Siemens Masterdrive VC/MC RS232 Cable Wiring & ExpressCard

Problem Definition: DriveMonitor Cannot Connect to Masterdrive VC/MC via ExpressCard Serial Adapter

The reported fault is unambiguous and reproducible. A Toshiba L40 laptop equipped with an ExpressCard-to-serial bridge runs DriveMonitor 5.4 and SimoVis without issue when communicating with SIMATIC S7-300 and S7-400 CPUs over MPI/PPI. The same software, same adapter, and the same USB/PCMCIA stack cannot establish a USS session with a SIMOVERT MASTERDRIVES Vector Control (VC) or Motion Control (MC) drive in the 6SE70 family. A second laptop with a native DB-9 COM1 connects to the same drive using the identical wiring harness and enumerates all parameters correctly inside DriveMonitor.

This pattern isolates the failure to the ExpressCard-to-serial bridge, not the cable, the drive, or the parameterization tool. Engineers must verify two independent variables before replacing hardware or sending the drive for repair:

1. Cable pinout against the SIMOVERT MASTERDRIVES X300 serial port specification. A cable that passes MPI traffic to a CPU will not necessarily establish USS communication with a 6SE70 drive because the pinout is non-standard.
2. ExpressCard bridge timing, buffering, and FIFO handling against the USS frame structure. Many consumer-grade USB/ExpressCard serial adapters break RS485 half-duplex turnaround or do not support the control line toggling the 6SE70 board expects.

Drive Identification: Masterdrive VC vs MC vs Micromaster

Siemens uses two distinct product families that are frequently confused in the field. Correct identification is mandatory because cable pinout, default USS address, parameter tree, and commissioning software differ between them.

The Masterdrive serial port X300 is electrically an RS485 interface, but Siemens specifies a direct RS232-to-drive cable that does not include an external converter. This is possible because the CUVC/CUMC board implements the line transceiver and termination on-board and accepts the RS232 levels from the PC when the cable wiring is built to specification. The cable pinout is not a null-modem, not a straight-through, and not the same as the MPI cable used on S7-300/400.

Hardware and Software Prerequisites

Before opening any diagnostic session, confirm the following items are available and verified:

• Drive type identification: Order number from the drive nameplate must begin with 6SE70. Record the full MLFB (e.g., 6SE7016-1TA61-Z) and the firmware version visible on the CUVC/CUMC display at power-up (r060 or similar).
• PC with native COM port or a known-good USB-to-serial bridge (FTDI FT232R-based recommended, Prolific PL2303 HXA/XA only).
• Siemens DriveMonitor 5.4 SPx for 6SE70 drives. DriveMonitor is bundled inside SIMATIC DriveMonitor, available from Siemens support portal under order number 6SX7005-0AB00 or successor media. SimoVis is a separate parameterization/graphing tool commonly used for commissioning and trend capture.
• Point-to-point cable matching the wiring table in the next section. Cables labeled "Siemens 6SE7090-0XX84" or built to the wiring specification below are acceptable.
• Drive powered up with the CUVC/CUMC LED state machine in the "Ready" condition (green LED steady, no fault on PMU display).
• Administrator rights on the Windows host to set COM port number, latency timer, and to disable power management on the ExpressCard slot.

Serial Cable Pinout and Wiring Specification

The reference cable wiring for a PC RS232 DB-9 female to a SIMOVERT MASTERDRIVES X300 DB-9 male is shown in the table below. This wiring is point-to-point only; do not use it on a multi-drop USS bus where a 6SN11xx terminal block is required.

Build rules:

1. Use shielded twisted pair, 24 AWG minimum. Connect the shield to metal connector housing on the drive end only; do not ground at the PC end to avoid ground loops.
2. Pins 1, 5, and 9 are shorted together on the drive-side DB-9 male connector. This is a hardwired short inside the connector backshell, not a function of the drive. Pin 5 on the drive side is not used in the three-wire configuration but must be tied to the reference ground to maintain the termination network.
3. Maximum cable length is 10 m for the RS232 path to the drive's internal transceiver. Beyond this, signal integrity degrades; use a Siemens RS485 repeater (6SE7090-0XX84) for longer runs.
4. Do not connect PC pins 4 (DTR), 6 (DSR), 7 (RTS), 8 (CTS), or 1 (DCD) to anything. Leaving them open on the PC side is mandatory. Bridging these lines on the PC side will cause some ExpressCard adapters to assert loopback or carrier-detect incorrectly.

The cable referenced in the field report as having been tested with S7-300/400 will not work for the Masterdrive because the S7-300/400 MPI cable is a null-modem with control-line bridges that bring pins 4-6 and 7-8 together. That cable is acceptable for MPI/PPI to the PLC but cannot be used on the 6SE70 X300.

ExpressCard-to-Serial Bridge Failure Modes

Several chipset-level failure modes appear when an ExpressCard-to-serial bridge is substituted for a native UART. Each one is independently capable of breaking USS communication while leaving MPI/PPI traffic functional. Verify the chipset of the ExpressCard adapter before deeper troubleshooting.

Specific failure modes observed on Toshiba L40 with ExpressCard:

1. Latency timer too high. Windows default is 16 ms. USS uses 9600 bit/s with frame turnaround < 5 ms. Setting FtdiLatencyTimer to 1 ms in Device Manager / Advanced settings for the COM port resolves timeout frames.
2. Driver power management. Windows may suspend the ExpressCard controller after 30 s of idle. Disable USB selective suspend for the root hub and uncheck "Allow the computer to turn off this device to save power" on the COM port power management tab.
3. COM port renumbering after reboot. DriveMonitor stores the COM port in the project file. If the ExpressCard enumerates as COM5 today and COM7 tomorrow, DriveMonitor will timeout. Fix COM port to COM2 or COM3 via Device Manager / Advanced / Com port number.
4. Even parity bit truncation. Some low-cost PL2303 clones drop the parity bit. USS uses 8E1. Enable parity bit forcing in the driver registry or replace the adapter.
5. Inadequate line driver. CH340/CH341 boards commonly output 3.3 V TTL levels instead of true RS232 +/- 5 V to +/- 12 V swings. The X300 driver board expects at least +/- 3 V at the connector; some TTL boards fall outside this range, producing intermittent framing errors that look like "drive not responding".

