The Free On-line Dictionary of Computing (30 December 2018):

serial line

    Wires which connect two serial ports
   carrying serial data consisting of sequential bits represented
   by one of two voltages.

   A common electrical specification for the signals on a serial
   line is RS-423.

   (ftp://ftp.acorn.co.uk/pub/documents/appnotes/231-245/234.ps).

   (1995-02-02)