Overview
The Siemens PC/PPI cable (catalog number 6ES7901-3CB30-0XA0) is a multi-function programming interface designed primarily for the SIMATIC S7-200 series PLCs. Although most engineers know it as a programming cable, the device contains an onboard RS-232-to-RS-485 converter that can be repurposed as a general-purpose serial converter when configured in Freeport mode.
This mode disables the cable's PPI/MPI/Profibus protocol intelligence and passes data through transparently between the PC's COM port (RS-232) and a downstream RS-485 device. Typical field applications include:
- Connecting a PC to a third-party RS-485 Modbus RTU slave (weigh scale, barcode reader, variable frequency drive, sensor).
- Communicating with SIMATIC S7-200 in Freeport (user-defined) protocol when paired with XMT/RCV instructions in the ladder program.
- Interfacing RS-232 equipment to a half-duplex RS-485 bus (e.g., a panel meter, energy meter, or encoder).
To repurpose the cable successfully, the engineer must set the onboard DIP switches correctly and select the matching PG/PC interface in the MicroWin programming software.
Cable Identification and Hardware
The 6ES7901-3CB30-0XA0 cable is the standard multi-master USB/RS-232 PC/PPI variant, supplied with a 9-pin sub-D female connector for the PC side and a 9-pin sub-D male connector wired to the S7-200 PPI port. Internally, the cable carries:
- An RS-232 transceiver for the PC side.
- A Siemens-proprietary ASIC/protocol engine for PPI, MPI, and Profibus operation.
- An RS-485 transceiver for the PLC side.
- A bank of six DIP switches that select baud rate, mode, and echo behavior.
The cable requires no external power when connected to an S7-200 (the PLC supplies 24 V from the PPI port). When used as a standalone RS-232/RS-485 converter, an external 24 V DC supply must be applied to pin 6 of the PPI connector to power the cable's internal electronics.
DIP Switch Configuration
The PC/PPI cable exposes six DIP switches inside the RS-232 connector housing. The switch bank is labeled SW1 through SW6, with positions numbered from left to right when the cable is laid flat with the PC connector on the left.
| Switch | Function | OFF | ON |
|---|---|---|---|
| SW1 | Baud rate bit 0 | See baud rate table | |
| SW2 | Baud rate bit 1 | ||
| SW3 | Baud rate bit 2 | See baud rate table | |
| SW4 | Reserved / 10-bit or 11-bit mode | 11-bit (PPI default) | 10-bit (Freeport default) |
| SW5 | Freeport / PPI mode select | Freeport (transparent) | PPI (intelligent) |
| SW6 | Remote / Local mode | Local (DCE echo on) | Remote (no echo) |
Baud Rate Selection (SW1, SW2, SW3)
| SW1 | SW2 | SW3 | Baud Rate (bps) |
|---|---|---|---|
| OFF | OFF | OFF | 115200 |
| ON | OFF | OFF | 57600 |
| OFF | ON | OFF | 38400 |
| ON | ON | OFF | 19200 |
| OFF | OFF | ON | 9600 |
| ON | OFF | ON | 4800 |
| OFF | ON | ON | 2400 |
| ON | ON | ON | 1200 |
Freeport vs. PPI Mode (SW5)
Switch 5 is the master selector that determines the cable's operating mode:
- SW5 = OFF (Freeport mode): The cable's protocol engine is bypassed. Data arriving on the RS-232 RXD line is converted directly to RS-485 differential signals and transmitted onto the bus, and vice versa. The cable functions as a transparent RS-232 to RS-485 converter. PPI/MPI/Profibus framing is not applied.
- SW5 = ON (PPI mode): The cable participates actively in the PPI token-passing protocol. It will not pass arbitrary bytes through; only valid PPI telegrams are accepted and forwarded. This is the default factory mode for S7-200 programming.
Remote vs. Local Mode (SW6)
Switch 6 selects between RS-232 DCE and DTE behavior:
- SW6 = OFF (Local / DCE): The cable echoes received data back to the PC. This is useful when the connected device is a half-duplex RS-485 slave that does not echo, allowing terminal programs and MicroWin to display traffic for diagnostics.
- SW6 = ON (Remote / DTE): The cable does not echo. Required when the downstream device already echoes characters (e.g., a GSM modem, terminal, or any device where the master must not see its own transmissions). Selecting this option prevents character collisions on the bus.
Recommended DIP Switch Settings for Freeport Conversion
For typical Modbus RTU or generic RS-485 communication, set the switches as follows:
- SW1, SW2, SW3 — match the baud rate of the target device (9600 is the most common starting point).
- SW4 = ON (10-bit character length, suitable for 8-N-1 framing).
- SW5 = OFF (Freeport, transparent conversion).
- SW6 = OFF (Local mode) when troubleshooting, or ON (Remote) when the slave already echoes or in production with a half-duplex bus.
For 8-E-1 or 8-O-1 framing used by some Modbus devices, set SW4 = OFF (11-bit mode) and ensure the PC COM port parity matches the slave.
