windows Disk Cleanup procedure.
2. Corrupted file system — interrupted writes to a project file produce a 0-byte block or a phantom handle. Run chkdsk C: /f /r per Microsoft Q&A: Cannot Complete a Download/Upload and the underlying disk-error KB article at Microsoft Support KB 929833.

Systematic Troubleshooting Flowchart

Use the following decision tree when "Upload/Download not done" or "No accessible nodes" is reported on a CPU 315-2DP:

Troubleshooting Matrix

Verification

After the suspected cause is repaired, perform the following verification sequence before returning the line to production:

1. Open STEP 7 / TIA Portal and run PLC → Display Accessible Nodes. The CPU 315-2DP must appear with its configured MPI address (default 2).
2. Perform a read-only upload (Upload Station to PG) and confirm the resulting project has a populated S7 Program folder with Blocks containing the expected OB/FB/DB.
3. Compare the uploaded project with the as-built documentation: the Block consistency check (PLC → Compare Blocks) must show no live differences.
4. Document the final PG/PC interface parameters in the maintenance log so the next engineer does not redo this exercise.

Frequently Asked Questions