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22 November 2021

What Is Differential Manchester Encoding?

Figure 1. Differential Manchester encoding is based on the
presence or absence of a transition, whereas Manchester 
encoding relies on the polarity of the transition.
Differential Manchester Encoding (DME) is an example of a differential, bi-phase encoding technology. DME is specified in the IEEE 802.5 standard for Token Ring local area network (LAN) topology. 

Manchester is categorized as bi-phase encoding because the signal is checked twice every bit interval, also called self-clocking. Each check is one “tick”, each bit interval equals two ticks of the clock. This removes the need for the separate clock signal that is required for Non-Return to Zero (NRZ) encoding. Instead, data and clock signals are combined into a single, two-level, self-synchronizing data stream. The clock can be "extracted" by measuring the timing of the edges.

Figure 2. In Manchester encoding, the polarity of the
transition that occurs mid-interval determines the logic.
In Manchester encoding, we see a digital modulation scheme where voltage transitions rather than voltage levels are used to represent 1s and 0s. In IEEE 802.3 Manchester, a low-to-high transition occurring in the middle of the bit interval represents logical 0, while a high-to-low transition represents logical 1 (the Thomas variant reverses this logic). Significant transitions always occur in the middle of the bit interval to ensure clock synchronization. Transitions at the start of a period are only used to reset the polarity to achieve the proper transition for the next bit. This increases error detection capabilities, compared to NRZ, but also increases the bandwidth needed to transmit the signal at the same data rate, making it a better candidate for short-distance applications.

Figure 3. Even inverted, DME signals result in same logic.
The most prominent feature that distinguishes DME from classic Manchester encoding is that, in DME only the presence or absence of a transition during the bit interval is important, not the polarity. The presence of a transition represents a logical 0, while the absence of a transition represents a logical 1. Whether the signal goes line-high or line-low depends simply on its state the previous bit interval. This increases bit rate at lower bandwidths, because one bit is guaranteed to occur every interval. It also helps with data recovery in noisy environments, like automotive, because DME allows for a data stream to be inverted, yet still be properly decoded, unlike classic Manchester where the polarity is significant. 

DME is used for 10Base-T1S Automotive Ethernet, a short-distance, low-bandwidth application with either a point-to-point or bus topology up to 25 m. 

Teledyne LeCroy offers QualiPHY compliance test solutions for 10Base-T1S, including a QPHY-10Base-T1-TDR option that automates all required MDI S-parameter tests using the WavePulser 40iX.

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