TIA Portal S7-300 PC Adapter RS232: Resolving MPI Detection

Problem Overview

Engineers migrating from STEP 7 V5.3/V5.5 to TIA Portal frequently encounter a complete loss of online access to legacy S7-300 CPUs when their only physical interface is the legacy PC Adapter RS232/MPI (order number 6ES7 972-0CA23-0XA0). The cable is physically present, the COM port is detected by Windows, and the same hardware has worked reliably under STEP 7 V5.x for years — but Online > Accessible nodes in TIA Portal reports no devices, and the PC Adapter does not appear in the PG/PC Interface selection tree.

Symptom summary across affected configurations:

• COM1/COM3 port visible in Windows Device Manager
• PC Adapter cable LED activity confirmed on the MPI connector
• STEP 7 V5.3/V5.5 can establish an online connection to the same CPU without changes
• TIA Portal V11 SP2 Update 5 / V12 SP1 / V13 SP1 / V14 / V15.x / V16 / V17 / V18 — no PC Adapter entry available
• Accessible nodes scan returns empty result; Set PG/PC Interface dialog lists only TeleService and S7USB as possible interfaces

Affected Hardware and Software

Affected TIA Portal versions (all of them, including the current V18):

• TIA Portal V11 SP2 Update 5 (the version originally reported in the source case)
• TIA Portal V12 SP1
• TIA Portal V13 SP1 / SP2
• TIA Portal V14 SP1
• TIA Portal V15, V15.1
• TIA Portal V16
• TIA Portal V17
• TIA Portal V18 (current at time of writing)

Root Cause: Serial PC Adapter Driver Removed

Siemens has removed the serial port driver for the PC Adapter family from the TIA Portal installation. The new "PC Adapter Auto" / "PC Adapter MPI" / "PC Adapter PROFIBUS" entries in the Set PG/PC Interface dialog are implemented on top of a different driver stack that communicates through the USB HID layer. The legacy S7WSCAN.DLL-based driver that handled COM-port devices under STEP 7 V5.x is not installed with TIA Portal, and no download package is provided to restore it.

Why this was done:

1. End of life of the RS232 PC Adapter. The 6ES7 972-0CA23-0XA0 was phased out in favor of the USB variants. The successor 6ES7 972-0CB20-0XA0 uses a different microcontroller and presents itself as a USB Human Interface Device, not as a serial COM port.
2. Performance. The RS232 bus topped out at 115.2 kbit/s, while the USB cable supports 12 Mbit/s. The legacy driver could not meet the firmware download throughput targets of TIA Portal.
3. Driver consolidation. Maintaining two parallel driver stacks (serial + USB) increased the regression-test surface. Siemens chose the USB stack only.

Diagnostic: Confirming the Issue Is Driver-Related

Before replacing hardware, confirm that the issue matches the known driver-removal pattern:

1. Open the Windows Start Menu > Siemens Automation > SET PG-PC Interface (the TIA Portal version, not the legacy STEP 7 version).
2. Change the Access Point of the Application to S7ONLINE (STEP 7).
3. Examine the Interface Parameter Assignment Used list. The available interfaces will be limited to PC Adapter (Auto), PC Adapter (MPI), PC Adapter (PROFIBUS), TCP/IP, and any installed CP 56xx PROFIBUS boards. No COM port entry will appear.
4. Compare with the legacy STEP 7 V5.5 Set PG/PC Interface tool on the same machine. If STEP 7 V5.5 lists PC Adapter (MPI) <-> COM1 and TIA Portal does not, the diagnosis is confirmed.

Additional quick-check:

• Control Panel > Device Manager > Ports (COM & LPT) should show the Prolific PL2303 or FTDI bridge for the PC Adapter. If the COM port is present but TIA Portal cannot see it, the driver is functional but the TIA Portal driver stack does not bind to it — this is the expected behavior.

Solution Path 1: Replace the Cable with a USB-MPI Adapter

This is the lowest-friction solution and is the official Siemens recommendation. The USB PC Adapter is fully supported in all TIA Portal versions from V11 SP2 onward.

Recommended Replacement Part Numbers

Both variants plug into a USB-A port on the engineering station and terminate in a 9-pin D-sub MPI/PROFIBUS connector on the CPU end. No external power supply is required; the cable draws its operating voltage from the USB port.

Configuration Procedure

1. Install the cable using the latest driver from the Siemens support site. Do not rely on the Windows in-box driver — the TIA Portal driver stack expects the vendor-specific driver package.
2. Open TIA Portal and load the project containing the S7-300 station.
3. Select the S7-300 CPU in the project tree, right-click and choose Go online > Target system > PG/PC type > PC Adapter (Auto).
4. Open Online > Accessible nodes. The S7-300 CPU will appear within 10–30 seconds on the first scan.
5. If no device is found, open the Properties dialog of the PC Adapter and verify the following:
   • USB port is selected (auto-detected)
   • Transmission rate: Auto (recommended) or 187.5 kbit/s (CPU default MPI rate)
   • Highest station address: 31
   • Profile: MPI

Solution Path 2: PROFIBUS CP Card (CP 5611 / CP 5621)

For shop-floor engineering stations that need permanent PROFIBUS connectivity, or where the S7-300 project contains PROFIBUS slaves and DP diagnostics, install a Siemens CP card in the PC.

