Teledyne LeCroy Line Code and Symbolic Decoders - Manual (Page 9)

Instruction Manual
922663 Rev A 7
L EVELS D IALOG
This dialog specifies the levels at which signal transitions occur and hysteresis to help tune noisy signals.
Level Type - Whether Level will be given as Percentage of signal amplitude or Absolute number of volts.
Percent mode is easy to set up because the software immediately determines the optimal threshold.
However, on poor signals Percent mode can fail and lead to bad decodes; then it might help to use
Absolute mode. On very long signals, Percent Mode adds computational load. If performance is an issue,
it might be beneficial to switch to Absolute mode.
Level - Percentage of signal amplitude or absolute voltage at which the bit transition occurs. When
working in percent, all values are proportional to the 100% signal amplitude.
Hysteresis Type - Whether Hysteresis will be given as Percentage or absolute number of vertical
Divisions. You may give the Level in one mode (e.g., Absolute) and Hysteresis is in the other mode ( e.g.,
Percent).
Hysteresis - Margin of error; amount signal may rise or fall without affecting bit transition. Hysteresis is
fairly subtle as it should not dominate the rendering of the decoded information.
8b/10b and 64b/66b
8b/10b Overview
8b/10b encoding is not a protocol, but a widely used method to encode 8-bit data words within a 10-bit
symbol, or character. The extra bits are used to ensure the long-term ratio of 1s and 0s transmitted is
1:1; ensuring the serial data encoding is DC free.

Any bit transmission longer than five consecutive 1s or 0s is prohibited, which limits the
requirements for the lowest required bandwidth in the serial data transmission channel.

The difference in number between 1 bit and 0 bit transmissions is never more than two.
While there are theoretically 1024 (2 to the 10th power) different 8b/10b encoded bytes possible, far
fewer are allowed based on these aforementioned rules.
In order to maintain the DC-free nature of the signal, a running disparity counter is kept for each byte.
This count reflects the bias of 1s or 0s from the transmitted byte, and the 8b/10b encoder makes use of
the value of this running disparity counter to determine whether to encode the next byte as a +1 or -1
running disparity so as to keep the overall DC bias of the transmitted signal at zero. Thus, there are two
valid bit sequences for any byte, depending on the running disparity used. The Teledyne LeCroy 8b/10b
Decoder takes all this into account so that you don't have to.
Serial data standards that use 8b/10b encoding also define special symbols or control characters that
indicate start or end-of-frame, skips, link idles, or other protocol-specific non-data information. These
are commonly referred to as primitives. Many high-speed serial data standards, such as PCI Express,
SATA, SAS, Fibre Channel, etc. use 8b/10b as the underlying encoding method below the protocol layer.
Each standard defines its own set of primitives. Primitives convey more basic information than
contained in a full protocol decode, but they can be valuable as well for debugging or quality control
purposes.