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

Device Control

    One of the four ASCII characters, DC1, DC2,
   DC3, and DC4, once used to remotely control equipment
   (e.g. a paper tape reader) via electromagnetic switches.
   The characters were usually paired, DC1/DC3 turning one device
   on/off, and DC2/DC4 another.

   [Other examples of equipment?]

   (1996-08-20)