1. Overview: The S7 PC Connectivity Problem
Connecting a SIMATIC S7-300, S7-400, or ET200 station to a programming device (PG) or service PC requires a dedicated PC Adapter that translates between the PC's native bus (USB or RS232) and the SIMATIC field bus (MPI, PPI, or PROFIBUS DP). Siemens ships two principal MLFB variants for this role:
- 6ES7972-0CB20-0XA0 – PC Adapter USB A2 (USB 2.0 to MPI/PPI/PROFIBUS)
- 6ES7972-0CA23-0XA0 – PC Adapter RS232 (RS232 to MPI/PPI/PROFIBUS)
The RS232 PC Adapter is the legacy design and is increasingly impractical on modern industrial laptops and tablets that have removed the DB-9 connector. The USB A2 adapter is the current production model and the de-facto standard for new deployments. Field engineers frequently ask why the Siemens USB adapter commands a price premium (historically ~300 EUR list) over a generic USB-RS485 converter (sub-20 EUR) and whether a third-party cable will perform equivalently. The short answer: the Siemens PC Adapter is not a transparent electrical converter. It contains a microcontroller that runs the full MPI token-passing, PPI master, and PROFIBUS DP-V0/V1 protocol stack. A USB-RS485 dongle exposed as a virtual COM port cannot perform the Siemens access protocol, and STEP 7 / TIA Portal will not bind to it as a PC Adapter.
2. MPI, PPI, and PROFIBUS Protocol Stack
Understanding the cable role requires understanding the three protocols the PC Adapter must speak simultaneously on a shared RS-485 physical layer.
| Protocol | Physical Layer | Baud Rates | Typical Use | PLC Families |
|---|---|---|---|---|
| PPI (Point-to-Point Interface) | RS-485 half-duplex | 9.6 / 19.2 kbaud | S7-200 programming, HMI-to-S7-200 | S7-200 |
| MPI (Multi-Point Interface) | RS-485 half-duplex | 187.5 kbaud, 1.5 Mbaud | PG/PC ↔ S7-300/400, HMI | S7-300, S7-400, ET200M |
| PROFIBUS DP | RS-485 half-duplex | 9.6 kbaud – 12 Mbaud | Distributed I/O, drives, third-party DP slaves | ET200S, ET200M, S7-300/400 with DP master, SINAMICS |
All three protocols use the same 9-pin Sub-D connector and the same RS-485 differential pair. The PC Adapter must auto-detect the active baud rate on the segment during initialization (so-called "speed recognition"). The MPI default of 187.5 kbaud is the most common setting on a greenfield S7-300 MPI segment. If a PROFIBUS DP master is present and configured at 1.5 Mbaud, the PC Adapter must fall back to that higher rate. Auto-detection adds roughly 1.5–4 seconds of connection establishment time, depending on the configured segment profile.
Token passing on MPI uses a logical token ring. The PC Adapter, when bound to STEP 7, holds the token for the duration of an online operation, performs its request, and releases the token. The PLC is a passive node from a token-arbitration standpoint when the PG is online. This is why the PC Adapter does not need a network address in the traditional sense; it borrows an address (default 0) from the local STEP 7 PG/PC interface configuration.
3. PC Adapter Variants in the Siemens Catalog
Siemens has produced several PC Adapter hardware revisions over the lifecycle of STEP 7. The current shipping variants and their immediate predecessors are listed below.
| MLFB | PC Interface | Field Bus | Max Field Rate | Status |
|---|---|---|---|---|
| 6ES7972-0CB20-0XA0 | USB 2.0 Type-A | MPI / PPI / PROFIBUS DP | 12 Mbaud | Current production (USB A2) |
| 6ES7972-0CB35-0XA0 | USB 2.0 Type-A | MPI / PPI / PROFIBUS DP | 12 Mbaud | USB A2 with termination switch (discontinued in many regions) |
| 6ES7972-0CA23-0XA0 | RS232 DB-9 | MPI / PPI / PROFIBUS DP | 1.5 Mbaud (field), 38.4 kbaud (RS232 bottleneck) | Current production |
| 6ES7972-0CA10-0XA0 | RS232 DB-9 | MPI / PPI / PROFIBUS DP | 1.5 Mbaud (field), 19.2 kbaud (RS232) | Legacy, withdrawn |
Note that the 6GK1571-0BA00-0AA0 "IE/MPI Cable" is a different product category. It is a thin client that exposes an MPI node as a PROFINET proxy over Ethernet and is used for S7-300/400 programming over a plant Ethernet. It is not interchangeable with the PC Adapter USB A2 and does not speak PROFIBUS DP directly. Some field engineers confuse the two and attempt to use the IE/MPI cable for PROFIBUS diagnostics, which is not supported.
