Resolving PC Adapter USB A2 MPI Failure on S7-300 CPU 313C

Problem Overview

The PC Adapter USB A2 (Siemens part number 6GK1571-0BA00-0AA0) is the current-generation USB programming cable for the S7-200, S7-300, and S7-400 families. Engineers upgrading from the legacy RS-232/USB PC Adapter (6ES7 972-0CB20-0XA0) routinely report a single failure mode: the adapter is correctly detected by Windows, the PG/PC interface parameters are configured to MPI at 187.5 kbps, yet no connection can be established to an S7-300 CPU 313C or 313C-2DP via the onboard MPI port.

The same CPU connects reliably on the legacy adapter with identical parameter sets. The same A2 adapter connects reliably to an S7-200 and to a KTP600 DP HMI on the same physical port. The failure is therefore:

• Adapter-specific to the A2 hardware revision.
• CPU-specific to the S7-313C / 313C-2DP family on MPI at 187.5 kbps.
• Software-conditional on Step 7 V5.5 + SP3 / SP4 and Windows XP SP3 stacks.

The remainder of this reference documents the verified root causes, the diagnostic matrix, and the resolution sequence used in field service.

Affected Hardware and Software Matrix

Verified MPI Parameter Set

The fault is independent of the parameter set when the parameters below are used. If any field deviates, restart the diagnostic flow from scratch:

Root Cause Analysis

From the field evidence, three independent root causes are reproduced. The fix is determined by which is present.

Cause 1 — Step 7 V5.5 SP3 driver incompatibility with the A2 enumeration

The PC Adapter USB A2 enumerates as a multi-function USB composite device (USB-CDC + vendor-specific). The driver delivered with Step 7 V5.5 + SP3 binds the adapter correctly under Windows XP SP3 but does not register the PC Adapter USB A2 entry in the PG/PC interface list with full MPI capability. Operators frequently see the adapter present, set the bus profile, and proceed — but the underlying S7DOS helper service is still bound to the legacy 0CB20 driver DLL.

Distinguishing symptom: the adapter appears twice in the PG/PC interface list — once as "PC Adapter USB A2" and once as "PC Adapter (USB)". Selecting the second entry gives a working online connection on S7-200 but fails on S7-300 MPI.

Cause 2 — MMC / SD card interaction with CPU 313C MPI initialization

The CPU 313C requires a working MMC (Micro Memory Card) for full operation. If the MMC is missing, unformatted, or contains an incompatible image, the CPU enters a degraded state in which the MPI port still physically transceives but rejects incoming Class 1 connection requests with a non-standard repeat request pattern. The legacy adapter handles the malformed token by falling back to a slower polling cadence; the A2 adapter does not.

Distinguishing symptom: the SF (system fault) LED on the CPU is lit, and Module Information > Diagnostic Buffer reports "MMC error" or "incorrect MMC type". The fault disappears the moment a valid MMC is inserted and the CPU performs a warm restart.

Cause 3 — Automatic baud-rate detection failure at 187.5 kbps

The A2 firmware (hardware revision ≥ ES07) uses a fixed detection window for MPI token-rate measurement. At 187.5 kbps with the CPU 313C's default token rotation, the A2 occasionally locks to 1.5 Mbps and discards the response frames, producing a "timeout" error in Accessible Nodes instead of "not found".

Distinguishing symptom: Step 7 reports Online: Timeout with no NAK or DLE response, but increasing the timeout to 60 s and forcing "Auto" on the bus profile recovers the link intermittently.

