9-pin D-SUB Male/Stecker

  1:  CD  in  Carrier Detect
  2:  RXD in  Receive Data
  3:  TXD out Transmit Data             
  4:  DTR out Data Terminal Ready      (transmitter powered on)
  5:  SG      System Ground/Masse
  6:  DSR in  Data Set Ready           (listener powered on)
  7:  RTS in  Control/Request to Send  (May I send data ?)
  8:  CTS out Control/Clear to Send    (I'm ready to receive if high)
  9:  RI  in  Ring Indicator



If the data set wants to stop/interrupt the communication (eg. buffer overflow),
it drops CTS to LOW; the sender uses RTS to control the data flow.