Siemens PC Adapter USB: Configuring for S7-400 PROFIBUS DP

Overview

The Siemens PC Adapter USB is a compact protocol converter that links a programming device (PG) or notebook USB port to SIMATIC S7 MPI, PROFIBUS DP, and PPI networks. For S7-400 stations, the adapter enables online commissioning, program download, diagnostic readout, and firmware updates without requiring a CP (communications processor) card inside the PG. Two hardware revisions are common in the field: the legacy 6ES7 972-0CB20-0XA0 and the current PC Adapter USB A2 (6GK1571-0BA00-0AA0). Both terminate to the PLC's combined MPI/DP interface (X1) and operate at PROFIBUS bus profiles from 9.6 kbit/s up to 12 Mbit/s.

When STEP 7 V5.x or TIA Portal is used, the adapter is exposed to the engineering tool as PC Adapter (PROFIBUS) or PC Adapter (MPI) in the Set PG/PC Interface dialog. This article documents the wiring topology, the correct interface assignment, driver installation, slave IM restrictions, and the diagnostic workflow an engineer follows when bringing an S7-400 online from a USB-only PG.

Hardware Identification and Part Numbers

The PC Adapter USB A2 ships with a fixed PROFIBUS D-sub connector and ships a separate 0.3 m MPI pigtail cable. Both revisions enumerate as a Siemens vendor-specific USB device (VID 0x0908 / PID 0x0001 or 0x0002) and require the S7USB / PC Adapter driver distributed with STEP 7 and TIA Portal.

Technical Specifications

Prerequisites

1. STEP 7 V5.5 SP2+ or TIA Portal V13 SP1+ installed on the PG.
2. Administrator rights on the PG (the S7USB driver registers as a kernel-mode service).
3. A free USB 2.0/3.x Type-A port (avoid USB hubs during commissioning).
4. 0.3 m MPI cable (ships with the adapter) or a PROFIBUS DP cable with D-sub connector.
5. The PG/PC is not connected to other active bus masters on the same PROFIBUS segment while the adapter is plugged.
6. S7-400 CPU with combined MPI/DP port (X1) free, or a free DP port on an IM/CP module.

Driver Installation

1. Install STEP 7 / TIA Portal first; the S7USB driver is bundled with the engineering software and is not available as a stand-alone installer.
2. Connect the PC Adapter USB A2 to the PG's USB port. Windows Plug-and-Play will detect Siemens PC Adapter USB A2 and locate the driver automatically when the engineering suite is installed.
3. Verify the device appears in Device Manager → USB controllers as Siemens PC Adapter USB A2. If a yellow triangle is shown, right-click → Update driver → Browse for driver software on the computer → point to %ProgramFiles%\Siemens\Automation\SIMATIC_OAM\addons\s7usb (STEP 7 V5.x) or the TIA Portal equivalent.
4. Confirm the three status LEDs (USB / MPI-DP / Active) illuminate: USB solid green, MPI-DP flashing on traffic, Active green when the PG is master on the bus.

Configuring Set PG/PC Interface

1. Launch STEP 7 Manager (or TIA Portal → Online → Accessible devices).
2. Open Options → Set PG/PC Interface.
3. From the Interface parameter assignment used dropdown select PC Adapter (PROFIBUS) for PROFIBUS DP traffic, or PC Adapter (MPI) for legacy MPI traffic. PC Adapter (Auto) is acceptable for first-time discovery; the tool will probe both profiles.
4. Click Properties and verify:
   • PROFIBUS tab → transmission rate matches the bus profile of the S7-400 segment (default 1.5 Mbit/s for DP, 187.5 kbit/s for MPI).
   • Bus profile = DP (standard) or MPI.
   • PG/PC is the only master on the bus = enabled (service case).
   • Address = default 0 for the PG; the S7-400 MPI/DP address is set in HW Config → CPU properties → PROFIBUS interface.
5. Click OK → OK → Diagnostic. The diagnostic dialog shows Active and Found masters/slaves; the PC Adapter should report Online.

