Troubleshooting S7-200 PPI Connection via PC Adapter USB A2

1. Problem Description

When commissioning or maintaining a Siemens SIMATIC S7-200 CPU (21x or 22x series), engineers frequently encounter a situation where the legacy PC/PPI cable communicates correctly with STEP 7 Micro/Win, but the newer SIMATIC PC Adapter USB A2 (Siemens order number 6GK1571-0BA00-0AA0) fails to establish a PPI/MPI session. The failure typically presents as one of the following Micro/Win SP9 error messages after pressing the "Communications" button:

• "Could not find the access point"
• "Cannot reach the CPU"
• "The connection to the PLC has been interrupted"
• "Access to the S7-200 CPU is denied"
• Silent timeout with no response after 30+ seconds

Because the S7-200 still responds over the PC/PPI USB cable, the CPU itself, its 24 VDC supply, the program integrity, and the RS-485 port are confirmed functional. The fault therefore lies in one of three domains: (a) the PC Adapter USB A2 driver stack, (b) the Set PG/PC Interface mapping in Micro/Win SP9, or (c) the physical RS-485 signalling parameters (address, baud rate, termination, protocol).

2. Root Cause Analysis

The PC Adapter USB A2 has supported PPI since firmware/driver version 1.2, but the implementation differs from the PC/PPI cable in three observable ways:

If Micro/Win SP9 still references the PC/PPI cable's COM port, the PC Adapter USB A2 will appear "connected" at the OS level but will never receive PPI frames. Conversely, if the wrong PG/PC interface entry is selected, Micro/Win times out silently.

3. Hardware Identification & Specifications

Verify that the adapter in hand is the A2 revision and not the obsolete original PC Adapter USB (without "A2"). The A2 revision uses a more recent FTDI bridge and adds a built-in terminating resistor switch.

Reference the official Siemens SIMATIC PC Adapter USB A2 product page on the Siemens Industry Online Support portal for the binding datasheet and current driver package.

4. Prerequisites

1. STEP 7 Micro/Win V4.0 SP9 (last service pack released for the S7-200 product line; order number 6ES7810-2CC03-8YX0 or later SP9 update 6ES7810-2CA03-8YX0).
2. SIMATIC PC Adapter USB A2 driver package installed on the engineering workstation. On Windows 7 Pro x64, install the driver package (Siemens provides an unsigned or WHQL-signed variant; allow the installation through Device Manager "Update Driver" if Windows prompts).
3. Administrator privileges on the Windows 7 x64 host. Micro/Win SP9 must write to HKLM\SOFTWARE\Siemens\SINEC and the PG/PC interface store.
4. A 24 VDC power supply wired to the S7-200 CPU (terminals L+ and M) capable of delivering at least the CPU's inrush current plus any expansion I/O.
5. An RS-485 PROFIBUS drop cable with 9-pin D-sub male connector on the adapter end. Standard PROFIBUS purple cable (Siemens 6XV1830-0EH10) or any shielded twisted-pair with characteristic impedance 150 Ω is acceptable.

5. Pre-Connection Checklist

1. Confirm the S7-200 CPU is powered. The RUN, STOP, and SF LEDs must show the expected pattern. If SF is solid red, resolve the system fault before any communication attempt.
2. Verify the CPU's PPI/MPI address. The factory default for the S7-200 is address 2. A CPU whose address has been changed by user program or by a previously installed Micro/Win project will not respond at address 2.
3. Verify the CPU's baud rate. Factory default is 9.6 kbps PPI on Port 0 for CPU 21x, and 9.6 kbps PPI on Port 0/1 for CPU 22x unless reconfigured.
4. Inspect the 9-pin D-sub on the adapter. Pin 3 = RxD/TxD-P (B-line, red conductor), Pin 8 = RxD/TxD-N (A-line, green conductor), Pin 5 = shield/drain, Pins 6 = +5 V (do not connect), Pin 9 = +5 V (do not connect).
5. Decide on RS-485 termination. The adapter has a slider switch on its side that inserts the 220 Ω termination network. Enable termination only at the physical ends of the RS-485 segment; with a single CPU directly connected, leave termination enabled on the adapter side and check whether the CPU's Port has its own built-in termination (CPU 226/224 XP have switchable terminations on Port 0 and Port 1).

