DP83256VF datasheet(15/144 Pages) NSC

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30 Functional Description (Continued)

TABLE 3-3 Abbreviations used in the

Repeat Filter State Diagram

IDLE

Force Idle

true when not in Active

Transmit Mode

Represents the symbols R or S or T

TPARITY

Parity error

Data symbols (for C e 0 in the PHY-MAC

interface)

Data portion of a control and data symbol

mixture

Any symbol (ie don’t care)

Violation symbols or symbols inserted by

the Receiver Block

Idle symbols or symbols inserted by the

Receiver Block

ALSZILSZ

Active Line State or Idle Line State (ie

PHY Invalid)

ALSZILSZ Not in Active Line State nor in Idle Line

State (ie PHY Valid)

Halt Symbol

Reset Symbol

Set Symbol

Frame ending delimiter

Frame start delimiter

Idle symbol (Preamble)

Code violations

The Repeat Filter complies with the FDDI standard by ob-

serving the following (see

Figure 3-5 )

1 In Repeat State violations cause transitions to Halt State

and two Halt symbol pairs are transmitted (unless JK or Ix

occurs) followed by transition to Idle State

2 When Ix is encountered the Repeat Filter goes to the Idle

State during which Idle symbol pairs are transmitted until

a JK is encountered

3 The Repeat Filter goes to the Repeat State following a JK

from any state

The END State which is not part of the FDDI PHY standard

allows an R or S prior to a T within a frame to be recognized

as a violation It also allows NT to end a frame as opposed

to being treated as a violation

SMOOTHER

The Smoother is used to keep the preamble length of a

frame to a minimum of 6 Idle symbol pairs

Idle symbols in the preamble of a frame may have been

added or deleted by each station to compensate for the

difference between the Receive Clock and its Local Clock

The preamble needs to be maintained at a minimum length

to allow stations enough time to complete processing of one

frame and prepare to receive another Without the Smooth-

er function the minimum preamble length (6 Idle symbol

pairs) cannot be maintained as several stations may con-

secutively delete Idle symbols

The Smoother attempts to keep the number of Idle symbol

pairs in the preamble at 7 by

Deleting an Idle symbol pair in preambles which have

more than 7 Idle symbol pairs

andor

Inserting an idle symbol pair in preambles which have

less than 7 idle symbol pairs (ie Extend State)

The Smoother Counter starts counting upon detecting an

Idle symbol pair It stops counting upon detecting a JK sym-

bol pair

Figure 3-6 describes the Smoother state diagram

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