4. Siemens 6ES7972-0CB20-0XA0 (USB A2) Detailed Specifications
The current production USB A2 adapter is the default choice for S7-300/400 programming on modern PCs.
| Parameter | Value |
|---|---|
| MLFB | 6ES7972-0CB20-0XA0 |
| PC interface | USB 2.0 Full Speed (12 Mbit/s), Type-A connector |
| Field interface | 9-pin Sub-D, RS-485, isolated |
| Galvanic isolation | Yes (between USB and field bus) |
| Supported protocols | MPI, PPI, PROFIBUS DP-V0/V1 master (passive PG) |
| Max PROFIBUS baud | 12 Mbaud |
| Max MPI baud | 1.5 Mbaud (no segment, point-to-point up to 12 Mbaud with active DP master) |
| Power | USB bus-powered (5 V, <500 mA) |
| Status LEDs | PWR (USB active), ACT (data traffic), ERR (segment error / baud mismatch) |
| Supported OS | Windows 7 SP1, Windows 8.1, Windows 10 (LTSC 2019/2021), Windows 11; Linux via libnodave / S7netplus (community drivers, not Siemens-supported) |
| STEP 7 V5.x support | STEP 7 V5.4 SP5 and later; V5.6 SP1 recommended |
| TIA Portal support | TIA Portal V13 SP1 and later; V15.1 / V16 / V17 / V18 verified |
| Driver package | "SIMATIC PC Adapter USB A2" driver (separate installer from STEP 7) |
| Typical list price (EU) | ~ 290 – 360 EUR (2024) |
The cable length from the PC Adapter body to the 9-pin Sub-D is fixed at 3 m. The USB upstream cable is hard-wired. A 5 m USB extension using an active hub is generally reliable; passive 5 m extensions are not recommended because the USB signal integrity degrades and the PC Adapter may drop the bus at random.
5. Siemens 6ES7972-0CA23-0XA0 (RS232) Detailed Specifications
The serial PC Adapter remains in production for legacy workstations, hardened industrial PCs that have retained a DB-9 COM port, and installations where the customer mandates a non-USB field interface for cyber-security reasons.
| Parameter | Value |
|---|---|
| MLFB | 6ES7972-0CA23-0XA0 |
| PC interface | RS232 DB-9 male, DCE-style pinout |
| Field interface | 9-pin Sub-D, RS-485, isolated |
| Galvanic isolation | Yes (between RS232 and field bus) |
| Supported protocols | MPI, PPI, PROFIBUS DP |
| Max PROFIBUS baud | 1.5 Mbaud (field bus); 38.4 kbaud (RS232 link) |
| RS232 baud | 38.4 kbaud (auto-selected on newer firmware) |
| Power | RS232 signals powered + optional 24 V DC jack for field-bus termination bias |
| Status LEDs | PWR, ACT, ERR |
| STEP 7 / TIA support | Same as USB A2; both adapters share the STEP 7 "PC Adapter" PG/PC interface type |
The fundamental limitation of the RS232 PC Adapter is the RS232 link bandwidth. The 38.4 kbaud ceiling on the PC side becomes the throughput bottleneck even when the field segment is configured at 1.5 Mbaud. The PC Adapter's internal firmware buffers data between the two rates. A typical 200-byte block transfer takes approximately 50 ms over RS232 versus 0.3 ms over the USB A2 at 1.5 Mbaud field rate. For online monitoring of a high-tag-count HMI tag list or for uploading a 2 MB project archive, the time difference is user-perceptible.
Two practical considerations apply to the RS232 PC Adapter:
- USB-to-RS232 bridge adapters (FTDI FT232, CH340, CP2102) will not work. STEP 7 selects the COM port by an internal registry key (HKLM\SOFTWARE\Siemens\AUTSW\...\S7ONLINE), and only COM ports backed by the native UART on a PC/AT-class chipset will be accepted. Most modern laptops lack a native COM port, which forces the engineer to use the USB A2 anyway.
- The RS232 PC Adapter requires the field bus to provide its own termination. The cable does not include a switchable terminator; the on-segment terminator on the PLC or the last DP slave must be enabled.
