Teledyne LeCroy Serial Data Debug Solutions - Manual (Page 45)

Operator's Manual
919586 RevA 45
Serial Pattern Encoding Schemes - 8b/10b
Serial Pattern Encoding Schemes Overview
8b/10b
8b/10b encoding is not a protocol, but a widely used method to encode 8-bit 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. Serial data standards using 8b/10b encoding also define special symbols or
control characters indicating start or end-of-frame, skips, link idles, or other protocol-specific non-data
information. 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 their own set of
special symbols or control characters.
The remainder of this section provides a brief introduction to the various protocols available as part of LeCroy's
Serial Data Debug Solutions for convenience.
Using the 8b/10b Option
Using the 8b/10b Option 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. Also, 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.
Furthermore, an additional requirement is that the difference in number between 1 bit and 0 bit transmissions
is never more than two. Theoretically, there are 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 LeCroy 8b/10b decoder takes all this into account so that the
user doesn'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 their 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.
8b/10b Decode Setup Detail
For general information about using controls shown on the main Serial Decode dialog, refer to Accessing The D
and TD Supported Protocol Toolsets (on page 13).
8 B /10 B D ECODE B ASIC , F ILTER , AND D VPT R IGHT -H AND D IALOGS
Access the Serial Decode dialog by touching Analysis → Serial Decode on the menu bar.
Touch the corresponding Setup... button for your decode. The Decode Setup... along with corresponding right-
hand dialogs are shown. Select the 8b/10b Decode protocol from the Protocol control.
Additional setup details for sending 8b/10b Decode to ProtoSync involve selecting a single source (trace) for the
data, and configuring the right-hand dialogs for Basic and Filter.