DP83255AVF datasheet(9/96 Pages) NSC

9 / 96 page

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 symbols as shown in

Figure 3-3

Parity Bit

Control Bit

Data Bits

FIGURE 3-3 Byte-Wide Data

PARITY CHECKER

The Parity Checker verifies that the parity bit in the Data

The parity checking 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 symbols from

the Configuration Switch into their respective 5-bit codes

See Table 3-2 for the Symbol Encoding list

REPEAT FILTER

The Repeat Filter is used to prevent the propagation of

code violations in data frames 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

Details on Repeat Filter operation are described in Section

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) may not 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

More details on the operation of the Smoother can be found

in Section 85

LINE STATE GENERATOR

The Line State Generator allows the transmission of the

PHY Request data and can also generate and transmit Idle

Master Halt or Quiet symbol pairs which can be used to

implement the Connection Management procedures as

specified in the FDDI Station Management (SMT) docu-

ment

The Line State Generator is programmed through Transmit

bits 0 to 2 (TMk20l) of the Current Transmit State Regis-

ter (CTSR)

Based on the setting of these bits the Transmitter Block

operates in the Transmit Modes where the Line State Gen-

erator overwrites the Repeat Filter and Smoother outputs

See Table 3-3 for the listing of the Transmit Modes

TABLE 3-2 4B5B Symbol Encoding

Symbol

4B Code

Outgoing 5B

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

(Starting

1101

11000 and

Delimiter)

10001

(Ending

0100 or

01101

Delimiter)

0101

(Reset)

0110

00111

(Set)

0111

11001

TABLE 3-3 Transmit Modes

Active Transmit Mode

Normal Transmission Mode

Off Transmit Mode

Transmit Quiet symbol pairs

and disable the Fiber Optic

Transmitter

Idle Transmit Mode

Transmit Idle symbol pairs

Master Transmit Mode

Transmit Halt-Quiet symbol

pairs

Quiet Transmit Mode

Transmit Quiet symbol pairs

Reserved Transmit

Reserved for future use If

Mode

selected Quiet symbol pairs

will be transmitted

Halt Transmit Mode

Transmit Halt symbol pairs

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