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

Synchronous Data Link Control

    (SDLC) An IBM protocol.

   A discipline conforming to subsets of the ADCCP of ANSI
   and the HDLC of the International Organization for
   Standardization.  SDLC manages synchronous, code-transparent,
   bit-serial communication which can be duplex or
   half-duplex; switched or non-switched; point-to-point,
   multipoint, or loop.

   Compare Binary Synchronous Communication.

   (1995-03-22)