Null modem, also called crossover, is a term associated with serial (RS-232) cables. A standard serial cable, also called an AT cable, has the wires inside the cable running straight through. Take a DB9 cable as an example. Pin 1 on one end of the cable would be connected to Pin 1 on the other end. Then Pin 2 to 2, 3 to 3, and so on. Null modem cables are serial cables that use an alternative pinout for different functionality.

 

Key Takeaways

 

  • A null modem cable is used for direct serial device-to-device communication.
  • Null modem cables are commonly associated with RS-232 interfaces.
  • The main difference from a straight-through cable is the crossing of transmit and receive signals.
  • DB9 and DB25 are common connector formats for legacy serial applications.
  • Not all null modem cables use the same pinout.
  • Basic cables may use only TX, RX, and signal ground.
  • Other applications may require additional hardware flow-control connections.
  • Always verify connector type, gender, pinout, and flow-control
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