Selecting Siemens PC Adapter USB A2 for PROFIBUS S7 Programming

Selecting the Siemens PC Adapter USB A2 for PROFIBUS S7 Programming: Power, Compatibility, and Field-Proven Choices

Field engineers commissioning, troubleshooting, and maintaining SIMATIC S7 systems routinely hit the same wall: the PROFIBUS PC adapter they pulled out of the cabinet needs 24 V DC, the target CPU will not source 24 V on the PROFIBUS connector, and the service laptop is the only thing within reach that has a USB port. The choice between the legacy PC Adapter USB, the USB A2, the CP 5711, and a passive Ethernet/PROFINET tap is governed by a small set of constraints: bus-side power availability, target device class, and firmware support in TIA Portal / STEP 7. This reference consolidates catalog numbers, voltage rails, device-class behavior (S7-300/400, ET 200, SINAMICS), and the official Siemens accessories that resolve the 24 V supply problem, so the correct adapter is selected the first time.

1. Adapter Generations and Catalog Numbers

Siemens has shipped four overlapping generations of PC-to-PROFIBUS programming adapters. The differences matter because the on-board power architecture changed between the original PC Adapter USB and the USB A2.

The defining change between the PC Adapter USB (6ES7 972-0CB20-0XA0) and the PC Adapter USB A2 (6GK1 571-0BA00-0AA0) is power routing. The A2 draws 5 V DC directly from the USB host, eliminating the 24 V rail that the legacy adapter demanded either from the target device's PROFIBUS connector or from an external power supply. For the engineer in the cabinet, this means one less cable and one less failure mode.

2. Power Supply Architecture

The PROFIBUS connector on a Siemens device is a 9-pin D-sub whose pin 6 (or pin 5 in some OEM wiring) typically carries 24 V DC isolated supply out of the device. The original PC Adapter USB used that 24 V to power its RS485 transceiver and galvanic isolation. Devices that did not source 24 V on the connector — including most SINAMICS drive Control Units, certain ET 200S head modules, and older S7-200 CPUs — required an external supply.

The USB A2 moves the DC-DC conversion to the host side: 5 V from the laptop USB is stepped up internally to power the isolated RS485 side. Result: the adapter works on any device with a compliant PROFIBUS terminator and a 9-pin D-sub, regardless of whether the device sources 24 V on pin 6.

3. Device-Class Compatibility Matrix

Behavior differs by device class because of (a) whether the device sources 24 V on the PROFIBUS connector, (b) whether the CPU/MPI/PROFIBUS interface exposes a usable programming channel, and (c) Siemens firmware behavior that has changed across CPU firmware versions.

4. SINAMICS-Specific Behavior

The X126 PROFIBUS interface on SINAMICS S120 / G120 / G120D is functionally a DP slave-only port; the SINAMICS Starter / TIA Portal STARTER plugin treats the host PC as a class-2 master for commissioning. The legacy PC Adapter USB behaves erratically on this port because the SINAMICS RS485 transceiver requires a tight supply rail (24 V ± 5 %) and the legacy adapter's internal DC-DC draws 30–40 mA from the bus. The drop on long cable runs exceeds the SINAMICS port's supply tolerance.

The USB A2 is the documented and verified replacement. Commissioning steps:

1. Connect USB A2 to the service laptop. Do not connect to SINAMICS yet.
2. Open TIA Portal (V15.1 or later) or Starter V5.4 SP3. Confirm the adapter is detected: Online → Accessible nodes lists the adapter as a USB node with PROFIBUS capability.
3. Set the A2's PG/PC interface to S7ONLINE (STEP7) → PC Adapter USB A2 (PROFIBUS).
4. Set the baud rate to match the SINAMICS X126 configuration (typically 1.5 Mbps or 3 Mbps on S120 commissioning runs).
5. Connect the 9-pin PROFIBUS cable to X126. Power on the SINAMICS line module first so the Control Unit has bus voltage at its own terminals.
6. Run Online → Accessible nodes. The SINAMICS node address (default 0, set via DIP switches on the Control Unit) should appear within 10 s.

5. ET 200 and Distributed I/O Requirements

ET 200 head modules (IM151 / IM154) do not source 24 V on the PROFIBUS connector — they are pure slaves with no bus-side supply function. The legacy PC Adapter USB will power up the transceiver, but only if the engineer provides 24 V externally to the adapter's 24 V terminal block. The USB A2 eliminates the 24 V run and is the correct tool for any ET 200S / ET 200pro / ET 200SP DP head.

For ET 200pro, the M12 PROFIBUS connector variant (Siemens 6GK1 905-0AA00, M12 D-coded) is required. The A2 ships with a standard 9-pin D-sub PROFIBUS pigtail; an M12-to-D-sub adapter cable is needed for the M12 segment.

