Resolving OP73 Transfer via PC-PPI Cable in WinCC Flexible 2008

The SIMATIC OP73 is a 3-inch monochrome operator panel from the Siemens SIMATIC HMI family, designed for compact machine applications and S7-200 PLC integration. Transferring a compiled WinCC Flexible 2008 project to the OP73 over an RS232/PC-PPI cable is a routine commissioning task that fails in a specific, recurring pattern: the cable's RX and TX LEDs blink during the operation, the transfer dialog in WinCC Flexible shows progress, and then the operation aborts with a "connection to the panel could not be established" or "transfer aborted" error. This article documents the root cause, the exact configuration values, and the field-tested diagnostic sequence for resolving the issue.

OP73 Hardware and Compatibility Reference

The OP73 is the smallest monochrome panel in the SIMATIC HMI OP family. It predates the TP (Touch Panel) and Comfort lines and remains in service on legacy S7-200 machines. New configurations should use a KTP400 Basic or TP700 Comfort panel; the OP73 is supported only for maintenance of existing installations. The official Siemens Industry Online Support portal hosts the OP73 device manual, firmware images, and WinCC Flexible compatibility matrices.

Root Cause Analysis

The transfer failure has three layered causes, ordered by probability:

1. Baud rate mismatch. The 6ES7 901-3CB30-0XA0 PC/PPI cable is set to 115200 bps via DIP switches 1 and 2 both ON. The OP73 cannot communicate at 115200 bps. The cable transmits frames at 115200 bps; the OP73 samples the bus at a different rate (187.5 kbps by default), never sees a valid byte, and never responds. RX/TX activity on the cable reflects the PC's transmission only, not successful two-way communication.
2. Wrong access point in PG/PC interface. The source configuration sets the access point to "WinCC MPI" instead of "WinCC flexible" or "WinCC PPI". The MPI access point wraps project data in MPI frames, which the OP73 rejects even when the baud rate is correct. The PC/PPI cable does not perform protocol conversion; it only adapts the electrical levels and frames the data as PPI.
3. Panel not in Transfer screen. The OP73 must display the "Transfer" setup screen to accept a download. If the panel is showing the project runtime or is in any other menu, it polls for incoming connections but will not accept a project transfer without operator confirmation on the system menu's Transfer entry.

PC/PPI Cable 6ES7 901-3CB30-0XA0 DIP Switch Configuration

The 6ES7 901-3CB30-0XA0 is the RS232-to-RS485 PC/PPI cable for connecting a PC COM port to a SIMATIC S7-200 or OP-series HMI panel. The cable contains an RS232/RS485 transceiver and a microcontroller that handles PPI protocol framing. It draws power from the RS232 DTR and RTS lines; no external power supply is required.

The cable has an 8-position DIP switch on the side of the housing. The first four switches control baud rate, PPI/Freeport mode, and master/slave behavior. Switches 5 through 8 are reserved and must remain OFF.

Transfer Topology and Wiring

The physical layer of an OP73 transfer consists of three nodes: the engineering PC running WinCC Flexible, the PC/PPI cable, and the OP73 panel. The PC/PPI cable is a protocol converter that wraps user data in PPI frames on the RS485 side and exposes a transparent RS232 UART on the PC side. The PC/PPI cable does not require any driver software beyond the Windows COM port driver for the RS232 variant. The following SVG diagram shows the topology, baud rate, and protocol assignments for a correct configuration.

The OP73 connector pinout is a 9-pin Sub-D male. Pin 3 is RS485 B (inverting), pin 8 is RS485 A (non-inverting), and pin 5 is signal ground. The PC/PPI cable's PPI end matches this pinout directly; no crossover is required. If using a custom cable, ensure that the A/B pair is not crossed and that 120 ohm termination is installed at the far end of the bus if the cable is longer than 50 m.

WinCC Flexible 2008 Transfer Channel Configuration

WinCC Flexible 2008 SP3 is the final release to support the OP73 panel. The transfer channel is configured in two locations: the project transfer settings dialog and the PG/PC interface control panel. Both must agree on the access point, COM port, and baud rate.

