Resolving PC-Adapter USB Power Issue with S7-400 CPU 412-2 PCI in a PC Box 620

This reference consolidates the field-proven workarounds for one of the most common first-time-commissioning problems on slot PLCs: the Siemens PC-Adapter USB (MPI/DP) cable appears dead (no LED activity) when connected to an S7-400 CPU 412-2 PCI embedded in a SIMATIC PC Box 620. The root cause is platform-specific power-supply behavior on the MPI/DP interface, and there are three clean engineering remedies. Each is documented below with pinout detail, hardware catalog numbers, STEP 7 V5.4 PG/PC-interface configuration, and a verification procedure.

1. Problem Description

The reported symptom is reproducible on first power-up or after a CPU battery / memory-card replacement:

• CPU 412-2 PCI is correctly seated in the PCI slot of the SIMATIC PC Box 620.
• CPU is in RUN or STOP (status LEDs normal, no SF/BF faults).
• PC-Adapter USB (catalog 6ES7 972-0CB20-0XA0) is plugged into the CPU's integrated MPI/DP port.
• The opposite end is connected to the service laptop's USB port.
• Result: PC-Adapter USB does not show the active LED, and STEP 7 V5.4 Accessible Nodes / PLC > Upload Station to PG fails with timeout.

The same PC-Adapter USB has been validated on an S7-300 CPU and is known good. This confirms the issue is not cable or driver related.

2. Affected Hardware & Software Stack

3. Root Cause: MPI/DP Power-Supply Behavior, S7-300 vs. S7-400

The Siemens PC-Adapter USB is a bus-powered device at the MPI/DP end. It draws the 24 V it needs for its RS-485 transceiver and on-board logic directly from the PLC's MPI connector on pin 2 (M24V) and pin 7 (P24V). This is convenient on most S7-300 CPUs (CPU 31x, CPU 31xC, CPU 31x-2 DP/PN) where 24 V is sourced onto the bus connector.

The S7-400 family — including the slot CPU 412-2 PCI used inside the PC Box 620 — does not feed 24 V onto pins 2/7 of its MPI/DP connector. The connector is galvanically isolated, and the bus is intended to be powered from an external termination / repeater or from a downstream station. With no 24 V present at the connector, the PC-Adapter USB has no supply rail, the LED stays dark, and STEP 7 never sees the bus rise.

3.1 Why the Same Cable Worked on S7-300

S7-300 MPI connectors are wired such that 24 V from the CPU's backplane is passed straight to pins 2/7. A PC-Adapter USB senses this rail and starts up. On the S7-400 (including the slot PCI variant), the connector is decoupled; the 24 V rail is present on the backplane but not at the front MPI/DP pins. This is the only behavioral difference a service technician needs to remember.

4. Solution A: Inject External 24 V DC into the MPI/DP Connector

The most direct fix is to feed a clean 24 V DC (±5 %, SELV) onto pins 2 and 7 of the CPU's MPI/DP sub-D connector using a flying lead or a dedicated power tap. Once the bus is energized, the PC-Adapter USB powers up and STEP 7 can handshake normally.

4.1 Required Hardware

• 24 V DC power supply, ≥ 100 mA continuous, ≥ 250 mA peak (PS 307, SITOP, or equivalent).
• Two-wire shielded cable, 0.5 mm² minimum, < 3 m run from PSU to the MPI/DP connector (to stay within the PC-Adapter USB bus-power loop).
• 9-pin sub-D male mating connector with solder cups (e.g. Siemens 6ES7 972-0BA30-0XA0 PROFIBUS connector, or a generic 6ES7 972-0BB20 stripped for power injection).

4.2 Pinout Reference (CPU MPI/DP Connector, 9-pin sub-D female)

4.3 Wiring Procedure

1. De-energize the PC Box 620 and the 24 V auxiliary supply. Lock-out / tag-out the upstream breaker.
2. Open the sub-D hood of a PROFIBUS connector (or a dedicated 9-pin male) and wire pins 2 and 7 to the 24 V supply: pin 7 → +24 V (P+), pin 2 → 0 V (M-).
3. Bond pin 5 (DGND / shield) to the connector housing and to the PSU 0 V at one end only to avoid ground loops.
4. Plug the wired connector into the CPU 412-2 PCI's MPI/DP port before the PC-Adapter USB daisy-chains into the same connector (use a PROFIBUS T-connector / 6ES7 972-0BB20 if both lines must coexist).
5. Power up the 24 V supply first; verify with a multimeter at the connector: +24 V between pins 7 and 2.
6. Power up the PC Box 620; the CPU 412-2 PCI comes up in STOP/RUN as before.
7. Plug the PC-Adapter USB into the second sub-D socket of the T-connector. The active LED on the adapter should illuminate within 1–2 s.

