This clause of the PCS specification consists of mapping each 8-bit input into a unique
10-bit output based on the current running disparity, which results in increased transition
density to assist the receiver to recover the clock. Running disparity (RD) is a measure of
l ’s and 0’s in the 10-bit output. RD is negative if the 10-bit output has more 0’s than l ’s
and positive if the 10-bit output has more 1 ’s than 0’s.
Figure 3.3 illustrates the 8 to 10-bit mapping. In this process, every 8-bit input is mapped
into a unique 10-bit output based on the current running disparity. The 10-bit output thus
determined is used for calculating the new running disparity, which in turn is used for
encoding the next 8-bit input. Thus every 8-bit input assumes one of two possible 10-bit
outputs based on the current running disparity. This process utilizes two lookup tables.
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Data code groups lookup table: This consists of 256 different data code groups ranging
from OO(Hex) to FF (Hex). Data code groups are mapped to their corresponding 10-bit
outputs from this table.
Special code groups lookup table: This consists of 12 different special code groups. Some
of the code groups relevant to the PCS are presented below.
K28.5: PCS Idle Character (/K/).
K28.3: PCS Idle Character (/A/).
K28.0: PCS Idle Character (/R/).
K28.4: PCS Sequence Character (/Q/).
K27.7: PCS Start Character (/S/).
K29.7: PCS Terminate Character (/T/).
These code groups are explained in detailed in section 3.4.
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