Siemens 6ES5 734-1BD20: USB Connection and STEP 5 VM Setup

The Siemens 6ES5 734-1BD20 is a PC-TTY programming cable used to connect SIMATIC S5 series PLCs to programming devices such as the PG 740 PIII. The original PG hardware is long obsolete, but the cable itself remains usable when paired with a USB-to-serial adapter and STEP 5 running in a virtual machine. This reference covers cable identification, TTY electrical interface behavior, USB-to-serial selection, STEP 5 versioning, and VMware/VirtualBox COM port mapping to bring a legacy S5 online from a current Windows laptop.

Cable Identification and Specifications

The 6ES5 734-1BD20 is part of the Siemens SIMATIC S5 accessory family. It is a shielded multi-conductor cable with a 25-pin D-sub (DB-25) on the PC end and a 9-pin TTY connector on the PLC end. The DB-25 was chosen because the original Siemens PG 7xx programming devices used a 25-pin COM port. The 9-pin end is sized to match the TTY programming port on the S5-90U and S5-95U CPUs. For S5-100U and larger CPUs, which use a 15-pin TTY port, an adapter or a different cable variant (such as 6ES5 734-2xxx) is required.

The cable is sometimes listed in the third-party market under the same Siemens part number. Listings on the cable confirm the 25-pin PC connector and the 9-pin TTY connector on the S5 end.

TTY vs RS-232: Why a Direct Serial Cable Will Not Work

The 6ES5 734-1BD20 is not a straight RS-232 extension. The S5 programming port uses a 20 mA current loop (TTY) interface, which is electrically distinct from the voltage-based RS-232 used on PC COM ports. The cable contains an active or passive converter that translates between the two electrical standards.

The TTY interface in the S5-90U and S5-95U is opto-isolated. The current loop driver inside the CPU sources or sinks 20 mA on the transmit and receive pairs, and the cable converts that to RS-232 levels on the PC side. Because TTY is isolated, ground potential differences between the laptop and the PLC cabinet are tolerated, which is one of the reasons Siemens chose TTY for the S5 platform.

AS511 Protocol Layer

Above the electrical layer, the S5 CPU speaks the AS511 protocol (also called the S5 PG protocol). AS511 is a master-slave, byte-oriented protocol where the PG (master) polls the CPU (slave) for block contents, status, and force values. The protocol is fixed at 9600 bit/s, 8 data bits, even parity, 1 stop bit. STEP 5 (running on the PC) is the only master; the CPU does not initiate traffic.

Key AS511 frame characteristics:

• Frame delimiter: 0x02 (STX) for commands, 0x03 (ETX) for data blocks.
• Checksum: 8-bit additive checksum on payload bytes.
• Retransmit on checksum mismatch: master retries up to 3 times before declaring error.
• Block types: OB (organization block), PB (program block), FB (function block), DB (data block), SB (sequence block), FX (extension block).

The 6ES5 734-1BD20 does not perform any protocol conversion; it only converts the electrical layer. STEP 5 speaks AS511 directly to the CPU's TTY port. This is important when troubleshooting: a port with the wrong baud rate, parity, or stop bits will result in AS511 timeout, not silent corruption.

Prerequisites for Modern PC Connection

To bring the 6ES5 734-1BD20 online from a current laptop, the following items are required:

1. Working cable: Inspect the 6ES5 734-1BD20 for crushed jackets, broken connector backshells, or oxidized pins. Damage to the inline TTY converter will prevent communication even if the conductors are continuous.
2. USB-to-serial adapter: A VCP-capable adapter based on FTDI FT232, FT230X, or similar proven chipset. Avoid unbranded adapters using CH340 or unknown chipsets for industrial legacy work.
3. DB-9 to DB-25 straight-through adapter: Most USB-to-serial adapters output a DB-9 male. The 6ES5 734-1BD20 has a DB-25 male. A 1:1 straight-through gender changer is required. Do not use a null-modem adapter.
4. STEP 5 software: STEP 5 V6.6 HF02 is the last shipping release and supports Windows XP and Windows 7 32-bit. Older V7.x versions are 16-bit and run on DOS or Windows 3.11.
5. Virtualization platform: VMware Workstation Pro/Player, Oracle VirtualBox, or Hyper-V with a Windows XP or Windows 7 32-bit guest. Native Windows 7 32-bit host is workable but unsupported by Microsoft.
6. PLC access: Physical access to the S5-90U/95U CPU's 9-pin TTY port, or the corresponding 15-pin port plus an adapter for larger S5 models.

