Siemens OP177B USB Host-to-Host Cable: Programming WinCC flexible

1. Overview of USB Host-to-Host Transfer on the OP177B

The SIMATIC OP177B is a 5.7-inch monochrome operator panel from the OP 170 family. It exposes a single USB 1.1 host (Type-A) port on the underside of the unit, intended primarily for connecting USB memory sticks to perform project upload/download, recipe export, and firmware updates. When the panel must be commissioned or re-loaded from a desktop engineering station running WinCC flexible, that same USB port is reused as a serial-style data channel — but only when paired with an active USB host-to-host bridge cable on the PC side. The OP177B itself does not accept a conventional straight-through USB-A to USB-A extension cable, and connecting one will not damage the hardware but will produce a permanent 'Could not establish connection' error from WinCC flexible and zero enumeration on the Windows device manager.

The reason is electrical and protocol-level: USB is a polled, master-slave bus. Two USB hosts cannot negotiate roles over a passive cable. A bridge cable contains an active protocol converter IC (commonly a Prolific PL-25A1, MCT USB-232 or similar USB-USB bridge) plus a small microcontroller, drawn from the 5 V VBUS pin of each host port. From the operating system's point of view, the PC enumerates one virtual COM port or USB device for each end of the bridge, and the two hosts exchange data through that bridge instead of over a direct electrical link.

This article documents the exact cable specification, driver stack, WinCC flexible transfer channel parameters, and field-proven troubleshooting steps required to reliably transfer a compiled .hmi or .fwx project to an OP177B over USB.

2. Cable Architecture: Why a Plain USB-A to USB-A Cable Fails

A standard USB 2.0 Type-A to Type-A cable sold for printer or hard-drive use is wired pin-for-pin straight through (VCC, D-, D+, GND). It assumes one end is a host (Type-A downstream) and the other is a device (Type-B or Mini-B). Two Type-A connectors therefore imply two hosts, which violates the USB specification. Plugging two hosts together creates contention on the 5 V rail, shorts the D+/D- differential pair through two output drivers, and is in principle a hazard to both ports' over-current protection.

A USB host-to-host bridge cable differs in three ways:

1. Active electronics. A small PCB with a USB bridge controller sits in the middle of the cable. Each Type-A plug is wired only to one side of the bridge; the two sides never touch electrically.
2. Power isolation between ends. The bridge powers itself by drawing minimal current from each VBUS and isolates the data lines through the controller. Some designs include ESD/TVS protection on each port.
3. Driver stack. The bridge enumerates as a virtual USB device or virtual COM port on each host. Each side needs its own driver; the Windows side is the one documented by Siemens on the WinCC flexible DVD.

The official Siemens FAQ document "FAQ USB Transfer" (entry ID 19142034) states: "Connect the USB host-to-host cable to the USB port of the PC. Do not connect the operator panel on the other end until the driver is installed." This sequence matters because Windows must register the bridge controller and assign a port before the panel-side enumeration is attempted; otherwise the OP177B boot ROM cannot handshake.

3. OP177B USB Interface Specifications

The OP177B is also referred to in catalogs as 6AV6642-0BA01-1AX1 (mono, 5.7") and 6AV6642-0BC01-1AX1 (color, 5.7"). The USB host-to-host transfer procedure is identical for both. Note that later Comfort Panels (TP700/1500/1900) replaced this mechanism with a different loader that uses standard USB Type-B device ports and a USB mini-B cable — the bridge cable described here applies only to the OP 170 / OP 177B / OP 277 generation running Windows CE panels configured under WinCC flexible.

4. Prerequisites

Before initiating a transfer, verify the following:

1. Engineering PC running Windows XP SP3, Windows 7 SP1 (32-bit preferred), Windows 10 with WinCC flexible 2008 SP3 or later. 64-bit Windows is supported by WinCC flexible 2008 SP3 but the legacy USB bridge driver is unsigned in some builds; you may need to enable test-signing mode or use a 32-bit Windows virtual machine.
2. WinCC flexible installation with the WinCC flexible Product DVD or downloaded ISO mounted. The USB driver is located in the path Support\USB\WinCE_USB_Host_to_Host\ on the DVD.
3. USB host-to-host bridge cable, USB 2.0 compliant, with active electronics. Field-proven examples include the Lindy USB 2.0 Easy Transfer Cable (article 31987) and the Belkin F3U138 or equivalent Prolific/MCT-based cable. Avoid cables that present as a generic mass-storage device; the WinCC flexible loader requires a USB device class that the panel boot ROM recognizes.
4. Compiled project matching the OP177B target. In WinCC flexible, choose Project > Compiler > All and verify there are zero errors before opening the Transfer dialog.
5. OP177B in Transfer mode: power on the panel and from the loader screen select Transfer. The control panel will display the "Waiting for transfer connection" prompt. The Control Panel is accessible from the OP177B loader via the gear icon at the bottom-left of the screen, or by holding the upper-left corner during boot on firmware V1.1.0 and later.
6. No USB hub between PC and bridge cable. Connect the cable directly to a root-hub port on the PC, preferably a rear motherboard port. Front-panel header ports and external hubs introduce enumeration timing problems that can mask successful transfers.

5. Installing the USB Host-to-Host Driver

The driver installation is the most common point of failure. Follow the sequence strictly.

