DP83256 datasheet(13/144 Pages) NSC

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30 Functional Description (Continued)

DATA REGISTERS

Data from the Configuration Switch is stored in the Data

Registers The 10-bit byte-wide data consists of a parity bit

a control bit and two 4-bit data symbols as shown below

Parity Bit Control Bit

Data Bits

FIGURE 3-4 Byte-Wide Data

The parity is odd parity The control bit determines whether

the Data bits represent Data or Control information When

the control bit is 0 the Data field is interpreted as data and

when it is 1 the field is interpreted as control information

according to the National Semiconductor control codes

PARITY CHECKER

The Parity Checker verifies that the parity bit in the Data

The parity is enabled and disabled through bit 6 (PRDPE) of

the Current Transmit State Register (CTSR)

If a parity error occurs the Parity Checker will set bit 0 (DPE)

in the Interrupt Condition Register (ICR) and report the error

to the Repeat Filter

4B5B ENCODER

The 4B5B Encoder converts the two 4-bit data symbols

from the Configuration Switch into their respective 5-bit

codes

See Table 3-2 for the Symbol Encoding list

TABLE 3-2 4B5B Symbol Encoding

Symbol

4B Code

5B Code

0000

11110

0001

01001

0010

10100

0011

10101

0100

01010

0101

01011

0110

01110

0111

01111

1000

10010

1001

10011

1010

10110

1011

10111

1100

11010

1101

11011

1110

11100

1111

11101

0000

11110 or

11111

JK (Starting

1101

11000 and

Delimiter)

10001

T (Ending

0100 or

01101

Delimiter)

0101

R (Reset)

0110

00111

Note

The upper group of symbols are sent with the ControlData pin set to

Data while the bottom grouping of symbols are sent with the ControlData

pin set to Control

REPEAT FILTER

The Repeat Filter is used to prevent the propagation of

code violations to the downstream station

Upon receiving violations in data frames the Repeat Filter

replaces them with two Halt symbol pairs followed by Idle

symbols Thus the code violations are isolated and recov-

ered at each link and will not be propagated throughout the

entire ring

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