4.4 Verifying the Injection

5. Solution B: Use a CP5711 (Self-Powered USB MPI/PROFIBUS)

The CP5711 is a USB-attached MPI/PROFIBUS communications processor that draws its operating power from the USB port of the laptop rather than from the MPI/DP bus. This eliminates the external 24 V requirement entirely and is the cleanest solution for service laptops that visit many S7-400 sites.

5.1 When to Choose CP5711

• Service work, ad-hoc uploads, no 24 V supply available at the cabinet.
• Frequent re-use of the same cable across S7-300, S7-400, and ET 200S stations.
• Higher MPI/DP baud rates (up to 12 Mbps on PROFIBUS) and improved diagnostics over PC-Adapter USB.

5.2 Setup Outline

1. Install SIMATIC NET PC software 2008 SPx or later (ships with the CP5711 CD or DVD).
2. Plug the CP5711 into the laptop's USB 2.0 port; allow Windows to enumerate.
3. Open SIMATIC Manager → Options → Set PG/PC Interface…
4. Select CP5711.PROFIBUS.1 or CP5711.MPI.1 as the access point "S7-Online" and confirm.
5. Click Accessible Nodes; the CPU 412-2 PCI should appear.

6. Solution C: Use the Internal CP5611 of the PC Box 620

Many PC Box 620 systems (and the closely related Panel PC 670) ship with a Siemens CP5611 PROFIBUS/MPI interface on the motherboard or installed as a PCI half-card. This CP is bus-powered (5 V from the PCI bus) and does not require 24 V on the MPI/DP connector. If the system has STEP 7 / SIMATIC Manager installed locally, you can attach a flying lead directly from the CP5611 sub-D to the CPU 412-2 PCI MPI/DP port and run an online session without any external hardware.

6.1 Configuration Procedure

1. Open SIMATIC Manager → Options → Set PG/PC Interface…
2. Select CP5611 (PROFIBUS) as the access point "S7-Online".
3. Click Properties and verify the PROFIBUS/MPI address (default 0 for the PG) and the baud rate (1.5 Mbps default for MPI; 12 Mbps for PROFIBUS — both should auto-negotiate with the CPU 412-2 PCI's MPI port).
4. Apply and close.
5. Run Accessible Nodes or PLC → Upload Station to PG.

6.2 When Solution C Is Not Applicable

• No STEP 7 / SIMATIC Manager installed on the Box PC (image built without PCS 7 or SW controller).
• CP5611 absent in the specific Box PC variant (some low-cost builds omit the option board).
• Laptop-only service workflow — the CP5611 is not portable to the laptop.

7. STEP 7 V5.4 PG/PC Interface — Decision Matrix

8. Verification Procedure After Power-Injection

1. LED check: PC-Adapter USB "Active" LED is on solid; no flashing red.
2. Driver check: Windows Device Manager → Network adapters → "Siemens PC Adapter USB" → no Code 10, Code 43, or Code 28.
3. Bus check: Run SIMATIC Manager → Options → Set PG/PC Interface → Diagnostics. The PROFIBUS/MPI Diagnostics window should show "Active", the connected CPU's MPI address, and a non-zero bus statistics counter (frames received).
4. Node discovery: PLC → Accessible Nodes lists the CPU 412-2 PCI with its MAC/S7 address and firmware version.
5. Project upload: PLC → Upload Station to PG completes with a green progress bar; a new S7 project is generated under the configured path.
6. Online compare: PLC → Compare Online/Offline shows the expected block list (OB1, FB, FC, DB, SFB, SFC, system data).

9. Troubleshooting Matrix

10. Best Practices and Field Notes

• Standardize on CP5711 for all S7-400 service laptops. It removes the 24 V dependency, supports higher baud rates, and is the recommended replacement for the aging PC-Adapter USB.
• Label the 24 V jumper at the MPI connector with a small tag ("External 24 V — remove before hot-swap") to prevent a future technician from disconnecting the live 24 V mid-commissioning.
• Avoid backfeeding 24 V onto a PROFIBUS segment that already has an active PROFIBUS terminator with its own 24 V — this creates a 24 V conflict and can damage the PC-Adapter USB's input protection diode over time.
• Document the PG MPI address (default 0) and the CPU MPI address (default 2) in the cabinet drawing; many S7-400 upload failures are simply address collisions left over from a previous commissioning.
• Use a fused 24 V tap (0.5 A slow-blow) when injecting power to satisfy UL/CE cabinet practices.
• Verify battery / EPROM state after the first successful upload — many first-time-commissioning visits coincide with a dead backup battery, which can wipe the project on power-down.