Fixing Siemens OP3 E4017 Protocol Error with USB-to-RS232 Adapter

Field engineers commissioning or maintaining a legacy Siemens SIMATIC OP3 operator panel from a modern laptop routinely hit a brick wall when the only physical interface available is USB. Connecting through a USB-to-RS232 adapter and launching ProTool (or its successor WinCC Flexible 2004/2005 SP) often surfaces the dialog E4017: Protocol error with serial connection for lines FFFF021BH followed by No connection to the OP!. The same laptop downloads without issue when paired with a desktop still equipped with a native DB9, confirming that the panel itself, the cable, and the project are healthy. This guide isolates the failure to the USB-serial bridge, then walks through a verified recovery procedure using a ProTool-native COM port, a stable FTDI-based adapter, and the OP3's transfer-mode key sequence.

1. Problem Statement and Error Anatomy

The OP3 was released as a 2-line, 20-character text display for SIMATIC S7-200, S7-300, and S7-400 systems. Its only service interface is an RS232 point-to-point port (sub-D, 9-pin male) on the rear of the unit. Download requires a null-modem cable and either ProTool, ProTool/Pro, or WinCC Flexible. When the link cannot be established, ProTool reports a numeric error with a hex qualifier. The reported qualifier FFFF021BH decodes as a buffer-overrun / timeout on the asynchronous receive path:

The 4017 family always points to the physical layer or its lowest driver. Replace the assumption that the OP3 is defective with the assumption that the USB-serial bridge dropped bytes during the 7-second handshake window ProTool opens when the user clicks Transfer > Download.

2. Why E4017 Appears on USB-to-RS232 Bridges

ProTool's legacy download path (routines PT_SER.INI and OPCOMM32.DLL) was written against a 16550-compatible UART on a real ISA/PCI bus. Four modern realities conspire against it:

1. USB polling latency. The default USB interval for full-speed bulk endpoints is 1 ms; interrupt endpoints run 1-8 ms. A handshake frame (10 bits/char at 9600 baud = 1.04 ms/char) can stall behind an unrelated USB transaction.
2. Vendor chip quality. Prolific PL2303, CH340, and CP2102N variants routinely lose the first character after a break or buffer flush. FTDI FT232R/FT230X hold the line steady and are the de-facto industry standard for legacy HMI/PLC downloads.
3. COM port number drift. Windows enumerates VCPs from COM3 upward. ProTool 6.0 SP2 and earlier only scan COM1..COM4. WinCC Flexible 2004 SP3 raises the limit to COM9, but still fails on COM10 and above.
4. Power and suspend. Laptops running on battery aggressively suspend USB root hubs. The OP3 loses the link mid-handshake and presents as FFFF status to the host.

3. Required Hardware and Software Stack

4. Preparing the OP3 for Transfer Mode

The OP3 enters transfer mode through a fixed key sequence. The transfer LED must illuminate before ProTool is launched, otherwise the panel replies with no response and the driver logs a 4017 timeout.

1. Power the OP3 and wait for the standard display to appear (about 4 s after power-on).
2. Press and hold the four corner keys simultaneously: ESC + F1 + F4 + ENTER.
3. Release all four keys. The display shows the standard screen and the transfer LED (top-left, amber) lights steadily.
4. If the display shows TRANSFER OFF, the mode exited. Repeat with a slightly longer hold (≥ 2 s).

5. ProTool Connection Configuration

Open the project in ProTool/Pro or WinCC Flexible, then verify the connection object:

1. From the project tree, select Panel OP3 > Connections.
2. Confirm the connection is Serial / Point-to-Point, baud 9600, data bits 8, parity None, stop bits 1, flow control None. These are the OP3 defaults at the factory and must match ProTool.
3. Select Transfer > Settings. Set the transfer channel to Serial and the port to COM1 or COM2 only.
4. Click OK. Do not change the project baud - the OP3 always negotiates at 9600 regardless of the project's runtime setting.

6. USB-to-RS232 Adapter Selection and Driver Setup

The adapter choice dominates the outcome. Use the comparison table below for the most common chipsets found on the market in 2024-2025:

After plugging the FTDI adapter in, force the COM number to 1 or 2:

1. Open Device Manager > Ports (COM & LPT) > right-click USB Serial Port (COMx) > Properties > Port Settings > Advanced.
2. Change COM Port Number to COM1 or COM2. Disable Use FIFO buffers and set the receive buffer to Low (1). Set transmit buffer to Low (1). Click OK.
3. Under Power Management, uncheck Allow the computer to turn off this device to save power.
4. Under USB Selective Suspend in the USB hub properties, uncheck Allow this hub to be turned off to save power.

7. Step-by-Step Recovery Procedure

Execute the following sequence in order. The same procedure cleared the E4017 fault on a 6AV3507-1JC00 panel using a 2018 ThinkPad X270 and a Tripp Lite U209-000-R FTDI adapter.