Selecting a USB-to-Serial Converter for STEP 5

STEP 5 communicates through a Win32 COM port handle. The application calls CreateFile, SetCommState, ReadFile, WriteFile, and WaitCommEvent on the COM device name. A USB-to-serial adapter must therefore expose a virtual COM port that the OS enumerates as COM1, COM2, COM3, etc. STEP 5 can then address it through the standard Windows COM API.

Recommended adapter selection criteria:

• Chipset: FTDI FT232R, FT232H, or FT230X is the most reliable for industrial legacy work. The FTDI VCP driver is signed and stable across Windows 7 through Windows 11. Prolific PL2303 adapters are widely available but have had driver-signing issues on 64-bit Windows; install the correct version (PL2303 HXA/HXD/RA/SA/TA/TB) explicitly.
• Isolation: For shop floor connection to a live S5, an optically isolated USB-serial adapter provides a second isolation barrier in addition to the TTY isolator. Recommended for any installation where the laptop is grounded to a different source than the PLC cabinet.
• DB-9 output: Most industrial-grade adapters output a DB-9 male. Pair with a straight-through DB-9 female to DB-25 male adapter to mate with the 6ES5 734-1BD20's PC end.
• FIFO buffer: Adapters with larger FIFOs (e.g., FT232H 1 KB) help at higher interrupt rates. The S5 at 9600 baud is not a bottleneck.
• Driver source: Always install the manufacturer's signed VCP driver, not the generic Microsoft CDC driver that Windows may auto-select on FT232 adapters.

DB-9 to DB-25 pin mapping for a straight-through adapter (for reference when building or verifying a custom adapter):

DB-25 pins 8 (CD), 15 (TXC), 17 (RXC), and 22 (RI) are not used by the 6ES5 734-1BD20. Leave them unconnected on the adapter.

Installing the USB-to-Serial Driver

Driver installation procedure for a Windows 10/11 64-bit host:

1. Connect the USB-to-serial adapter to the host. Do not connect the 6ES5 734-1BD20 cable yet.
2. Allow Windows to detect the new hardware. On first connection, Windows may auto-install a generic driver. Open Device Manager, expand Ports (COM & LPT), and identify the new entry.
3. If the entry shows as USB Serial Device (Microsoft CDC) instead of the manufacturer's VCP driver, right-click and choose Update driver → Browse my computer for driver software → Let me pick from a list of device drivers on my computer.
4. Download the latest VCP driver from the manufacturer's website (FTDI 2.12.36 or later for FT232). Run the setup executable or point the manual driver install to the extracted INF file.
5. Confirm the new virtual COM port appears with the correct manufacturer description (e.g., USB Serial Port (COM3)).
6. If a specific COM port number is required (e.g., COM1 or COM2, which STEP 5 often defaults to), right-click the entry, choose Properties → Port Settings → Advanced, and select a free COM port number from the dropdown. Note that COM1 and COM2 are commonly occupied by internal UARTs on legacy motherboards but are free on most modern laptops.

After driver installation, run a quick test outside the VM. Open PuTTY or Tera Term on the host at 9600 8E1 connected to the assigned COM port. Short pins 2 and 3 on the DB-9 output of the adapter with a wire. Typed characters should echo back. This proves the USB-to-serial hardware and host driver are functional before adding the VM layer.

STEP 5 Compatibility and Virtualization

STEP 5 is no longer distributed or supported by Siemens. The last shipping release, STEP 5 V6.6 HF02, was designed for Windows 95/NT 4.0/2000/XP. It is informally used on Windows 7 32-bit with some registry compatibility shims. Newer Windows releases (10/11 64-bit) cannot run STEP 5 natively; a virtual machine is required.

Yes, STEP 5 installs and runs in a virtual machine. The success of the connection depends on how the virtual COM port is mapped to the host's physical or virtual USB-serial adapter. VMware Workstation has the most mature COM port passthrough; VirtualBox works but occasionally loses COM state on host sleep/wake.

