Differential Manchester encoding
A more esoteric version of Manchester encoding is a scheme called Differential Manchester encoding (DME). Think of it as Manchester encoding on steroids. DME is a more efficient encoding scheme because it requires less bandwidth than standard Manchester encoding. The overhead of transmitting a data stream using DME is less because it doesn't require a preamble, which is used by the DPLL to lock onto the clock frequency. Because of this, DME can be found in networks, such as fast Ethernet over copper twisted-pair wiring.

DME differs from standard Manchester encoding in one simple way: Manchester encoding represents binary data based on a positive or negative edge transition at each bit boundary. DME represents data by the presence or absence of a transition between two bit boundaries. Simply stated, if a transition occurs between a bit boundary, it's represented as a binary 0. An absence of a transition signifies a binary 1.

As a complement to this reintroduction to the basics of Manchester encoding for low-bit serial network applications, a second article is available online at Embedded.com. The article will leverage from the theory presented here and offer a practical, real-world example that illustrates the simplicity of implementing Manchester encoding into a real embedded design.

Robert Guastella is a senior controls engineer for Tennant Company in Minneapolis, Minnesota. He has over 22 years of experience in hardware and software design on products ranging from industrial controls, to digital servo drives, to automotive electronics. Guastella holds a BSEE from Lawrence Technological University, as well as an MBA from Oakland University, both located in Detroit, Michigan. He can be reached at robert.guastella@tennantco.com.