Abstract: A method for testing a signal path for mark ratio tolerance includes generating a varying test pattern by selecting between a first pattern and a second pattern according to a defined sequence; and sending the varying test pattern over the signal path. An apparatus disposed in a communication system includes a selection circuit for generating a varying test pattern to send over the signal path, the selection circuit generating the varying test pattern by selecting between a first pattern and a second pattern according to a select sequence signal, and a sequencer coupled to the selection circuit, the sequencer providing the select sequence signal to the selection circuit, the sequencer generating the select sequence signal according to a mode value. The mark ratio tolerance of a system can be tested, varying the data density of one portion of the signal path while maintaining a constant data density on another portion of the signal path.