6. Power Supply Accessories

For engineers who must continue using the legacy 6ES7 972-0CB20-0XA0 (e.g., existing fleet that has not been migrated), Siemens offers an external plug-in power supply set:

The set includes a 24 V wall-wart with a right-angle 2-pin connector that mates with the 24 V terminal block on the legacy adapter's pigtail. This allows the legacy adapter to operate against any device, regardless of whether the device sources 24 V on its PROFIBUS port. The set is the official Siemens accessory — third-party 24 V supplies are discouraged because the legacy adapter's DC-DC converter does not tolerate reverse polarity or sustained over-voltage.

7. Decision Tree: Selecting the Right Adapter

The single-branch decision above collapses to a simple field rule:

Default to the PC Adapter USB A2 (6GK1 571-0BA00-0AA0) for any new deployment. The 5 V USB power architecture removes the device-class constraint that complicates the legacy adapter. Keep one legacy 6ES7 972-0CB20-0XA0 plus the 6ES7 972-0CA00-0XA0 plug-in supply set as a service fleet for legacy S7-200 / S7-300 CPU work where the A2's firmware is not yet recognized by older TIA Portal versions.

8. Field Commissioning Procedure (USB A2 + TIA Portal V17)

8.1 Prerequisites

• PC Adapter USB A2, catalog 6GK1 571-0BA00-0AA0, with current firmware (loadable via TIA Portal Help → Adapter firmware).
• TIA Portal V16 SP1 or later. V17 is the reference build for this procedure.
• 9-pin PROFIBUS cable, terminated at both ends if the adapter is a bus terminator. Standard 9-pin SiNet cable (6XV1 830-0EH10) in the right length class.
• Service laptop with USB 2.0 or USB 3.0 port. USB hubs are not supported on the A2 — connect direct.

8.2 Configure the PG/PC interface

1. Open the Windows Control Panel → Set PG/PC Interface applet (32-bit) or TIA Portal's Online → PG/PC Interface dialog (64-bit).
2. Select the access point S7ONLINE (STEP 7).
3. For the interface, choose PC Adapter USB A2.PPROFIBUS.1.
4. Click Properties. Set the PROFIBUS address of the PG/PC to an unused value (default 0 is rarely safe).
5. Set the baud rate. The A2's autobaud mode works on most modern S7 firmware; manual override to 1.5 Mbps is recommended for SINAMICS and ET 200SP DP.

8.3 Establish the online connection

1. Click Online → Accessible nodes. The bus scan will list all reachable DP nodes within 30 s.
2. If the target CPU does not appear, check the segment terminator switch on the A2; it must be OFF if the cable continues to another segment, ON if the A2 is the segment end.
3. Confirm the bus voltage on the target device. On SINAMICS, the X126 port lights the LINK LED only when the Control Unit is in RUN or commissioning state.

9. Verification Checklist

10. Troubleshooting Matrix

11. Related PROFIBUS Adapters Outside the S7 Programming Use Case

Although the focus here is the PC programming adapter, PROFIBUS DP also appears in retrofit communication modules for low-voltage power distribution. A representative product is the PXR-PCAM PROFIBUS DP communication adapter module, used with PXR 20/25 electronic trip units on circuit breakers. The PXR-PCAM sits on the CAM-PXR link of the breaker and exposes the trip unit's measurement data (currents, voltages, trip events, breaker status) as a DP slave. Engineers integrating power monitoring into a TIA Portal project alongside S7 logic use the same PROFIBUS cable and the same DP address space; the only difference is the GSD file (Siemens-supplied, embedded in TIA Portal hardware catalog). Refer to the Eaton PROFIBUS DP Communication Adapter Module documentation (IL120009EN) for GSD file identifier, bus termination, and power-supply requirements specific to the PXR-PCAM. Power-supply requirements on the PXR-PCAM are independent of the PC adapter choice — the trip unit's CAM-PXR link powers the module, no 24 V is needed on the PROFIBUS connector.

12. Migration Note: PROFIBUS to PROFINET

For greenfield deployments in 2024 and later, the engineering default is PROFINET on S7-1200 / S7-1500 / ET 200SP PN. The PC adapter question simplifies to a single Ethernet cable and a managed switch. The PROFIBUS toolchain remains relevant only for:

• Service work on installed S7-300 / S7-400 systems predating the PN migration;
• SINAMICS S120 / G120 systems with PROFIBUS option modules (PROFINET variants are an extra-cost option);
• Existing DP segments where upgrading the DP slaves is not in scope.

For all three, the USB A2 remains the correct PC-side adapter.

13. Quick-Reference Order Codes

14. Frequently Asked Questions