Source : Free On-Line Dictionary of Computing

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)