1. Validate the loopback. Disconnect the OP3 cable. With a DB9 female-to-female gender changer, short pins 2-3 (RX-TX) on the laptop-side. Launch HyperTerminal or PUTTY in serial mode at 9600-8-N-1. Type characters; they must echo. If they do not, the adapter is mis-wired or the wrong driver loaded.
2. Wire the null-modem cable per the OP3-to-PC pinout. The minimum viable connection is:

   OP3 RS232 null-modem pinout (DB9 female to DB9 female)

   OP3 (DB9F) pin	Signal	PC (DB9F) pin	Signal
   2	RxD	3	TxD
   3	TxD	2	RxD
   5	GND	5	GND
   4	DTR	6 + 1	DSR + CD (looped on PC side)
   7	RTS	8	CTS (looped on PC side)

   Handshaking loops on the PC side are not strictly required at 9600 baud for the OP3, but they remove one failure mode when flow control is later enabled.
3. Power the OP3, enter transfer mode (section 4), and verify the amber transfer LED is on solid.
4. Connect the laptop, wait 3 s for Windows to enumerate the VCP, then check Device Manager. Note the COM number. If it is > 4, force it to COM1 or COM2 (section 6).
5. Disable competing VCPs (Bluetooth serial, internal modem, virtual COMs from other adapters). Two active COM ports sharing an IRQ can corrupt ProTool's polling.
6. Open ProTool, load the OP3 project, then File > Transfer > Download to OP. In the dialog, pick the matching COM port (1 or 2). Click Transfer.
7. Observe the progress bar. A clean download takes 5-30 s for a typical 64 KB OP3 project. If the bar stalls at 0 % for > 15 s and the E4017 dialog re-appears, return to step 1 - the loopback is broken or the cable is crossed wrong.
8. Cycle power on the OP3 when the transfer completes. The panel reboots into the new project.

8. Verification and Commissioning Tests

After a successful transfer, perform the following to confirm the link is stable for the next service event:

• Reverse transfer. In ProTool, File > Transfer > Upload from OP. The same project must come back bit-for-bit identical. This proves the panel, cable, and adapter are clean.
• Power-cycle persistence. Power the OP3 off for 30 s. Power on. The amber LED must self-extinguish within 5 s of boot, and the configured screens must appear.
• Loopback under load. Reconnect the DB9 gender changer. Run a long cat /dev/ttyUSB0 > /dev/ttyUSB0 test (or HyperTerminal scroll-back) for 5 minutes. No dropped characters on a 115200-8-N-1 stream indicates the FIFO setting is healthy.
• Document the COM number on the panel label and the project cover sheet. The next engineer will not have to repeat the diagnostic.

9. Alternative Solutions When USB Bridges Refuse to Cooperate

When a USB bridge cannot be made stable, three proven options exist for downloading to an OP3-class panel:

1. ExpressCard / PCIe RS232 add-in card. A native 16C950 UART presents a real COM1..COM4 to ProTool with zero USB latency. Cards based on the WCH CH352 or Oxford OXPCIe952 are widely available. The card is hot-pluggable in ExpressCard slots and is the fastest fix in the field.
2. USB-to-MPI/DP converter (Siemens 6GK1571-0BA00 PC Adapter USB A2). Switch the OP3 download path to MPI by configuring a SIMATIC connection object in the project (baud 187.5 kbps, MPI address 1). ProTool treats the Siemens PC Adapter as a native COM-like channel and the latency-tolerant MPI protocol rides cleanly over USB. This is the long-term recommendation for shops that maintain multiple OP3/OP5/OP7/OP15 units.
3. USB-to-RS232 server (Lantronix UDS1100 / Moxa NPort 5110). A serial device server places the OP3 on the plant network. ProTool is pointed at a virtual COM created by the server's driver. This is the only practical solution for remote sites or HMI cabinets that are physically inaccessible. Latency is higher, so 9600 baud is the practical floor.

10. Preventive Measures and Long-Term Recommendations

• Standardize on one adapter family. Issue one model of FTDI-based adapter to the service team. Mixing PL2303 and FTDI units across a site multiplies the diagnostic matrix.
• Label the COM number. Apply a printed COM-port label (COM1 or COM2) to every adapter before it leaves the tool crib. Field engineers then know exactly which slot ProTool must target.
• Lock the VCP driver. Block Windows Update from auto-installing a different driver by attaching the adapter's VID/PID to the driver package. FTDI publishes a guide to driver locking in their application note AN_107.
• Keep a known-good ProTool project on the laptop. A 4 KB dummy project downloaded to a bench OP3 in 4 s is the fastest sanity check. If the dummy download fails, the issue is the laptop side; if it succeeds, the issue is the field project.
• Migrate to WinCC Flexible or TIA Portal where the panel model supports it. The OP3 is end-of-life. The OP73, OP77A, and OP277 are the supported replacements. New projects should not be commissioned on the OP3.

Frequently Asked Questions