Recommended VM configuration for STEP 5:

• Guest OS: Windows XP SP3 32-bit
• Disk: 20-40 GB (XP needs less; reserve space for STEP 5 projects)
• RAM: 1-2 GB (XP runs comfortably in 1 GB)
• Network: Bridged or NAT (only needed for file transfer to host)
• Serial port: 1, mapped to host COM port
• USB controller: present but not required for COM port mapping mode

Configuring the Virtual Machine COM Port

There are two methods to expose the USB-to-serial adapter inside the VM. Pick one, not both, to avoid device conflicts.

Option A: Map a Host COM Port (Recommended)

If the USB-to-serial adapter enumerates as COM3 on the host, the guest should be configured to use COM3. In VMware Workstation Pro or Player:

1. Power off the VM and open VM Settings.
2. Select Add → Serial Port → Next.
3. Choose Use physical serial port on the host and select COM3 (or whichever COM number the USB-serial adapter created on the host).
4. Check Connect at power on.
5. Inside the XP guest, the device shows up as the first free COM port, typically COM1 or COM2. STEP 5 will use this in-guest COM number, not the host COM number.
6. Verify in the XP guest's Device Manager → Ports (COM & LPT) that a Communications Port (COM1) or (COM2) entry is present.

VirtualBox equivalent: Settings → Serial Ports → Enable Serial Port → Port Number: COM1, Host Device: COM3 (Windows host) or /dev/ttyUSB0 (Linux host), Mode: Host Device.

Option B: USB Passthrough (Direct)

Some engineers prefer to pass the entire USB device into the guest, so the FTDI driver runs in the XP guest rather than on the host. In VMware: VM → Removable Devices → [FTDI adapter name] → Connect (Disconnect from host). The device disappears from the host and appears in the XP guest as a virtual COM port that STEP 5 can use directly.

This method is sometimes less stable than the COM port mapping because USB device hot-plug inside a VM can cause re-enumeration issues after VM suspend/resume. Use COM port mapping if the connection drops intermittently.

Setting Up STEP 5 Online Communication

With the VM running and the virtual COM port in place, start STEP 5 and configure the PG interface:

1. Launch STEP 5 in the XP guest.
2. Open the project or create a new S5 program file (STL, LAD, or CSF representation).
3. From the menu, select File → Set Interface (or the German Datei → Schnittstelle einstellen).
4. Select Online as the working mode.
5. Set the COM port to the in-guest COM number (typically COM1 or COM2 inside XP).
6. Set the baud rate to 9600, parity to Even, data bits to 8, stop bits to 1. The S5 programming port is fixed at 8E1.
7. Click Test to send a polling frame. If the S5 responds, the green Connected indicator appears in the status bar.

Default STEP 5 communication parameters for the S5 PG interface:

Verification and Acceptance Test

After the cable, USB-serial adapter, VM, and STEP 5 are all in place, run this checklist before going online with a live PLC.

1. Loopback test on host: Disconnect the 6ES5 734-1BD20 from the PLC. Short pins 2 and 3 on the DB-9 output of the USB-serial adapter with a wire. Open PuTTY on the host at 9600 8E1 to the assigned COM port. Typed characters should echo back. This proves the USB-to-serial hardware and host driver are good.
2. Loopback test in guest: Repeat step 1 with PuTTY running inside the XP guest connected to the in-guest COM port. This proves the VM COM passthrough is functional.
3. STEP 5 self-test: In STEP 5, choose Online → Test. The software polls the bus; without a connected S5 you should get a timeout error, not a port-not-found error. A timeout confirms STEP 5 is talking to the COM port.
4. Live connection: Power the S5 PLC, connect the 6ES5 734-1BD20 to the PLC's TTY programming port, then click Online in STEP 5. The CPU type, rack number, and program status should populate within a few seconds.
5. Block upload: Upload a single FB or PB block from the S5 to the STEP 5 project. This exercises the full AS511 path including error detection, checksum verification, and retransmission.
6. Block download (with caution): Download a small test block to verify the write path. Do not download to a running production process without a backup and a controlled stop sequence.

Troubleshooting Matrix

Cable Care and Known Failure Modes

The 6ES5 734-1BD20 contains inline conversion electronics. Failure modes observed in the field include:

• Crushed cable near the strain relief, breaking one of the current loop conductor pairs.
• Burned-out TTY opto-isolator from a miswired parallel TTY port on a third-party interface.
• Loose DB-25 backshell screws causing intermittent RXD/TXD.
• Corroded 9-pin connector pins on the PLC side from long-term storage in a humid cabinet.
• Open circuit in the TTY current source, manifesting as no response from the CPU even when the host loopback is good.

