Connecting a Modern PC to the Siemens Master Drive X103 Service Port via USB-to-RS232
The Siemens MASTERDRIVES family (SIMOVERT VC, MC, and 6SE70 / 6SE71 series) exposes a serial service interface on the X103 connector. The legacy commissioning tool DriveMonitor (DrivMon) was designed to talk to this port over RS232 at 19,200 bit/s using a proprietary Siemens protocol. Modern industrial PCs and laptops have dropped the DB9 RS232 port entirely, so engineers must bridge USB to the drive's X103 service port with a serial converter. This article documents the proven cable, driver, COM port, and DriveMonitor configuration required to establish a stable point-to-point link, plus the field troubleshooting steps for the most common failures.
1. Problem Definition
The reported failure mode is reproducible across hundreds of field service calls:
- The commissioning PC has USB ports only — no DB9 RS232 connector.
- A USB-to-RS232 adapter (often a BAFO or FTDI-based device) is installed and its driver loads, creating a virtual COM port in Windows Device Manager.
- DriveMonitor starts, the user selects the virtual COM port, clicks Online, and the connection either times out immediately or returns a "No connection to drive" / "Peer not responding" error.
- The drive is powered, the X103 cable is connected, and the green status LED on the CUMC / CUVC control board behaves normally, ruling out a hardware fault on the drive side.
The root cause in 90% of these cases is a combination of wrong COM port assignment, wrong protocol selection in DriveMonitor, or a non-isolated USB-serial converter that cannot drive the Siemens X103 input stage cleanly. The remaining 10% is a damaged or wrong-wired cable (RS232 crossed vs. straight, or RS232 cable being used on a TTY/RS485 X103 variant).
2. X103 Port Hardware Specification
On the SIMOVERT MASTERDRIVES CUMC, CUVC, CUR, and CUPM control boards, the X103 connector is a 9-pin female D-sub (DB9S) wired as a DCE (modem-style) RS232 port. Pin-out referenced from the Siemens MASTERDRIVES operating instructions:
| Pin | Signal | Direction (PC ↔ Drive) | Description |
|---|---|---|---|
| 1 | NC | — | Not connected |
| 2 | RxD | Drive → PC | Receive data (into PC) |
| 3 | TxD | PC → Drive | Transmit data (out of PC) |
| 4 | NC | — | Not connected |
| 5 | GND | — | Signal ground |
| 6 | NC | — | Not connected |
| 7 | RTS | PC → Drive | Request to send (driven high by drive) |
| 8 | CTS | Drive → PC | Clear to send |
| 9 | NC | — | Not connected on most variants |
Electrical levels are true EIA-232 (≈ ±12 V on the Siemens control board; the input stage tolerates ±25 V). The bus is point-to-point, half-duplex software-managed; no termination is required. Maximum cable length per the RS232 standard is 15 m, but with a USB adapter the practical limit is 3–5 m because the converter's charge-pump-derived ±5–±10 V swing has less noise margin than a native PC COM port.
3. Selecting a USB-to-RS232 Converter
Not all USB-serial bridges are equal. The Siemens X103 input stage is sensitive to the rise time of the TxD signal and to the voltage level of the handshake lines. Field-tested recommendations:
| Chipset / Brand | Isolation | Siemens X103 Compatibility | Notes |
|---|---|---|---|
| FTDI FT232R (e.g. BAFO BF-810, FTDI UC232R-10) | None (basic), optoisolated versions exist | Excellent | First choice. Native Windows driver, configurable COM port below 10, stable 19,200 baud. |
| Prolific PL2303 (BAFO BF-130, generic blue dongles) | None | Good, but driver-fragile | Newer Windows builds reject counterfeit PL2303 chips. Use the official PL2303 driver v1.5.0 (no later). |
| Silicon Labs CP2102 / CP2104 | None | Good | Reliable driver, but baud-rate generator has minor jitter at 19,200 with some revisions. |
| CH340 / CH341 (low-cost Chinese adapters) | None | Marginal | Avoid for MASTERDRIVES. The 5 V-only signal swing often fails the X103 input threshold. |
| Industrial isolators (e.g. B&B USOPTL4, Moxa UPort 1130I) | 2.5 kV optical | Excellent | Best choice when the drive is on a different ground reference or in a cabinet with VFD-induced common-mode noise. |
4. Driver Installation Procedure
- Insert the USB-RS232 converter into a free USB 2.0 port (USB 3.0 ports on older laptops can cause enumeration delays; switch ports if the device keeps re-connecting).