Step 1: Configure the PG/PC Interface

Open "Set PG/PC Interface" from the Windows Control Panel. Under "Access Point of the Application", select WinCC flexible or WinCC PPI. Do not select "WinCC MPI" or "WinCC Profibus" — those access points wrap the project data in MPI or Profibus frames that the OP73 cannot decode. The PC/PPI cable does not perform protocol conversion; it only adapts the electrical levels and frames the data as PPI.

Under "Interface Parameterization Used", select PC/PPI cable (PPI) and click Properties. Verify:

• Connection: COM1 (or whichever COM port the cable is on)
• Baud rate: 187.5 kbps
• Address: 0 (the PC's PPI master address)
• Timeout: 3 s (default)

Step 2: Configure the Project Transfer Settings

In WinCC Flexible 2008, open the project and select Project → Transfer → Transfer Settings. Configure:

Click "Transfer" to initiate the download. The output window logs each PPI frame exchange and the transfer progress. If the OP73 does not respond within the timeout, the output window displays the error "Connection to the panel could not be established" and aborts the transfer.

OP73 Panel Transfer Mode Setup

The OP73 must be in the Transfer setup screen before any PC-initiated project download will succeed. The Transfer screen puts the panel in a state where it actively listens for PPI connect requests and accepts project data on receipt of a valid password (if one is configured in the project).

1. Apply 24 V DC to the OP73 and wait for the project runtime or system menu to appear.
2. Press the system menu key (the top-left function key) to enter the OP73 main menu.
3. Use the cursor keys to navigate to the Transfer entry and press ENTER.
4. The Transfer screen displays the current channel 1 status, baud rate, PPI address, and the message "Waiting for transfer...".
5. Leave the panel on this screen. Do not press ESC; the transfer screen must remain active for the duration of the download.

Step-by-Step Diagnostic Procedure

Apply the following sequence in order. Each step verifies a single layer of the transfer stack and isolates the failing layer before moving to the next. The order is from physical layer (cable, COM port) up to application layer (WinCC Flexible project).

1. Verify the Cable Type and COM Port

Confirm that the cable is the RS232 PC/PPI cable 6ES7 901-3CB30-0XA0. The USB variant 6ES7 901-3DB30-0XA0 uses a different driver set and is not interchangeable without reconfiguring the access point. The RS232 cable does not require a driver beyond the standard Windows COM port driver.

Open Windows Device Manager and confirm the COM port number assigned to the cable. Open a terminal program (PuTTY, Tera Term, or the built-in Windows HyperTerminal) at 187500 bps, 8 data bits, no parity, 1 stop bit. Short the cable's RS485 pins 3 and 8 together (this loops back the transmitted data to the receiver). Characters typed in the terminal should appear in the receive window. This confirms the cable is functional at 187.5 kbps.

2. Set the Cable DIP Switches to 187.5 kbps

Power off the PC and the panel. Set DIP switches 1 and 2 on the PC/PPI cable to OFF (187.5 kbps). Set switch 3 to OFF (PPI local master mode) and switch 4 to OFF (master). This is the canonical configuration for OP73 project transfer. Apply a small label to the cable housing with the switch position and the date; this prevents accidental reconfiguration.

3. Configure WinCC Flexible Transfer Settings

Open the project in WinCC Flexible 2008 SP3. Select Project → Transfer → Transfer Settings. Configure the parameters as listed in the table above. Save the project before transferring. The transfer dialog uses the compiled project binary, not the source project, so any change to the project requires a recompile before transfer.

4. Configure the PG/PC Interface

Open "Set PG/PC Interface" from the Control Panel. Under "Access Point of the Application", select "WinCC flexible" or "WinCC PPI". Under "Interface Parameterization Used", select "PC/PPI cable (PPI)" and click Properties. Verify the COM port and baud rate match the previous step.

5. Put the OP73 in Transfer Mode

On the OP73, enter the system menu, navigate to "Transfer", and confirm. The panel displays the Transfer screen showing the current channel parameters. Do not press ESC; the transfer screen must remain active.

6. Initiate Transfer

Return to WinCC Flexible and click "Transfer". The output window displays the progress. RX and TX LEDs on the cable blink as frames are exchanged. A successful transfer completes with a "Transfer completed" message; the panel restarts and displays the new project.

Error Message Reference

The following table maps the error messages reported by WinCC Flexible during a failed OP73 transfer to their most likely causes and resolutions.