6. Driver Installation on Windows 7 Pro x64

1. Insert the PC Adapter USB A2 into a USB 2.0 port. Avoid USB 3.0 hubs that may not enumerate the device correctly on legacy systems.
2. Open Device Manager. On first insertion, the adapter appears under Other devices > SIMATIC PC Adapter USB A2 with a yellow exclamation mark.
3. Right-click and select Update Driver Software > Browse my computer for driver software. Point to the directory containing the SIMATIC PC Adapter USB A2 driver (typically C:\Program Files\Siemens\Automation\Drivers\PC_Adapter_USB_A2 after running the driver setup, or the unpacked folder from the Siemens support download).
4. Confirm the warning that the driver is unsigned, if shown, by clicking Install this driver software anyway.
5. After installation, the adapter must appear under Universal Serial Bus controllers as SIMATIC PC Adapter USB A2, and a virtual network adapter entry SIMATIC PC Adapter USB A2 (NDIS) appears under Network adapters. Both entries are required.
6. Reboot the workstation. Micro/Win SP9 caches the PG/PC interface registry; a stale cache causes spurious "interface not installed" errors.

7. Micro/Win SP9 Set PG/PC Interface Configuration

The single most common cause of "no communication" with the PC Adapter USB A2 is a wrong entry in Micro/Win's Set PG/PC Interface dialog. Micro/Win must explicitly point to the adapter's NDIS interface, not to a COM port.

1. Launch Micro/Win SP9. From the left navigation pane, click Communications, then click the Set PG/PC Interface hyperlink in the Communications dialog. Alternatively, run set-pc-interface.exe from the Micro/Win installation directory.
2. In the interface list, locate the entry that reads PC Adapter (PPI) or PC Adapter (MPI). Do not select PC/PPI cable (PPI) or any COM-based entry.
3. Click Properties to open the adapter properties dialog.
4. On the PPI tab, set the following values:
   • Transmission rate: 9.6 kbps (CPU 21x default) or 19.2 kbps (common CPU 22x setting).
   • Highest station address: 31
   • Profile: PPI
5. On the Local Connection tab, verify that the Connection to target/PG is set to the PC Adapter USB A2 (NDIS) interface. If multiple NDIS entries are shown, ensure only one PC Adapter instance is installed.
6. Click OK and then OK again to confirm the PG/PC interface selection.

8. Establishing the Connection in Micro/Win

1. Click Communications > Communications in Micro/Win SP9 to open the Communications dialog.
2. Double-click the Double-click to refresh field. Micro/Win broadcasts an MPI/PPI Identify frame and waits for a token-passed response from the CPU.
3. Within 5 to 30 seconds, the CPU should appear in the list with its CPU type (e.g., CPU 224, CPU 226 XM) and its address.
4. Click the CPU entry, then click OK to bind the project to that CPU.
5. Verify the link by selecting PLC > Information. The dialog returns the CPU model, firmware version, and the user program checksum. If the firmware version field shows ???, the connection is half-established and indicates a baud-rate or address mismatch.

9. S7-200 Protocol and Baud Rate Compatibility Matrix

The PC Adapter USB A2 must be bound to the correct physical port. Connecting to Port 1 while Micro/Win searches Port 0 produces a silent timeout. Use PLC > System Block > Communication Ports in Micro/Win SP9 to inspect the assigned port and address.

10. RS-485 Cabling and Termination Detail

The PC Adapter USB A2 expects a PROFIBUS-compliant 9-pin D-sub pinout. Wiring the cable with the older PC/PPI USB cable pinout (which swaps Pins 3 and 8 in some revisions) is a common field wiring error.