6. Throughput, Latency, and Baud Rate Analysis
The selection of cable directly affects online operations: project download, program block upload, and online tag refresh. The table below summarizes the effective throughput for each adapter at typical MPI/DP field rates.
| Adapter | Field Bus Rate | Effective Throughput (typical project block download) | Approx. Time for 1 MB upload |
|---|---|---|---|
| 6ES7972-0CB20-0XA0 (USB A2) | 187.5 kbaud MPI | ~ 17 KB/s | ~ 60 s |
| 6ES7972-0CB20-0XA0 (USB A2) | 1.5 Mbaud MPI/DP | ~ 130 KB/s | ~ 8 s |
| 6ES7972-0CB20-0XA0 (USB A2) | 12 Mbaud DP | ~ 700 KB/s (USB 2.0 ceiling) | ~ 1.5 s |
| 6ES7972-0CA23-0XA0 (Serial) | 187.5 kbaud MPI | ~ 3.5 KB/s | ~ 5 min |
| 6ES7972-0CA23-0XA0 (Serial) | 1.5 Mbaud MPI/DP | ~ 4 KB/s (RS232 bound) | ~ 4.5 min |
The 38.4 kbaud RS232 link on the serial PC Adapter imposes a hard ceiling of approximately 3.5–4 KB/s regardless of the field-bus rate. The USB A2 does not exhibit this limitation because the USB link is 12 Mbit/s and the protocol-overhead is handled by the adapter's microcontroller.
For modern S7-300/400 firmware updates over MPI (firmware ≥ V3.x blocks can exceed 4 MB), the USB A2 is the only practical option. Engineers who attempt firmware update on a serial PC Adapter at 1.5 Mbaud will see STEP 7 report "Online: Time-out during read/write" errors at block boundaries where the PLC re-enters Run-Stop-Run state.
7. STEP 7 V5.x PG/PC Interface Configuration
After the Siemens USB A2 driver is installed, the adapter is selected through the Set PG/PC Interface dialog in STEP 7 V5.x.
- Open Start → SIMATIC → STEP 7 → Set PG/PC Interface or Options → Set PG/PC Interface from inside the SIMATIC Manager.
- Under Access Point of the Application, select
S7ONLINE. - Under Used Interface Parameterization, select
PC Adapter (MPI)orPC Adapter (PROFIBUS). The(AUTO)option forces auto-detection against the connected segment and is recommended for service work where the field rate is unknown. - Click Properties. The PC Adapter MPI Properties dialog opens.
| Parameter | Recommended Value | Notes |
|---|---|---|
| PG/PC is the only master on the bus | Unchecked (default) | Check only if the PLC is the sole node and the PG is connected directly |
| Address | 0 (default) or 1 | Must not collide with a PLC MPI address on the segment |
| Transmission rate | 187.5 kbaud (default for MPI) | Set to 1.5 Mbaud if the segment is a DP segment |
| Highest MPI address | 31 (default) | Set to 126 for DP segments |
| Timeout | 10 s | Increase to 30 s on noisy plant segments |
The S7ONLINE access point is the routing layer that connects the STEP 7 application (LAD/FBD/STL editor, NetPro, hardware diagnostic) to the PC Adapter driver. The CP 5611/CP 5621 PCI cards and the USB A2 driver both register themselves under S7ONLINE; switching between them is a one-click operation in the PG/PC Interface dialog.
8. TIA Portal Online Access Configuration
TIA Portal V15 and later expose the same adapter under Online → Accessible devices. Configuration path:
- Open the TIA Portal project. From the menu, select Online → Accessible devices.
- In the "Online access" tree, expand the entry PC Adapter [MPI/PROFIBUS] [USB] or the COM port entry for the serial adapter.
- Right-click the entry → Properties. The same transmission rate, address, and timeout parameters from STEP 7 V5.x are exposed in a nearly identical dialog.
- Click Flash LED to confirm physical connectivity: the ACT LED on the PC Adapter should pulse for ~2 seconds.
9. Third-Party and Replacement Cable Considerations
Several third-party manufacturers build cables that advertise "compatible with 6ES7972-0CB20-0XA0". Most isolate the bus, expose a 7-pin or 9-pin PROFIBUS connector, and bind to the Siemens PC Adapter USB driver. One example is the USB MPI Programming Cable for S7-200 / 300 / 400 – a 7-pin isolated replacement that is a direct functional substitute for the Siemens-branded adapter at a fraction of the list price.