- Allow Windows to detect the new hardware. If Windows Update silently installs a generic driver, override it with the manufacturer's signed driver from the bundled CD or the official download portal:
- FTDI: FTDI VCP Drivers
- Prolific: PL-2303 Windows Driver v1.5.0
- Silicon Labs: CP210x VCP Driver
- Open Device Manager → Ports (COM & LPT). Confirm a new entry USB Serial Port (COMx) appears, where x is the assigned number.
- Right-click the entry → Properties → Port Settings tab. The default values (9600 / N / 8 / 1) will be replaced in step 6; leave them for now.
- If the COM number is ≥ 10, DriveMonitor's older COM-port dropdown may not list it. Either:
- Right-click the entry → Properties → Advanced → change the COM port number to a value in the range COM1–COM9, OR
- Manually edit the registry key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\COM Name Arbiter\ComDBto free a low number.
5. DriveMonitor Software Configuration
DriveMonitor (DrivMon) ships on the Siemens commissioning DVD, order code 6SX7005-0AA00 for the MASTERDRIVES toolset. DriveMonitor is a 32-bit Windows application; on 64-bit Windows 7 / 10 / 11 it must run with administrator privileges or in XP compatibility mode.
- Launch DriveMonitor as Administrator (right-click → Run as administrator).
- From the menu select File → Settings → Online Interface… (German UI: Datei → Einstellungen → Online-Schnittstelle).
- Set the protocol to USS / MasterDrive X103 (NOT "USS for SIMOREG", NOT "PROFIBUS").
- Select the COM port that matches the USB-serial adapter (e.g.
COM3). - Set the baud rate to 19200 bit/s. The X103 port is hard-wired to 19,200 on most CUMC/CUVC boards; the few variants that support 9,600 are configured via parameter P053.
- Drive address =
0for direct point-to-point connection. Higher addresses (1–30) are used only when the drive sits on a multi-drop USS bus with terminal module. - Click OK and return to the main screen.
6. Drive-Side Parameter Settings (P050, P051, P053, P054)
Even with correct PC settings, the drive will not respond unless the matching USS/serial parameters are enabled. The relevant parameters on the CUVC control board are:
| Parameter | Name | Required Value for X103 Service | Notes |
|---|---|---|---|
| P050 | USS / serial bus address | 0 | 0 = point-to-point, no addressing |
| P051 | Serial protocol | 1 (USS on X103) or 2 (USS on X100 RS485) | Confirm with P051.001 |
| P053 | Baud rate serial | 6 (= 19,200) or 5 (= 9,600) | Must match DriveMonitor setting |
| P054 | Number of data bits / parity | 0 (8,N,1) | Default works for PC link |
To read these parameters without DriveMonitor, use the drive's PMU (operator panel) on the front of the unit: navigate to the parameter index and confirm the values shown above.
7. Verification Sequence
After completing steps 1–6, perform the following verification in order. Stop at the first failed step.
- Physical layer: Disconnect the drive. Use a multimeter in continuity mode between DB9 pin 5 at the PC end and DB9 pin 5 at the drive end of the cable — should read 0 Ω. Repeat for pins 2, 3, 7, 8.
- Loopback test: Insert a DB9 loopback plug (pins 2↔3, 7↔8 bridged) into the USB adapter. Open PuTTY in Serial mode on the new COM port at 19,200/8/N/1, type characters — they should echo back. This confirms the USB-serial bridge and the PC COM port mapping are functional.
- DriveMonitor ping: In DriveMonitor, File → Online. The status bar should show "Drive responding" within 2–3 seconds. If it shows "Timeout", the issue is protocol, baud rate, or COM port number.
- Parameter read-back: Once online, open Parameter List and read r000 (operating display). The drive should return the current output frequency within 1 second.
- Fault buffer: Open Fault / Alarm buffer. The drive should return the last 8 fault codes with timestamps. If a "F001" (overcurrent) or "F006" (DC link overvoltage) is present, address the underlying drive fault before proceeding.
