DP83261AVF datasheet(20/132 Pages) NSC

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50 Functional Description (Continued)

551 Odd Symbols Handling

When the first T symbol of a frame is received as the sec-

ond symbol of a symbol pair (the T symbol is received off-

boundary) the Ring Engine corrects this condition by send-

ing out the symbol sequence TSII This symbol sequence

indicates the end of the frame and that an error has been

detected in the frame Note that this is a low probability

error event

Reception of symbols other than R S and T during the

Frame Status processing is also a low probability event

This event is handled slightly differently on the first byte of

the Ending Frame Status

After the first byte of the Ending Frame Status if either the

first symbol is not RorS or the second symbol is not Ror

SorT

an Idle symbol pair (II) is transmitted

56 SMT FRAME PROCESSING

All SMT frames are handled as all other frames with the

exception of the SMT Next Station Addressing (SMT NSA)

frame NSA frames are used to announce this station’s ad-

dress to the next addressed station The current SMT proto-

col requires stations to periodically (at least once every 30

seconds) transmit an NSA frame Since the Broadcast ad-

dress is used and every station is required to recognize the

broadcast address the downstream neighbor will set the A

Indicator A station can determine its upstream neighbor by

finding NSA frames received with the A Indicator received

as R By collecting this information from all stations a map

of the logical ring can be built

Additionally only the station that sets the A Indicator is per-

mitted to set the C Indicator on such frames In this way the

station that sends out the NSA frame can determine if the

next addressed station copied the frame by examining the

returning C Indicator

57 MAC FRAME PROCESSING

Upon the reception of a valid MAC frame (Claim Beacon or

Other) the Ring

Operational flag is reset and the Ring En-

gine enters the Idle Claim or Beacon State Received Claim

and Beacon frames are processed as defined in the Stan-

dard (See Appendix A) unless inhibited by the bits in the

Option Register

571 Claim Token Process

Receive

When a Claim frame is received its Frame Type is reported

(Claim frame) along with the type of Claim frame

There are three types of Claim frames

Claim

Higher

Claim and Lower

Claim

AMy

Claim frame is a Claim frame with a Source Address

that matches this station address and the T

Bid

Rc in the

INFO field is equal to this station’s TREQ

A Higher

Claim frame is a Claim frame with a Source Ad-

dress that does not match this station address and the

Bid

Rc in the INFO field is greater than this station’s

TREQ

A Lower

Claim frame is a Claim frame with a Source Ad-

dress that does not match this station address and the

Bid

Rc in the INFO field is less than this station’s

TREQ

Transmit

Claim frames are transmitted continuously while in the

Claim State

Claim frames are generated by the Ring Engine unless an

Immediate Claim Request is present at the MAC Request

Interface Even if an Immediate Claim Request is present at

the MAC Request Interface at least one Claim frame must

be generated by the Ring Engine before Claim frames from

the Interface are transmitted

For internally generated Claim frames the Information field

is transmitted as the 4-byte Requested Target Token Rota-

tion Time

The Information field of a Claim frame consists of the sta-

tion’s Requested Target Token Rotation Time In the Ring

Engine implementation

TREQ is programmable with

2048 ms resolution and a maximum value of 134 seconds

Claim Protocol

Entry to the Claim state occurs whenever token recovery is

required The Recovery Required condition occurs when

TRT expires and Late Flag is set

TVX expires

A Lower Claim frame or My Beacon frame is received

Entry to the Claim state may be blocked by enabling the

Inhibit Recovery Required option (bit OptionITR)

The Claim state is entered (even if OptionIRR e 1) with a

Controlrequest (Claim) (Set FunctionCLM to 1)

While in the Claim state

Claim frames are transmitted continuously

If a Higher Claim frame is received the station exits the

Claim state and enters the IDLE state In this state it then

repeats additional Higher Claim frames

If a Lower Claim frame is received this station continues

to send its own Claim frames and remains in the Claim

state

Eventually if a logical ring exists the station with the short-

est TREQ on the ring should receive its own Claim frames

the My Claim frame This completes the Claim Token Pro-

cess This one station then earns the right to issue a token

to establish an Operational ring

An option is provided to remain in the Claim state if this

station won the Claim Token Process by enabling the Inhibit

Token Release Option (bit OptionITR)

DP83261AVF Datasheet(PDF) 20 Page - National Semiconductor (TI)

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