Engineers evaluating a third-party cable should validate the following:
| Criterion | Siemens Genuine | Quality Third-Party | Risk in Cheap Clones |
|---|---|---|---|
| Galvanic isolation (USB ↔ field) | Yes, verified | Yes, in well-reviewed models | Often missing; ground loops damage PLC port |
| PPI / MPI / PROFIBUS support | All three | All three in advertised models | Some clones support PPI only, not MPI/DP |
| 12 Mbaud DP support | Yes | Yes in current-gen clones | Older clones cap at 1.5 Mbaud |
| Windows 10 / 11 signed driver | Yes, Microsoft WHQL | Varies; some use the Siemens-signed driver in compatibility mode | Unsigned drivers fail on secure boot / Windows 11 22H2 |
| TIA Portal V17 / V18 compatibility | Verified | Usually works; field reports of intermittent ACT LED at 12 Mbaud on some clones | Driver-level instability during long uploads |
| Warranty and RMA | 24 months via Siemens regional office | 6 – 12 months via reseller | None |
For warranty service and field support on critical production systems, the Siemens genuine adapter is the lowest-risk choice. For training, lab, and bench work, a quality third-party cable delivers equivalent functionality at 30 – 50% of the list price.
10. VIPA, ABB, and Non-Siemens PLC Compatibility Warnings
| PLC Family | Cable Required | Why Siemens PC Adapter Fails |
|---|---|---|
| VIPA SPEED7 (System 100V, 200V, 300S+) | VIPA "green cable" (VIPA 950-0KB01 / 950-0KB10) or standard Ethernet for newer models | Different MPI token implementation; VIPA SPEED7 uses a non-standard bus allocation that the Siemens adapter cannot negotiate |
| Allen-Bradley SLC 500, MicroLogix | 1747-CP3 / 1761-CBL-PM02 | DF1 full-duplex protocol, not MPI |
| Schneider Modicon M340 / M580 | Modbus USB or Ethernet | Modbus RTU / TCP, not MPI/PROFIBUS |
| ABB AC500 | TK502 / Ethernet | CS31 protocol or Modbus, not MPI |
| Siemens S7-1200 / S7-1500 | PROFINET (CP 1623, IE/MPI cable, or industrial Ethernet) | No MPI/DP hardware interface; PROFINET only |
| LOGO! 8 | Ethernet or LOGO! USB cable (6ED1057-1AA01-0BA0) | LOGO! soft PPI is not MPI-compatible |
Engineers migrating from VIPA SPEED7 to genuine Siemens S7 must replace the VIPA green cable. Conversely, an engineer pulling a VIPA SPEED7 from a panel and assuming the existing Siemens PC Adapter will reach it will see "Online: Connection to partner cannot be established" from STEP 7 even with a perfect physical link.
11. Driver Installation and Windows Compatibility
The Siemens PC Adapter USB A2 driver is not included by default in Windows. It is provided on the STEP 7 / TIA Portal installation media and is also downloadable from the Siemens Industry Online Support portal. Install the driver before plugging the adapter in for the first time.
- Insert the STEP 7 V5.6 DVD or mount the TIA Portal V17 / V18 installer ISO. Run
DriveBased\01.SIMATIC_PC_Adapter_USB_A2\setup.exe(path may vary by version). - Follow the installer prompts. The driver registers a kernel-mode filter driver (
s7ousba2.sys) and a virtual S7ONLINE access point. - Plug in the PC Adapter. Windows Device Manager should show SIMATIC PC Adapter USB A2 under the Siemens AG device class. The COM port is intentionally not enumerated; the adapter does not appear as a serial device.
- Verify with the PC Adapter Diagnostics tool (Start → SIMATIC → PC Adapter USB Diagnostics). The tool reports the field-bus signal quality, the detected baud rate, and the number of frame errors since the last reset.
12. Cable Selection Procedure for New Projects
Use the following decision tree when specifying a PC Adapter for a new S7-300/400 deployment.
- Identify the PLC family. If the target is S7-1200/1500, the PC Adapter is the wrong tool – spec PROFINET from the start.
- Confirm the programming device has a usable port. Modern laptops without a DB-9 should default to the USB A2 (6ES7972-0CB20-0XA0).
- Confirm the STEP 7 / TIA Portal version. STEP 7 V5.4 SP5+ and TIA Portal V13 SP1+ support the USB A2. Older versions of STEP 7 may require the legacy USB PC Adapter (6ES7972-0CB20-0AA0) or the serial PC Adapter.
- Confirm the field-bus rate. If the segment is a PROFIBUS DP network at 1.5 Mbaud or 12 Mbaud, the USB A2 is mandatory; the serial PC Adapter will time out on large block transfers.
- Confirm the OS. Windows 7 SP1 and later are supported. Windows 11 22H2 requires driver ≥ V2.0.3 (shipped with TIA V17 and later).
