18.104.22.168.3. 8B/10B Encoder Idle Character Replacement Feature
The idle character replacement feature is used in protocols such as Gigabit Ethernet, which requires the running disparity to be maintained during Idle sequences. During these Idle sequences, the running disparity has to be maintained such that the first byte of the next packet always starts when the running disparity of the current packet is negative.
When an Ordered Set, which consists of two code-groups, is received by the 8B/10B encoder, the second code group is converted into /I1/ or /I2 so that the final running disparity of the data code-group is negative. The first code group is /K28.5/ and the second code group is a data code-group other than /D21.5/ or /D2.2/. The ordered set /I1/ (/K28.5/D5.6/) is used to flip the running disparity and /I2/ (/K28.5/D16.2/) is used to preserve the running disparity.