8B/10B Encoding/Decoding; Overview; 8B/10B Encoder; 8B/10B Decoder - Xilinx RocketIO User Manual

8B/10B Encoding/Decoding

8B/10B Encoding/Decoding

Overview

The RocketIO transceiver has the ability to encode eight bits into a 10-bit serial stream using
standard 8B/10B encoding. This guarantees a DC-balanced, edge-rich serial stream, facilitating DC-
or AC-coupling and clock recovery.
that change purpose, depending on whether 8B/10B is bypassed or enabled.

8B/10B Encoder

A bypassable 8B/10B encoder is included in the transmitter. The encoder uses the same 256 data
characters and 12 control characters (shown in
used for Gigabit Ethernet, XAUI, Fibre Channel, and InfiniBand.
The encoder accepts 8 bits of data along with a K-character signal for a total of 9 bits per character
applied. If the K-character signal is High, the data is encoded into one of the twelve possible
K-characters available in the 8B/10B code. (See
Low, the 8 bits are encoded as standard data. If the K-character input is High and a user applies other
than one of the twelve possible combinations, TXKERR indicates the error.

8B/10B Decoder

An optional 8B/10B decoder is included in the receiver. A programmable option allows the decoder
to be bypassed. When it is bypassed, the 10-bit character order is as shown in
The decoder uses the same table that is used for Gigabit Ethernet, Fibre Channel, and InfiniBand.
The decoder separately detects both "disparity errors" and "out-of-band" errors. A disparity error
occurs when a 10-bit character is received that exists within the 8B/10B table
but has an incorrect disparity. An out-of-band error occurs when a 10-bit character is received that
does not exist within the 8B/10B table. It is possible to obtain an out-of-band error without having
a disparity error. The proper disparity is always computed for both legal and illegal characters. The
current running disparity is available at the RXRUNDISP signal.
The 8B/10B decoder performs a unique operation if out-of-band data is detected. Should this occur,
the decoder signals the error, passes the illegal 10 bits through, and places them on the outputs. This
can be used for debugging purposes if desired.
The decoder also signals reception of one of the twelve valid K-characters
way of the RXCHARISK port.
In addition, a programmable comma detect is included. The comma detect signal RXCOMMADET
registers a comma on the receipt of any plus-comma, minus-comma, or both. Since the comma is
defined as a 7-bit character, this includes several out-of-band characters. RXCHARISCOMMA
allows the decoder to detect only the three defined commas (K28.1, K28.5, and K28.7) as plus-
comma, minus-comma, or both. In total, there are six possible options, three for valid commas and
three for "any comma."
Note that all bytes (1, 2, or 4) at the RX FPGA interface each have their own individual 8B/10B
indicators (K-character, disparity error, out-of-band error, current running disparity, and comma
detect).
RocketIO™ Transceiver User Guide
UG024 (v2.3.2) June 24, 2004
Table 2-10, page 63, shows the significance of 8B/10B ports
Appendix B, "8B/10B Valid
Table B-2, page
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1-800-255-7778
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141.) If the K-character input is
Figure 2-14, page
(Table B-1, page
(Table B-2, page 141) by
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