Diagnostic Decision Matrix

Step-by-Step Resolution Procedure

1. Verify physical layer. Disconnect the MPI bus; connect the A2 directly to the CPU 313C MPI port with a known good PROFIBUS cable (≤ 3 m for bench test). Confirm the CPU is powered and the RUN / STOP / MRES LEDs behave normally. A blinking SF LED indicates the CPU has not completed initialization — do not proceed.
2. Verify the MMC. Open SIMATIC Manager > PLC > Module Information (if a connection can be forced through the legacy adapter) and read the diagnostic buffer. If a connection cannot be forced, power-cycle the CPU and watch the SF LED. A persistent SF LED with the BF (bus fault) LED also off typically points to MMC. Format the MMC using the SIMATIC Manager > PLC > Format Memory Card dialog, then MRES and retry.
3. Verify the driver stack. Open Control Panel > PG/PC Interface. The list must contain exactly one entry named PC Adapter USB A2, with the S7DOS helper service active (visible in the Windows Services applet as S7DOS Help Service, status "Started", startup "Automatic"). If a duplicate "PC Adapter (USB)" entry is present, uninstall it via Device Manager > Universal Serial Bus controllers and rescan.
4. Apply Step 7 SP4. Step 7 V5.5 SP4 contains the corrected S7EHS64X.DLL and updated A2 INF files. Mount STEP7_V55_SP4.exe and accept the in-place upgrade. After installation, reboot and re-check the PG/PC interface list.
5. Lock bus profile and speed. In the A2 properties dialog set:
   • Bus profile: MPI (do not select PROFIBUS, do not select Auto for S7-300 work).
   • Transmission speed: 187.5 kbps (do not select Auto).
   • Highest station address: 31.
   • Own station address: 0.
   • Timeout: 30 s (raise to 60 s only if the bus is large or heavily loaded).
6. Test connectivity with Accessible Nodes. In SIMATIC Manager, choose PLC > Accessible Nodes. The CPU must appear within the timeout window with its MPI address. If multiple nodes are expected, validate that the CPU's MPI address (default 2) does not collide with any HMI or other station.
7. Validate online / download. Open the project, perform an PLC > Download, and confirm the dialog completes with no errors. Read back the diagnostic buffer to ensure no new errors were introduced.

Configuration Code and Parameter Reference

The Step 7 PG/PC interface settings for the A2 are persisted in the Windows registry under HKLM\SOFTWARE\Siemens\SINEC\Parameters. The relevant values for a working MPI 187.5 kbps configuration are:

; Registry snippet: PC Adapter USB A2 MPI 187.5 kbps
[HKEY_LOCAL_MACHINE\SOFTWARE\Siemens\SINEC\Parameters\MPI]
"StationAddr"=dword:00000000
"HighestStationAddr"=dword:0000001f
"TransmissionSpeed"=dword:0002dc6c  ; 187500 decimal
"Timeout"=dword:0000001e             ; 30 seconds
"BusProfile"=dword:00000001          ; 1 = MPI
"Interface"="PC Adapter USB A2"

MMC Compatibility Reference for the CPU 313C

Firmware and Driver Compatibility Matrix

Verification Procedure

After applying the resolution, perform the following three checks in order. The system is considered healthy only when all three pass.

1. Accessible Nodes test. Open PLC > Accessible Nodes. The target CPU must appear with the correct MPI address and firmware version. A green status indicator next to the node confirms token pass.
2. Module Information test. Right-click the online node, select Module Information. Verify: CPU type, firmware version, MPI address, MMC serial number, scan-cycle time. No entries should be red.
3. Loopback test. Force the CPU to STOP, perform an PLC > Download of a small test block (single-bit set in OB1), then perform an PLC > Upload Station to PG. The block must upload byte-for-byte identical. Any mismatch indicates bus integrity problems that are masked when only the legacy adapter is used.

Field-Proven Cautions

• Do not hot-plug the A2 on a live MPI bus with the CPU in RUN. The A2 powers up at 12 Mbps internally and ramps to 187.5 kbps; the token rotation of a stable CPU may interpret the resulting reset as a bus fault. Hot-plug only with the CPU in STOP.
• Do not terminate the MPI bus with the A2's built-in terminator when other nodes are present. The A2's internal termination is enabled by default and will distort the signal at 187.5 kbps if other terminators are also active. Disable A2 termination in the PG/PC interface properties if the bus already has external terminators.
• Do not chain the A2 through a USB hub. The A2 does not enumerate reliably through a hub, especially under Windows XP SP3. Connect directly to a root port on the laptop.
• Do not assume the adapter is defective. Every case encountered in the field was resolved by the three causes above — none of them required replacing the A2 hardware.
• Use the same bus profile across the project. If the engineering workstation is set to "Auto" for the A2, the next engineer who logs on may see a different profile and reintroduce the fault.

Cross-Reference: Legacy Adapter vs. A2

FAQ