MicroWin PG/PC Interface Configuration
MicroWin (STEP 7-Micro/WIN) V4.0 SP9 or earlier uses the Set PG/PC Interface utility to bind a serial port to a communication protocol. The recommended steps:
- Open MicroWin and navigate to Communications → Set PG/PC Interface.
- On the Interface Parameter Assignment dialog, double-click PC/PPI cable (PPI) if programming an S7-200 in PPI mode.
- Click Properties and select the correct COM port (typically COM1).
- Set the transmission rate to match SW1/SW2/SW3 on the cable (e.g., 9600 bps for default PPI, 19200 bps for higher-speed PPI).
- Under Transmission timeout, set to 1000 ms and Number of retries to 3 for a typical first-try configuration.
- Click OK and double-click the refresh icon to search for the S7-200 station.
When the cable is in Freeport mode (SW5 OFF) and connected to a third-party RS-485 device, MicroWin's PPI driver will not detect anything on the bus. To verify the cable as a transparent converter, use a generic terminal program such as Windows HyperTerminal, PuTTY, or the Modbus Poll master instead of MicroWin.
RS-485 Pinout on the PPI Connector
The male 9-pin D-sub on the PLC side of the cable follows the Siemens PPI/MPI standard pinout:
| Pin | Signal | Direction | Function |
|---|---|---|---|
| 1 | — | — | Shield (optional) |
| 2 | — | — | Not connected (24 V return on early revisions) |
| 3 | RS-485 B / Data+ / RxD/TxD+ | Bidirectional | Non-inverting differential line |
| 4 | RTS (TTL) | Output | Direction control for RS-485 transceiver |
| 5 | DGND | — | Digital ground (5 V reference) |
| 6 | +5 V / +24 V | Input (from PLC) | Cable power supply |
| 7 | — | — | Not connected |
| 8 | RS-485 A / Data- / RxD/TxD- | Bidirectional | Inverting differential line |
| 9 | — | — | Not connected |
For connection to a generic RS-485 slave, wire only pins 3, 5, 6, and 8. The 24 V supply on pin 6 (when connected to an S7-200) powers the cable. If using the cable without an S7-200 host, apply +5 V DC regulated to pin 6 referenced to pin 5; this is the standard practice for bench-top RS-232/RS-485 conversion work.
RS-232 (PC) Connector Pinout
The female 9-pin D-sub on the PC side follows a DCE layout:
| Pin | Signal | Direction (cable side) |
|---|---|---|
| 2 | RXD | Output to PC |
| 3 | TXD | Input from PC |
| 4 | DTR | Input from PC |
| 5 | SGND | — |
| 6 | DSR | Output to PC |
| 7 | RTS | Input from PC |
| 8 | CTS | Output to PC |
Standard straight-through serial cable works for most PCs because both ends follow the DTE/DCE convention. For laptops without RS-232, use a USB-to-RS-232 adapter based on FTDI FT232 or equivalent; avoid generic CH340 clones for industrial work where noise immunity matters.
Freeport Mode Use Cases
Freeport mode is the only configuration in which the cable behaves as a transparent converter. Practical applications include:
- Modbus RTU master on a PC talking to a weigh indicator, VFD, or power meter over RS-485. The cable does not interpret the protocol — Modbus Poll or any Modbus master library handles the framing.
- S7-200 in Freeport protocol with XMT/RCV ladder instructions, communicating with a barcode scanner or serial printer. In this case, the cable still acts as a transparent bridge between the S7-200 PPI port (now used as a generic UART) and the third-party device.
- GSM modem gateway: historically used with SW6 = ON (Remote) to attach a wireless modem to an RS-485 bus for telemetry.
Switching Between PPI and Freeport Programmatically
The 6ES7901-3CB30-0XA0 is a hardware-switched cable, not a software-switched one. There is no escape sequence to toggle Freeport mode. To change modes, physically open the cable housing and reset the DIP switches. Field engineers often mark the housing with paint to identify the current mode at a glance.
Verifying the Cable as a Transparent Converter
- Connect the cable to a PC COM port and to a known RS-485 slave (a USB-RS-485 sniffer on the opposite end is ideal for the test).
- Open PuTTY in Serial mode, set the COM port to the cable's baud rate, and set 8-N-1 flow control to None.
- With SW6 = OFF (Local) type a few characters. The characters should appear both locally (echoed by the cable) and at the sniffer.
- Have the slave transmit a response; the response should appear in the PuTTY window.
- If no data is received, check SW5 (must be OFF for transparent mode) and confirm +5 V on pin 6 of the PPI connector.
