Digital transmission system

A terminal for receiving digital data in the form of a time multiplexed (TDM) data stream incorporating frames appearing at a predetermined frame period and each frame incorporating a frame word, the terminal comprising:

• (a) alignment detecting means including frame word detecting means for detecting the presence or absence of correctly aligned frame words and frame interval counter means for clocking said frame word detecting means once per frame;
• (b) reset means for resetting said frame interval counter means;
• (c) counting means operative for reaching a first predetermined state in response to said frame word detecting means indicating that a predetermined sequence of frame words has been correctly received;
• (d) master alignment circuit means held in an inalignment state when said counting means is in said first predetermined state;
• (e) means for setting said master alignment circuit means into an out-of-alignment state when said counting means changes to a second predetermined state in response to said frame word detecting means failing to detect a predetermined number of frame words in sequence;
• (f) search means for initiating a bit-by-bit search of the incoming data stream for the sequence of bits forming a frame word when an out-of-alignment condition has been detected and said master alignment circuit means is in said out-of-alignment state;
• and
• (g) delay means for delaying the start of said bit-by-bit search after an out-of-alignment state has been detected, said delay means being connected to said reset means.