The Siemens 6ES7901-3CB30-0XA0 is a PC/PPI multimaster cable for the SIMATIC S7-200 family, designed to connect the S7-200's RS-485 port to a serial PC interface (RS-232). It supports both the PPI protocol at 10/11 bit and ASCII Freeport mode, making it usable for direct programming and for communicating with third-party serial devices like GSM modems. The cable is powered from the CPU, so no external supply is needed at the PC end.
At 5.63 m, this cable is long enough to reach from a panel-mounted S7-200 to a nearby laptop or desktop PC without an extension. The maximum transmission rate of 187.5 kbit/s (selectable via DIP switch at 9.6, 19.3, or 187.5 kbit/s) matches the S7-200's PPI bus speed, so it won't bottleneck the programming or data-exchange rate. The RS-232 side does not require a baud-rate setting — the cable handles the conversion automatically. Three LED indicators: Tx (green) for RS-232 transmit, Rx (green) for RS-232 receive, PPI (green) for RS-485 transmit. If the PPI LED does not flash, check the PLC side or cable variant.