Troubleshooting Matrix
| Symptom | Likely Cause | Corrective Action |
|---|---|---|
| No communication in either direction | SW5 is ON (PPI mode) | Set SW5 OFF for transparent conversion |
| PC receives own transmissions, no slave data | SW6 is OFF but slave is half-duplex without echo | Set SW6 ON to disable echo |
| Garbled characters | Baud rate mismatch | Verify SW1/SW2/SW3 match slave baud rate and PC COM port setting |
| MicroWin cannot find S7-200 | SW5 set to Freeport by mistake | Set SW5 ON to return to PPI |
| Intermittent communication | Missing termination resistor on RS-485 bus | Add 120 Ω across pins 3 and 8 at both ends of the bus |
| No power on cable when used standalone | Pin 6 not energized | Apply +5 VDC between pins 5 and 6 of the PPI connector |
| Characters double-echoed | SW6 OFF and slave also echoes | Set SW6 ON (Remote mode) |
Comparison with Dedicated RS-232/RS-485 Converters
| Feature | Siemens 6ES7901-3CB30-0XA0 (Freeport) | Generic RS-232/RS-485 Converter (e.g., Advantech ADAM-4520) |
|---|---|---|
| Auto direction control | Hardware (RTS-based) | Hardware or software (selected by jumper) |
| Galvanic isolation | No | Yes (on industrial models) |
| Baud rate range | 1200 - 115200 bps | 300 - 115200 bps |
| External power required | Yes (5 V from PLC or external) | 9-30 VDC typical |
| PPI/MPI/Profibus protocol | Yes (mode-selectable) | No |
| Cost (used/new) | Moderate (Siemens brand premium) | Low |
The PC/PPI cable is most attractive when the engineer already has one in the toolbox and a temporary RS-485 gateway is needed. For permanent installations, a dedicated isolated converter is preferable because it provides 1500 V isolation between the RS-232 and RS-485 sides, protecting the PC's serial port from ground loops on long RS-485 runs.
Standards and Documentation
Refer to the official Siemens FAQ for the most current DIP switch mapping and application guidance:
- Siemens Support: How should you set the DIP switches on the PC/PPI cable for communication with the S7-200?
- Siemens Support: PC/PPI cable wiring and electrical characteristics
The RS-232 electrical specification and DTE/DCE conventions are defined in the TIA-232-F standard (formerly EIA-232). RS-485 electrical characteristics and bus topology are defined in TIA-485-A. Verify cable selection against the connected device's manual and the bus length and termination guidelines in those standards.
Field Commissioning Checklist
- Open the cable housing and identify SW1-SW6.
- Set SW1-SW3 to the baud rate of the downstream device (e.g., 9600).
- Set SW4 to ON (10-bit) for 8-N-1 framing, or OFF (11-bit) for parity use.
- Set SW5 to OFF (Freeport, transparent conversion).
- Set SW6 to OFF (Local) for first-time verification, then switch to ON (Remote) once the bus is stable.
- Connect the cable to the PC and to the RS-485 bus, paying attention to A/B polarity (Siemens pin 8 = A, pin 3 = B; some third-party devices reverse this convention).
- Apply power to the cable (via PLC or external +5 V on pin 6).
- Test communication with a terminal program. If responses are correct, switch to the application master (Modbus Poll, MicroWin Freeport, etc.).
FAQ
What does DIP switch 5 (Freeport/PPI) actually do on the Siemens 6ES7901-3CB30-0XA0?
Switch 5 selects between the cable's two fundamental operating modes. SW5 = ON engages the onboard PPI/MPI/Profibus protocol engine, allowing the cable to participate in token-passing and to program an S7-200. SW5 = OFF bypasses that engine entirely, leaving the cable to act as a transparent RS-232 to RS-485 converter suitable for Modbus RTU, Freeport protocol, or any generic serial traffic.
Can MicroWin still program the S7-200 when the cable is set to Freeport mode?
No. Freeport mode disables the cable's protocol intelligence, so MicroWin's PPI driver will time out and fail to find the PLC. To program or upload from the S7-200, return SW5 to ON and reset the baud rate switches to match your PG/PC interface (typically 9600 bps for default PPI). There is no software override — the DIP switch must be changed physically.
What is the difference between Remote and Local mode (DIP switch 6)?
Switch 6 controls whether the cable echoes received data back to the PC. Local (SW6 = OFF) = DCE-style echo, useful for diagnostics on half-duplex buses where the slave does not echo. Remote (SW6 = ON) = DTE-style, no echo, required when the downstream device already echoes characters (for example, a GSM modem or a terminal) to prevent collisions on the RS-485 bus.
Does the PC/PPI cable supply its own power when used as a converter?
No. The cable draws its operating voltage from pin 6 of the PPI connector, which is normally fed by the S7-200's 24 V supply through the PPI port. When used without an S7-200 host, apply +5 VDC regulated to pin 6 with reference to pin 5 (DGND), or connect the cable to a powered S7-200 or PPI tap that supplies this voltage.
What baud rates are available via SW1-SW3 in Freeport mode?
The eight standard rates are 1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200 bps. Select the rate that matches both the connected RS-485 slave and the PC COM port setting in your terminal program or Modbus master. 9600 bps is the most common default for Modbus RTU devices, while 19200 or higher is recommended for Freeport applications with larger payloads.