- If budget is constrained and the deployment is non-critical (training, lab, bench), spec a quality third-party isolated cable. For production systems with warranty and support requirements, spec the Siemens genuine adapter.
- Order two adapters. A spare avoids multi-day RMA-induced commissioning delays.
13. Troubleshooting Matrix and Field Diagnostics
| Symptom | Probable Root Cause | Resolution |
|---|---|---|
| Adapter LED off after plug-in | USB driver not installed / USB port underpowered | Reinstall driver; try a rear panel / powered hub USB port |
| ERR LED steady on | Field-bus segment has short or missing terminator | Verify terminator is ON at both ends; check for shield ground at one point only |
| ERR LED blinking | Baud rate mismatch between PG and segment | Set PG/PC interface to (AUTO) or to the segment's configured rate |
| STEP 7 reports "Cannot reach the partner" | Wrong MPI address on PG or wrong access point selected | Verify S7ONLINE access point; verify PG address does not collide with PLC |
| Connection drops after sleep/resume | USB selective suspend setting | Disable USB selective suspend in Windows Power Options for the USB root hub |
| Connection slow on USB A2 | Wrong field-bus type selected (PPI vs MPI vs DP) | Verify the segment profile; switch to PROFIBUS profile if the segment is DP at 1.5 Mbaud |
| Online works, project upload fails at ~ 50% | RS232 PC Adapter at 1.5 Mbaud field, block timeout exceeded | Use the USB A2 for uploads exceeding 500 KB; lower timeout to 30 s on noisy segments |
| Windows 11 22H2: "Unknown USB device (Device Descriptor Request Failed)" | Inbox driver claiming the adapter | Disable the inbox USB serial driver per Section 11 note; reinstall Siemens package |
| Online connects, but Force table is empty | Wrong PG/PC interface selected (e.g., S7ONLINE set to TCP/IP instead of PC Adapter) | Reassign S7ONLINE access point to PC Adapter (MPI) or (PROFIBUS) |
| Adapter works on PG #1 but not PG #2 | Driver installation corrupted on PG #2 | Uninstall via Programs and Features → SIMATIC PC Adapter USB A2, reinstall, reboot |
The Siemens PC Adapter Diagnostics tool provides a live bus monitor that decodes MPI/DP frames. It is the first tool to deploy when the ERR LED blinks or when online connections are intermittent. Save the diagnostic trace to a .log file before contacting Siemens support, as the file format is what the regional support desk will request.
14. Frequently Asked Questions
Can I use a generic USB-to-RS485 adapter as an MPI cable?
No. A generic USB-RS485 adapter exposes a virtual COM port and contains no Siemens protocol stack. STEP 7 binds to the S7ONLINE access point, which only the Siemens PC Adapter driver registers. A USB-RS485 dongle will appear in Windows as a serial port but STEP 7 will not list it as a usable interface.
Why is the Siemens USB A2 priced 2-3x a USB-Serial adapter?
Because the PC Adapter USB A2 contains a microcontroller that runs the full MPI token-passing, PPI master, and PROFIBUS DP-V0/V1 protocol stack. It is a protocol converter and bus master, not an electrical bridge. The list price reflects the firmware licensing, the ruggedized isolated housing, and the long-term component sourcing.
Does the PC Adapter USB A2 work with TIA Portal V18?
Yes. TIA Portal V13 SP1 and later support the 6ES7972-0CB20-0XA0. On TIA Portal V18 with Windows 11 22H2, install the driver package shipped with TIA V17 or download the latest signed driver from the Siemens Industry Online Support portal.
Can the PC Adapter program an S7-1200 or S7-1500 PLC?
No. S7-1200 and S7-1500 do not have an MPI or PROFIBUS DP port; they use PROFINET exclusively. Use a PROFINET-capable interface such as the CP 1623 PCI card, the SCALANCE USB PROFINET adapter, or any industrial Ethernet NIC bound to the PROFINET IO controller stack.
What is the maximum MPI/DP cable length supported by the PC Adapter?
The PC Adapter is a bus master, not a repeater, and the cable length to the first segment terminator must follow the PROFIBUS DP cable specification: up to 1,200 m at 9.6 / 19.2 / 45.45 / 93.75 kbaud, 1,000 m at 187.5 kbaud, 400 m at 500 kbaud, 200 m at 1.5 Mbaud, and 100 m at 12 Mbaud. For segment lengths beyond 100 m at high baud, install a PROFIBUS repeater (6ES7972-0AA01-0XA0).