Supported Cards

Commissioning the CP

1. Power down the PC and install the CP card in a free PCI or PCIe slot. For CP 5711, simply plug into a USB 2.0 port.
2. Power on the PC and let Windows enumerate the device. The SIMATIC NET drivers will be installed automatically if TIA Portal V13 or later is present.
3. Launch the SIMATIC NET Commissioning tool (Start > Siemens Automation > SIMATIC NET > Commissioning Wizard) and assign the CP to the S7ONLINE access point.
4. Restart the TIA Portal.
5. Open Online > Accessible nodes. The CP-based PC adapter will appear in the PG/PC Interface dialog as CP 5611 (MPI) or similar.

Solution Path 3: PROFINET / Industrial Ethernet

If the S7-300 CPU has a PROFINET interface — either integrated (CPU 31x PN/DP variants) or via a CP 343-1 — and the PC has a network port, this is the most future-proof path. TIA Portal uses standard TCP/IP and requires no special driver hardware.

Requirements

• CPU 315-2 PN/DP (6ES7 315-2EH14-0AB0) or equivalent with PROFINET interface
• CP 343-1 (6GK7 343-1EX30-0XE0 or current variant) for older CPU 31x that lack PROFINET
• Ethernet switch (managed or unmanaged) and standard Cat 5e / Cat 6 cabling
• Engineering station with a free Ethernet port (1 Gbit/s recommended)

Addressing Steps

1. Configure the CPU's PROFINET interface in the TIA Portal hardware configuration. Assign a unique IP address on the same subnet as the engineering station (e.g. 192.168.0.1 for the CPU, 192.168.0.100 for the PC). Subnet mask 255.255.255.0.
2. Set the PC's Ethernet adapter to DHCP or a static address in the same subnet.
3. Compile and download the hardware configuration to the CPU using the USB or CP-based PC Adapter (first-time setup only).
4. Switch the PG/PC interface to Intel(R) Ethernet Connection / TCP/IP with the S7ONLINE access point.
5. Select Online > Accessible nodes. The CPU will appear with its PROFINET MAC address.

Solution Path 4: Direct Ethernet Cross-Over to CPU 31x-2 PN/DP

For temporary engineering access without a switch, use a crossed Ethernet cable directly between the PC and the CPU's PROFINET port. Most modern Ethernet NICs auto-cross, so a straight cable typically works as well.

Direct connection addressing:

• PC: 192.168.0.100, mask 255.255.255.0, no gateway
• CPU: 192.168.0.1, mask 255.255.255.0

Temporarily disable the Windows firewall on the engineering profile to allow the S7 communication. Re-enable the firewall before returning the station to production.

Migration Strategy: RS232 PC Adapter Users in Legacy Plants

Plants that standardized on the RS232 PC Adapter for ten or more years often have a fleet of twenty or thirty cables and only a handful of engineering stations. The recommended transition path is:

1. Inventory the cables. Distinguish 6ES7 972-0CA23-0XA0 (RS232, not supported) from 6ES7 972-0CA11-0XA0 (RS232, also not supported) and 6ES7 972-0CB20-0XA0 (USB, supported).
2. Procure two USB PC Adapters per engineering station as the minimum. One for primary use, one as a hot spare. Order number 6ES7 972-0CB35-0XA0.
3. For shop-floor terminal PCs that cannot accommodate a USB cable (lockdown, USB ports disabled), install a CP 5621 PCIe card. This is the only PROFIBUS/MPI solution that does not depend on user-portable cables.
4. Retain the RS232 PC Adapters for legacy STEP 7 V5.5 SPx engineering stations that are still under maintenance. They remain functional under STEP 7 V5.5 for the full life of the platform.
5. Document the interface change in the engineering standard so that future commissioning engineers do not waste time searching for the legacy driver in TIA Portal.

Firmware and Driver Compatibility

The SIMATIC PC Adapter USB driver is bundled with TIA Portal from V13 SP1 onward. For V11 SP2 and V12, the driver must be installed separately from the support package. For TIA Portal V18, ensure the Windows 10/11 driver signature accepts the package — older driver releases (pre-V2.1) may not install on Windows 11 22H2 without driver-signing override.

Verification: Confirm Online Access to the S7-300

1. Open TIA Portal and load the project.
2. Select the S7-300 station in the project tree.
3. Right-click the CPU and choose Go online.
4. Confirm the online status icon in the status bar turns green and the diagnostic buffer of the CPU is readable.
5. Open Online > Accessible nodes. The CPU should appear within 30 seconds with its MPI/PROFINET/PROFIBUS address.
6. From the project tree, right-click the Program blocks folder and select Download to device. This verifies both directions of the online channel.

Successful verification points:

• Online status icon: green
• Diagnostic buffer time stamp matches engineering PC clock
• Block download completes without timeout
• Online & diagnostics > Operating mode shows the current CPU mode (RUN / STOP / STARTUP)

Troubleshooting Matrix

References to Siemens Documentation

• S7-300 CPU 31xC and CPU 31x: Technical specifications — official hardware manual covering all CPU 31x variants including the 6ES7 314-6CF01-0AB0 referenced in the source case
• SIMATIC S7 PC Adapter — Manuals — product manual index for the PC Adapter family, with the matrix of supported TIA Portal versions
• SIMATIC S7-300 S7-300 Programmable Controller — System Manual — system manual with chapter on MPI/PROFIBUS networking
• Programming and Operating Manual — STEP 7 Professional V18 — current TIA Portal online chapter that describes Set PG/PC Interface and the absence of the serial PC Adapter

FAQ