Source : Free On-Line Dictionary of Computing

level-sensitive scan design
     
        (circuit design) (LSSD) A kind of {scan design} which uses
        separate system and scan clocks to distinguish between normal
        and test mode.  Latches are used in pairs, each has a normal
        data input, data output and clock for system operation.  For
        test operation, the two latches form a master/slave pair with
        one scan input, one scan output and non-overlapping scan
        clocks A and B which are held low during system operation but
        cause the scan data to be latched when pulsed high during
        scan.
     
        	  ____
        	 |    |
         Sin ----|S   |
         A ------|>   |
        	 |   Q|---+--------------- Q1
         D1 -----|D   |   |
         CLK1 ---|>   |	  |
        	 |____|	  |    ____
        		  |   |	   |
        		  +---|S   |
         B -------------------|>   |
        		      |	  Q|------ Q2 / SOut
         D2 ------------------|D   |
         CLK2 ----------------|>   |
        		      |____|
     
        In a single latch LSSD configuration, the second latch is used
        only for scan operation.  Allowing it to be use as a second
        system latch reduces the silicon overhead.
     
        (1995-02-15)