DP84902MS datasheet(13/16 Pages) NSC

13 / 16 page

Functional Description (Continued)

17 RLL CODE

The (17) code used is based on US patent

4413251 via

cross-licensing with International Business Machines Cor-

poration (IBM ) Table II summarizes the decoding method

used for this device Nine SYNCDATA bits are used to de-

code the middle three SYNCDATA bits of the nine bit

stream into two NRZ output bits a Most Significant Bit

(MSB) and a Least Significant Bit (LSB) Bit 8 is the first

SYNCDATA bit shifted into the decoder The left-most col-

umn of the table (‘‘NRZIO OUTPUT BIT’’) identifies whether

the row represents the NRZ MSB or LSB This table identi-

fies the combinations which will produce a high logical level

If the code bits do not match the table a low logical state

will be produced Table III represents the same decoding

operation in a different format

Table IV summarizes the state diagram used by this device

to encode NRZ data into 17 coded data The table is read

from left to right during the encoding process To encode

data the ‘‘CURRENT STATE’’ (column 1) must be deter-

mined first

The initial ‘‘CURRENT STATE’’ is always zero This ‘‘CUR-

RENT STATE’’ selects a group of four rows in the table The

two NRZ input bits determine the exact row within the

group to be used Once the row is identified follow the row

to the right of the ‘‘NRZIO’’ column to locate the coded

output in the ‘‘17 OUT’’ column Continue by identifying the

next state in the ‘‘NEXT STATE’’ column immediately to the

right of the ‘‘17 OUT’’ column The number located in this

column is used as the ‘‘CURRENT STATE’’ for the next two

NRZ input bits This procedure is continued until all the NRZ

data is encoded

TABLE II Decoding State Table

NRZ OUTPUT

SYNCDATA (CODE BITS)

BIT

876543210

NRZIO0 (LSB) e 1

xxxx

xxx

NRZIO1 (MSB) e 1

xxxx

xxxxxx

TABLE III Decoding State Table

Encoded Read Data (SYNCDATA)

Decoded Data

Present

(NRZ Data)

MSB

xLSB

MSB

xLSB

MSB

xLSB

MSB

x LSB

0000

1000000

1000001

1000010

1000100

1000101

001

010000

010001

010010

010100

010101

100000

100001

100010

100100

100101

101

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