DriveMonitor 5.4 Configuration

After verifying the cable pinout and replacing or reconfiguring the ExpressCard adapter, configure DriveMonitor 5.4 as follows.

1. Launch DriveMonitor 5.4. Select Online → Connect to target system.
2. On the Interface dialog, set:
   • Interface type: USS / Serial interface
   • COM port: select the fixed COM port assigned to the ExpressCard (e.g., COM2)
   • Baud rate: 9600 bit/s (factory default for 6SE70)
   • Number of data bits: 8
   • Parity: Even
   • Stop bits: 1
3. Set the USS address to 0 for the first drive. DriveMonitor will scan a range if configured.
4. Click Connect. The PMU display on the drive should flicker briefly as the USS frame is acknowledged. DriveMonitor should report "Connection to drive established".

USS Protocol Parameters in the Drive

The 6SE70 parameters that govern USS serial communication must match DriveMonitor exactly. The most common combinations are documented in the table below. Use the PMU (parameter display) on the drive front panel to navigate if DriveMonitor cannot connect.

PMU navigation: Hold the P key on the CUVC/CUMC panel, press the up arrow to increment the parameter number, then press P to display the value. Use the up/down arrows to change. Press P again to confirm. The drive must be in state "Ready to switch on" (r001 = 0110 0111 bin) to accept parameter changes.

SimoVis Parameterization Setup

SimoVis is an optional visualization tool used for parameter list editing, trace capture, and commissioning diagnostics. It uses the same USS protocol as DriveMonitor but presents the parameter tree as a graphical control panel. Configuration steps are:

1. Launch SimoVis from Start → Siemens Automation → DriveMonitor → SimoVis. SimoVis ships with DriveMonitor 5.4 SP3 and later.
2. From the File → New menu, select MASTERDRIVES VC or MASTERDRIVES MC as the device type. Do not select MICROMASTER.
3. Open Settings → Interface and confirm the COM port, baud rate, and USS address match the values in the table above.
4. Click Go online. SimoVis will scan the USS address range and report all detected drives.
5. Use the Trace tab to monitor parameters r001 (drive state), r002 (output frequency), r024 (output voltage), and r028 (motor current) live while the drive is running.

Step-by-Step Commissioning Procedure

This procedure restores communication when DriveMonitor cannot connect to a 6SE70 VC/MC drive via an ExpressCard-to-serial bridge. Execute the steps in order. Do not skip verification points.

1. Verify drive identification. Confirm the nameplate reads SIMOVERT MASTERDRIVES with a 6SE70 order number. If the drive is a MICROMASTER 4 (6SE32) or MICROMASTER 6, stop and refer to the MM4/MM6 commissioning procedure instead.
2. Power the drive up and wait for the CUVC/CUMC board to complete its self-test. The PMU should display a value such as 0.00 Hz or the drive state code Sxx.
3. Build or verify the cable against the wiring table in the Serial Cable Pinout section above. Test continuity with a multimeter: PC pin 2 to drive pin 7, PC pin 3 to drive pin 2, PC pin 5 to drive pin 1, and drive pins 1-5-9 shorted together.
4. Plug the cable into PC COM port and drive X300 with the drive powered down. Confirm the cable seats fully. Hand-tighten the connector screws.
5. Configure the ExpressCard adapter:
   • Open Device Manager, locate the COM port under Ports (COM & LPT).
   • Open Properties → Port Settings → Advanced. Set COM port number to a low value (COM2 or COM3) so it does not renumber.
   • Set latency timer to 1 ms (FTDI drivers) or to 1 ms via registry for Prolific (HXA revision or newer).
   • Uncheck power management box.
6. Launch DriveMonitor 5.4 and configure the interface as in the DriveMonitor section above.
7. Click Connect. Observe the drive PMU display: if it flickers when DriveMonitor sends a frame, the link layer is alive.
8. Verify by reading parameter P700. The value displayed in DriveMonitor must match the drive PMU.
9. Save the project in DriveMonitor to retain the working COM port assignment.

Verification and Acceptance Test

After the connection is restored, perform the following verification steps to confirm reliable bidirectional communication before releasing the system.

1. Read-back test: Read parameter P095 (drive identity) and verify it matches the expected MLFB prefix 6SE70.
2. Write/read-back test: Write parameter P700 = 1, then read back. If the read-back returns 1, the write path is functional. Reset P700 = 0 to restore default.
3. Trace capture: Open the DriveMonitor trace buffer, record for 60 s while the drive is in standstill, then save the buffer. The trace should contain no USS CRC errors.
4. Hot-insert test: Unplug the ExpressCard and reinsert. Confirm COM port number does not change and DriveMonitor reconnects within 10 s.
5. Long-run stability test: Leave DriveMonitor online for 30 minutes with no user interaction. Any COM timeouts indicate buffer under-run on the ExpressCard bridge; replace the adapter.
6. Cold reboot test: Reboot the laptop, then launch DriveMonitor. Connection should establish without manual port reconfiguration.

Troubleshooting Matrix

The matrix below maps common symptoms observed in the field to root causes and the corrective action.