Transfer Timing Calculations and Baud Rate Formulas

Understanding transfer timing helps determine why a slow baud rate or large project produces a long transfer window. The relevant formulas are:

For a 128 KB project at 187.5 kbps, the theoretical minimum transfer time is approximately 7 seconds. Practical transfer times are 30 to 90 seconds because PPI framing, retries, and panel flash-write overhead add substantial latency. If the transfer appears to hang at a particular percentage, observe the TX LED on the cable: a steady or rapid-blinking TX LED indicates the PC is still sending; a dark TX LED indicates the PC has stopped and the transfer has timed out.

At 115200 bps (the wrong setting in the source scenario), the OP73 does not sample the bus at the correct bit timing and discards incoming frames. From the PC's perspective, the transfer appears to proceed normally until the first response is expected, at which point the timeout fires and the transfer aborts.

Common Pitfalls and Field-Tested Solutions

USB PC/PPI Cable, Cable Length, and EMI Considerations

If the engineering workstation has no RS232 port, the USB variant 6ES7 901-3DB30-0XA0 is required. This cable installs as a virtual COM port using the Siemens USB-to-Serial driver. The cable's DIP switch rules are identical to the RS232 cable: for OP73, set switches 1+2 to OFF-OFF (187.5 kbps), switch 3 to OFF, switch 4 to OFF.

Install the driver before plugging in the cable. After installation, confirm the COM port number in Device Manager. Windows 10 and Windows 11 typically assign COM3 or higher. The PG/PC interface configuration in WinCC Flexible must use the assigned COM port number, not the default COM1.

RS485 cable length is a function of baud rate. At 187.5 kbps, the maximum cable length is 1000 m (3280 ft) when using shielded twisted pair cable with proper termination per EIA-485. At 115200 bps, the maximum is also 1000 m. However, industrial environments with VFD-induced EMI can reduce the practical cable length to 50-100 m. If the OP73 is more than 50 m from the engineering station, use a shielded cable and add 120 ohm termination resistors at both ends of the RS485 bus.

The PC/PPI cable is not rated for outdoor or wet environments. If the OP73 is in a panel on a machine, bring the engineering station to within cable reach rather than extending the RS232 link with boosters. Avoid routing the PPI cable parallel to VFD output cables; cross VFD cables at 90 degrees and maintain at least 200 mm separation.

Migration, Safety, and Field Best Practices

The OP73 is not supported in TIA Portal V13 or later. WinCC Flexible 2008 SP3 remains the only HMI configuration software that supports the OP73, and WinCC Flexible 2008 is itself end-of-life as of 2017. If the workstation is upgraded to TIA Portal V16 or later, the OP73 project cannot be opened. Migration requires a panel swap to a SIMATIC TP700 Comfort or KTP400 Basic, and the project must be ported using the TIA Portal migration tool. The OP73's PPI-only interface means it cannot be reached over PROFINET, so the migration also requires a panel that supports Ethernet or PROFINET.

For new installations, specify a TP or Comfort panel rather than an OP73. The OP73 is a legacy product for maintaining existing S7-200-based machines.

Always disconnect the OP73 from any active PPI bus (e.g., S7-200 CPU port 0 or port 1) before performing a project transfer. The PC/PPI cable acts as a PPI master and will conflict with the S7-200 master on the same bus. If the OP73 is connected to an S7-200 port, power off the S7-200 or disconnect the PPI cable from the bus before connecting the PC/PPI cable. After the transfer, restart both devices to re-establish the bus.

Field-tested best practices for OP73 commissioning:

• Archive the WinCC Flexible project source (.hmi or .fwd) to a network share before each transfer. This allows recovery if the panel firmware is corrupted during a botched transfer.
• Label the PC/PPI cable with the current DIP switch settings and the date of last change. Switches can be toggled accidentally during transport.
• Maintain a known-good spare PC/PPI cable and a known-good spare OP73 panel on site. The fastest way to isolate a transfer failure is to swap the cable and panel with spares.
• Document the OP73 firmware version in the project source notes. When migrating between WinCC Flexible SP levels, the firmware version must match the SP's compatibility list.
• Use a dedicated COM port number (e.g., always COM3 for the PC/PPI cable) and disable other COM ports in Device Manager to avoid driver conflicts.

Recommended spare parts for OP73 maintenance:

FAQ