1. Insert the bridge cable's PC-end Type-A plug into the PC. Do not connect the OP177B to the other end yet. Windows should detect a new USB device and launch the Found New Hardware wizard.
2. If the wizard prompts for a driver location, point it at D:\Support\USB\WinCE_USB_Host_to_Host\ on the WinCC flexible DVD (substitute your actual drive letter). The driver package includes WinCEUSBLAN.inf and the corresponding .sys files.
3. Confirm that Windows Device Manager now lists a "USB Host-to-Host Bridge" device under Universal Serial Bus controllers. Some driver revisions enumerate a virtual Ethernet adapter over USB instead; both are valid.
4. Once the PC-side driver is installed, plug the cable's Type-A plug into the OP177B USB port. The panel will re-enumerate and the Windows device manager will refresh.
5. Reference: see Loading via USB interface (Panels, Comfort Panels, RT Advanced) for the corresponding procedure on TIA Portal V20 targets. Although that page documents Comfort Panels, the driver-install-then-connect sequence is identical.

6. Configuring the Transfer Channel in WinCC flexible

1. Open the compiled project in WinCC flexible.
2. Choose Project > Transfer > Transfer Settings.
3. In the left pane, select the OP177B target. The right pane lists available channels: Serial, MPI/Profibus, TCP/IP, and USB.
4. Set the mode to USB. Tick Enable transfer and Overwrite existing project if you want unattended replacement.
5. If the project contains a runtime-only portion (no image), additionally enable Allow delta transfer to reduce transfer time on large projects.

7. Step-by-Step Transfer Procedure

1. Power on the OP177B. Wait for the loader screen to appear (typically 15–25 seconds from cold start).
2. On the loader, tap Transfer. The status line shows "Waiting for transfer connection on USB".
3. Confirm the OP177B is in Transfer mode (not Start). The panel must be idle, not currently running a runtime image.
4. On the PC, in WinCC flexible, choose Project > Transfer > Transfer (or press Ctrl+F9).
5. WinCC flexible will list detected panels. Select the OP177B entry that matches the panel's order number (visible on the loader screen Control Panel > System > Device).
6. Click Transfer. A progress dialog shows block-by-block transfer of the project. Typical project size 4–10 MB transfers in 30–90 seconds on a USB 2.0 Full-Speed link.
7. When the dialog reports "Transfer completed successfully", tap Start on the OP177B loader to launch the new runtime.
8. If the panel reports "Could not establish connection", do not retry immediately. Unplug both ends, wait 5 seconds, and reseat starting from the PC side. Then repeat.

8. Verification and Post-Transfer Checks

1. Version check: on the OP177B, navigate to Control Panel > System > About and confirm the project version and compilation timestamp match the source .hmi.
2. Tag visibility: from the runtime screen, open a tag-forcing dialog (if enabled) and verify at least one PLC tag is reading correctly. This confirms not only the project loaded but that the connection to the PLC is intact.
3. Touch calibration: tap the four corner crosshairs during the loader's calibration screen; miscalibrated touch is a common side-effect of interrupted transfers.
4. Recipe area integrity: if the project uses internal recipes, navigate to Control Panel > Recipe View and verify a test record persists across a panel reboot.
5. Audit log: enable the diagnostic log in the loader (under Service > Log) and review for "USB enumeration OK" and "Transfer CRC OK" entries.

9. Troubleshooting Matrix

10. Cable Selection and Approved Vendors

Siemens does not sell a USB host-to-host bridge cable as a catalog part number. The OP177B loader ROM was designed to interoperate with any cable that enumerates as a USB device class compatible with the WinCC flexible loader — practically, this means any cable based on a Prolific PL-25A1, MCT WH-188, or similar transparent-USB bridge IC. Field-proven options:

• Lindy USB 2.0 Easy Transfer Cable (art. 31987) — confirmed working across multiple OP177B sites; commonly available in Europe and Middle East IT distributors.
• Belkin F3U138 — equivalent Prolific-based design; widely available in North America.
• Sitecom CN-061 — alternate vendor; verify the IC is PL-25A1 and not PL-2303 (which is a UART bridge, not a USB bridge).

The cable label is typically marked "USB Transfer Cable", "Easy Transfer Cable", "PC-to-PC USB Link Cable", or simply "USB Data Link Cable". It must always ship with a driver CD, because without a driver the cable is useless.

11. Field-Proven Engineering Notes

Three details that routinely cost an hour of commissioning time:

1. The bridge cable must be powered by the host it is plugged into. Do not use a Y-cable that draws VBUS from two ports simultaneously; the dual-feed confuses the bridge's power-management logic.
2. The OP177B does not show a USB icon during transfer. Unlike Windows CE transfer over Ethernet, there is no on-screen indicator on the panel beyond the loader text "Waiting for transfer connection". If that text disappears, the panel has accepted the link and is buffering.
3. Backup before overwrite requires free flash. The OP177B has 4 MB to 8 MB of flash depending on the variant. If a previous project plus its backup exceeds available space, the transfer will fail with "Insufficient memory" even if the new project alone fits. Delete prior backups from the loader's Backup/Restore menu first.

For long-term fleet deployment, standardize on a single cable model and stock at least two spares per site; bridge cable reliability in industrial panels is roughly 3–5 years before the active electronics fail from ESD or flex fatigue.

FAQ