Connecting to an S7-400 via the MPI/DP Port

The S7-400 CPU exposes a single combined MPI/DP connector on the front of the CPU (X1). The PC Adapter USB A2 connects to this port using the supplied 0.3 m MPI cable. The CPU's interface mode is set in HW Config → CPU properties → Interface → Type:

• MPI – default; the PG and any HMI on the same bus operate at 187.5 kbit/s or 19.2 kbit/s.
• DP-Master – the CPU is a PROFIBUS DP class 1 master; the bus runs at the configured DP baud rate (45.45 kbit/s to 12 Mbit/s).
• DP-Slave – the CPU is a DP slave to a different master; in this mode the PG must use the master's configured baud rate and may require token-handshake from the master before going online.
• Not networked – interface is disabled; the PG cannot go online unless the mode is changed first (this requires an existing project with a programming device port or a memory card).

For a first-time download to a brand-new CPU, leave the interface as MPI, attach the adapter, and let STEP 7 discover the CPU at the default MPI address 2. After the project is loaded and the PROFIBUS network is configured, you can switch the interface to DP-Master and reconnect at the higher DP baud rate.

Connection Topology

The PC Adapter USB A2 is a bus master and must be inserted at either end of the PROFIBUS segment or on a stub line with the bus termination enabled on the connector. The standard wiring for a single-CPU service case is:

[PG USB] --USB--> [PC Adapter USB A2] --PROFIBUS 0.3 m--> [S7-400 CPU X1]
                                       (terminator ON on adapter DP connector)

If the segment contains additional DP slaves (ET200S, ET200M, drives), the PC Adapter must be placed at one end of the segment with the bus terminator switched on, and the existing terminating resistor on the far end must remain on. The adapter has a slide switch on the D-sub hood to engage the integrated termination (220 Ω between A and B, plus 390 Ω pull-up/pull-down to 5 V on the 6ES7972-style connectors).

Slave IM Module Behaviour

A frequently reported field issue: the PC Adapter USB can go online with the S7-400 CPU directly on X1, but it cannot establish a connection when plugged into the DP port of a remote-slave IM module (for example, an IM 151 in an ET200S station that is itself a slave of a higher-level master). The reasons are architectural, not cable-related:

1. An IM module configured as a DP slave does not forward MPI/DP class 2 master requests through its backplane to a sibling CPU unless explicitly configured as a router.
2. The PC Adapter USB initiates an FDL (Fieldbus Data Link) class 2 connection using the PG/PC address. A DP slave's link layer does not propagate these acyclic services upstream.
3. For S7 routing, the CPU and the IM must have been programmed in advance to act as S7 routers. Without route entries in the project, the adapter is invisible to the CPU behind the slave IM.

Diagnostics and Verification

After configuring the Set PG/PC interface, run the following verification sequence to confirm end-to-end online capability:

1. Open Accessible nodes (STEP 7 V5.5: PLC → Accessible nodes; TIA Portal: Online → Accessible devices). The S7-400 CPU should appear with its MPI or DP address.
2. Double-click the discovered CPU. Online → Online & diagnostics opens the diagnostic buffer; verify the last entries are present and not all red.
3. From Online → Download to device, perform a software download. Observe the Active LED on the PC Adapter blinking rapidly during the transfer.
4. After download, Online → Monitor & Modify to read a tag (e.g. MW0) and confirm tag services pass through the adapter.
5. Check the diagnostic buffer for any Bus fault or Time-out entries with the same time-stamp as the operation.

Firmware and Driver Compatibility Matrix

Troubleshooting Matrix

Setting the PG/PC Address

The PC Adapter's default PROFIBUS address is 0. If the S7-400 segment has an existing master at node 0 (for example, an HMI panel), the adapter will be unable to acquire the token. In this case:

1. Open Set PG/PC Interface → Properties → PROFIBUS.
2. Set the Address field to a free node (e.g. 126 — the highest legal address and common for service tools).
3. Confirm with Diagnostic that the new address is reported and that the adapter wins arbitration.

Bus Profile Selection in Practice

Safety and Operational Notes

Related PROFIBUS Diagnostic Tools

For deeper bus health analysis beyond the PC Adapter's basic traffic LEDs, Siemens and the PROFIBUS product finder list class-2 diagnostic adapters that combine bus statistics with oscilloscope views. These are recommended when random disconnects persist after the steps above, as they expose signal-quality metrics (RSSI, edge count, errored frames) that the PC Adapter alone does not show.