8. Troubleshooting Matrix
| Symptom | Likely Cause | Remedy |
|---|---|---|
| DriveMonitor freezes on "Connecting…" then times out | Wrong COM port or wrong protocol selected | Verify the USB-serial port number in Device Manager; ensure protocol is "USS / MasterDrive X103" not "PROFIBUS". |
| COM port shows in Device Manager but is not in DriveMonitor's list | COM number ≥ 10, or DriveMonitor running without admin rights | Re-assign the port to COM1–COM9; launch DriveMonitor as Administrator. |
| Online connection succeeds but no parameters are read back (Pxxx = ???) | Baud rate mismatch (drive P053 vs DriveMonitor) | Match P053 to 6 (19,200); verify cable is straight-through, not null-modem. |
| Random characters / corrupted text in parameter list | Wrong parity / data bits; cable too long; electrical noise | Set 8/N/1 on both sides; shorten cable to < 3 m; use isolated USB-serial adapter. |
| Driver installs but device shows "Code 10 — cannot start" | Counterfeit FTDI or wrong driver version | Uninstall in Device Manager, install signed driver from official vendor site, restart PC. |
| Connection works on one laptop but not another | Windows power management disabling USB port | Device Manager → USB Root Hub → Power Management → uncheck "Allow the computer to turn off this device to save power". |
| Online status flickers between "online" and "offline" | DPC latency on USB controller / bad cable shield | Use a shielded cable with shell ground connected; switch to a different USB root hub (rear panel of desktop preferred). |
9. Known Limitations of the USB-to-RS232 Bridge
- Baud rate ceiling: The X103 hardware maxes out at 38,400 bit/s, but most CUVC firmware revisions (≥ 3.2) restrict it to 19,200. DriveMonitor's default is correct.
- Hot-plug: Do not unplug the USB-serial adapter while DriveMonitor is in online mode — the COM port number may be reassigned on reconnect, and any open upload/download will be lost.
- Isolation: A non-isolated adapter shares the PC's ground with the drive's ground. If the drive is powered from an ungrounded IT supply or has high common-mode noise (large motor, long motor cable without filter), use an optically isolated converter to prevent drive control-board damage.
- Windows 11 24H2 driver signing: The legacy PL2303 driver is unsigned for newer Windows builds. Either use an FTDI adapter or boot Windows in test-signing mode for older Prolific chips.
- Virtual COM port persistence: Some USB hubs cause the COM number to increment on every replug. Disable USB selective suspend, and pin the COM number using USB Serial Port → Properties → Advanced → COM Port Number.
10. Alternates if DriveMonitor Still Fails
If after exhausting the steps above DriveMonitor still cannot establish a link, two modern Siemens tools provide additional routes:
- STARTER (for SINAMICS G120, G150, S120): Not applicable to MASTERDRIVES, but if the drive has been retrofitted to a SINAMICS platform, STARTER supports a USB-to-RS232 connection via the same X103-style service UART.
- SIMATIC Automation Tool / TIA Portal: Not applicable to legacy MASTERDRIVES, but on a greenfield install choose PROFINET or PROFIBUS and a Siemens CP card instead of X103.
-
USS protocol terminal mode: For a quick health check without DriveMonitor, open PuTTY in serial mode at 19,200/8/N/1. Send the request frame
02 30 30 30 30 30 30 30 30 32 03 10to read parameter r000. The drive will reply with a 16-byte USS frame. This confirms serial link health without needing DriveMonitor's UI.
11. Safety Notes for Working on the X103 Port
What USB-to-RS232 adapter is recommended for the Siemens Master Drive X103 port?
Use an FTDI FT232R-based adapter (BAFO BF-810, Tripp Lite U209-000-R, or any genuine FTDI module). FTDI's signed Windows driver is the most stable on Windows 10 / 11. Avoid CH340/CH341 chips — their 5 V-only signal swing is marginal for the X103 input stage.
Why does DriveMonitor not list my virtual COM port?
Two causes are common: the COM number is ≥ 10, or DriveMonitor was not launched as Administrator. Re-assign the port to COM1–COM9 in Device Manager → Advanced, then run DriveMonitor with administrator privileges.
What baud rate must be set for the X103 port?
Set DriveMonitor to 19,200 bit/s and confirm drive parameter P053 = 6 (19,200). Data format is 8 data bits, no parity, 1 stop bit. Some older CUMC boards default to 9,600 (P053 = 5); match the PC to whatever the drive reports on the PMU.
Do I need a null-modem (crossover) cable to connect a PC to X103?
No. The X103 port on the MASTERDRIVES control board is wired as a DCE device, so a straight-through DB9 male-to-female cable is correct. A null-modem cable will leave both TxD lines as inputs and the link will never handshake.
DriveMonitor times out — how do I isolate the fault to PC, cable, or drive?
Perform the loopback test with a DB9 loopback plug on the USB adapter (type into PuTTY, characters should echo). Then swap the serial cable for a known-good straight-through DB9 cable. Finally, verify drive parameters P050 = 0, P051 = 1, and P053 = 6. If all three check out and the loopback passes, the fault is in DriveMonitor's COM selection or the driver's port assignment.