A quick bench test for the cable: power the S5 CPU and connect the 6ES5 734-1BD20. Measure the voltage across the TTY current loop pins on the PLC connector. A healthy TTY port shows roughly 5-24 V DC across the active pair, depending on the CPU variant. Zero voltage indicates either a cable break or a dead TTY driver in the PLC. In a dim room, a powered TTY opto-isolator in the cable glows faintly through the cable jacket; this is a quick way to spot an active cable and confirm power flow.

Replacing a Damaged PG 740 PIII

The PG 740 PIII is a Siemens-branded laptop from the 1990s, originally built around an Intel Pentium III processor. If it is damaged or has failed, the modern functional equivalent is any current Windows laptop with a USB port, a USB-to-serial adapter, a copy of STEP 5, and a virtualization layer. The replacement stack:

• PG 740 PIII equivalent: Any modern laptop running Windows 10/11 64-bit with a USB-to-serial adapter and a Windows XP or Windows 7 32-bit VM running STEP 5 V6.6.
• PG 720/730 equivalent: Same as above.
• Field PG replacement: A ruggedized laptop (e.g., Panasonic Toughbook, Getac, or Durabook) with the same stack, useful for shop floor commissioning where dust, moisture, or drop hazards are present.

Older PGs used a proprietary MPI/TTY port and could connect directly to the S5. Modern hardware cannot, hence the need for a USB-to-serial adapter plus the 6ES5 734-1BD20 cable in between. The advantage of the modern stack is that the laptop can also run TIA Portal for newer S7-300/400/1200/1500 commissioning, making it a single device for the entire SIMATIC estate.

Cable Variants and S5 Family Compatibility

The 6ES5 734 family includes several variants. Selecting the right one depends on the S5 CPU's TTY port form factor:

For modern USB-serial adapters that output DB-9 male, the 6ES5 734-1BD30 or 6ES5 734-2BF20 eliminates the need for a separate DB-9 to DB-25 adapter. If the existing 6ES5 734-1BD20 is intact, the 25-pin variant remains fully usable with a passive straight-through adapter.

DB-9 to DB-25 Adapter Specification

A passive 1:1 DB-9 female to DB-25 male adapter is the typical companion to the 6ES5 734-1BD20. The wiring must be straight-through: each DB-9 pin connects to its corresponding DB-25 pin. Use a fully populated adapter (all relevant pins wired) to ensure DTR, DSR, RTS, and CTS pass through. A minimal adapter that only wires pins 2, 3, and 5 will not work for the S5, because the 6ES5 734-1BD20 expects DTR/DSR handshaking from the host PC.

Avoid null-modem (crossover) adapters; they swap TD/RD and will produce timeout errors in STEP 5 even though the host loopback is good.

Sample VMware Workstation Configuration

The following is a complete vmx file snippet for mapping a host COM3 to a Windows XP guest as COM1:

# /path/to/step5vm.vmx
serial0.present = "TRUE"
serial0.fileType = "device"
serial0.fileName = "COM3"
serial0.autodetect = "FALSE"
serial0.startConnected = "TRUE"
serial0.yieldOnMsrRead = "TRUE"
guestOS = "winxppro"

The four serial0 parameters are the minimum required. The yieldOnMsrRead flag is important: it makes the guest yield CPU time when waiting for the COM port, avoiding a 100% guest CPU stall that some VMware versions produce on idle COM waits. Inside the XP guest, the mapped port appears as COM1 (the first free COM number in the guest namespace) and STEP 5 should be configured to COM1 at 9600 8E1.

Safety and Plant Floor Precautions

When connecting the 6ES5 734-1BD20 to a live S5 in a running process, observe the following:

• Process state: Putting an S5 into STOP for online editing will stop program execution. Many S5 processes are interlocked; verify with the process engineer that STOP is safe for the affected machine.
• Cable connection: The TTY port is hot-pluggable (the opto-isolator tolerates live insertion), but avoid plugging/unplugging while a block download is in progress.
• Block overwrite: STEP 5 will overwrite an existing block in the CPU when downloading the same block number. Always archive the current PLC program to a STEP 5 file on the host before any download.
• Power-off handling: The S5 PLC does not retain user program or data block contents across a power loss in the standard S5-90U/95U configuration (no battery-backed RAM option for these models). Uploading to STEP 5 is the only archive method.