A novel technique for trellis decoding of block both run length limited (RLL) and balanced codes on PR channels is described. The technique allows performance improvement without increment of decoder complexity. A simple technique for constructing RLL block error control codes (ECC) together with their minimal trellises is introduced. The procedure adapted for the design of such codes is based on taking a linear ECC and incorporating a maximum run length constraint by carefully modifying the basis code while retaining the minimum distance properties of the parent code. Such codes are particularly suited for magnetic recording applications where the (1-D) partial response (PR) channel provides a good model at low information density rates.