For a single-CPU point-to-point link, the cable between the PC Adapter USB A2 and the S7-200 Port 0 (or Port 1) connector must use a 9-pin male D-sub on the adapter side and the CPU's PPI connector (also 9-pin D-sub female) on the PLC side. PROFIBUS connectors with integrated termination should be set to the OFF position if the PC Adapter USB A2's internal termination is enabled, or to the ON position otherwise.

11. Verification Procedure

After establishing the link, run the following functional checks before resuming work on the project:

1. CPU Identification: PLC > Information returns a non-empty CPU model string and firmware version (e.g., CPU 226 / FW 2.01).
2. Read Clock: PLC > Time of Day Clock > Read Clock returns a timestamp matching the workstation clock within 5 seconds. A clock reading of 00:00:00 01-JAN-90 indicates the CPU has lost retentive memory or is in a fault state.
3. Upload Test: PLC > Upload produces a project matching the in-station program. Compare the uploaded block count against the project documentation.
4. LED Behavior: The PC Adapter USB A2 green LED remains steady, the yellow LED flickers during each PPI/MPI frame, and the red LED stays dark. A persistent red LED indicates CRC errors and usually points to baud-rate mismatch or termination problems.
5. Cycle Test: From Micro/Win, force an output coil (Debug > Force) and verify the corresponding output module LED activates. Release the force immediately after verification.

12. Troubleshooting Matrix

13. When the PC Adapter USB A2 Will Not Work

The PC Adapter USB A2 cannot communicate with S7-200 CPUs in the following scenarios. If any of these match the field installation, switch back to the PC/PPI USB cable for that task.

• CPU 212/214 with firmware older than V1.10: Early firmware has PPI implementation bugs that reject the PC Adapter's token-passed requests. Upgrade firmware or use the PC/PPI cable.
• CPU running as PROFIBUS DP slave with master configured elsewhere: The PC Adapter competes for the token with the active PROFIBUS master and the S7-200 will not respond to its polls. Remove the CPU from the PROFIBUS DP slave configuration during programming.
• CPU in RUN with password protection on system blocks: Some CPUs reject PC Adapter access to System Block while running. Place the CPU in STOP before connecting.

14. Field-Proven Commissioning Procedure

The following sequence has been validated on CPU 224, CPU 224 XP, and CPU 226 XM installations and reduces the typical diagnostic cycle from hours to minutes:

1. With the PC Adapter USB A2 disconnected, open Device Manager and confirm the driver is installed. A phantom device without driver binding cannot be activated later.
2. Power the S7-200. Record the address shown on the CPU display (digital CPUs) or read it with a known-good PC/PPI cable first.
3. Connect the PC Adapter USB A2 directly to the CPU Port 0 with a short (< 3 m) PROFIBUS drop cable. Enable termination on the adapter only; disable termination on the CPU Port 0 switch.
4. Open Set PG/PC Interface, select PC Adapter (PPI), set 9.6 kbps, set Highest Station Address to 31, set Profile to PPI. Apply.
5. Open Micro/Win Communications dialog, double-click to refresh. Confirm CPU appears.
6. If step 5 fails, place the CPU in STOP using the mode switch, then retry.
7. If step 6 still fails, try MPI profile at 187.5 kbps (CPU 22x only).
8. Once communication is established at the default baud rate, raise the baud rate in 19.2 kbps increments until the production baud rate is reached, re-verifying after each change.

15. Notes on Migrating from Micro/Win SP9 to STEP 7 V13 / TIA Portal

Although Micro/Win SP9 remains the only environment with full S7-200 programming capability, the PC Adapter USB A2 is fully reusable with STEP 7 V5.x and TIA Portal V13 onward for S7-300, S7-400, S7-1200, and S7-1500 communication. When migrating an S7-200 program to a TIA-supported platform, leave the PC Adapter USB A2 driver installed and bound to Micro/Win's PG/PC Interface entry; reinstall only after the Micro/Win project is archived. Concurrent use between Micro/Win and TIA Portal on the same workstation can corrupt the NDIS binding and requires a Set PG/PC Interface reconfiguration after switching applications.

Frequently Asked Questions