DP83266 datasheet(23/152 Pages) NSC

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50 Functional Description (Ring Engine) (Continued)

TABLE 5-3 Request Service Classes

RQRCLS

Name

Class

THT

Token

Notes

Capture

Issue

0000

None

0001

Apri

Async

enabled

non-rstr

THSH1

0010

Reserved

0011

Reserved

0100

Sync

disabled

any

captured

0101

Imm

Immediate

disabled

none

0110

ImmN

Immediate

disabled

none

non-rstr

0111

ImmR

Immediate

disabled

none

rstr

1000

Async

enabled

non-rstr

1001

Rbeg

Restricted

enabled

non-rstr

rstr

2 3

1010

Rend

Restricted

enabled

rstr

non-rstr

1011

Rcnt

Restricted

enabled

rstr

1100

AsynD

Async

disabled

non-rstr

1101

RbeginD

Restricted

disabled

non-rstr

rstr

2 3

1110

RendD

Restricted

disabled

rstr

non-rstr

1111

RcntD

Restricted

disabled

rstr

Note 1

Synchronous Requests are not serviced when RELRBCNR e 1

Note 2

Restricted Requests are not serviced when RELRMACR e 1

Note 3

Restricted Dialogues only begin when a Non-Restricted token has been received and transmitted

Note 4

Immediate Requests are serviced when the ring is Non-Operational These requests are serviced from the Data state if neither signal RQCLM nor RQBCN

is asserted If signal RQCLM is asserted Immediate Requests are serviced from the Claim State If signal RQBCN is asserted Immediate Requests are serviced

from the Beacon State RQCLM and RQBCN do not cause transitions to the Claim and Beacon States

On Transmit frames are checked to see that they are of a

minimum length If the end of a frame is reached before a

valid length is transmitted the frame will be aborted and a

Void frame will be transmitted (as with all aborted frames) A

MAC frame with a zero length INFO field will not be aborted

even though the Receiver will not recognize it as a valid

frame Frame lengths are not checked for the maximum al-

lowable length (4500 bytes)

TABLE 5-4 Valid Frame Length

Frame

Types

Short

Long

Notes

Address

(minimum number

of bytes)

Void

MAC

including a 4 byte

INFO field

non-MAC

including a 0 byte

INFO field

Also on the Transmit side the L bit in the FC field is

checked against the ESA and ELA bits in the Option Regis-

ter (if the SA Transparency option is not selected) to insure

that a frame of that address length can be transmitted If the

selected address length is not enabled the frame is aborted

at the beginning of the SA field If SA Transparency is se-

lected the frame is not aborted

When OptionIRPT is set and SAT and SAIGT are selected

or when ModeDIAG is set the minimum frame length is one

data byte

55 FRAME STATUS PROCESSING

Each frame contains three or more Control Indicators The

FDDI Standard specifies three the E A and C Indicators

When a frame is transmitted the Control Indicators are

transmitted as R (Reset) symbols If an error is detected by

a station that receives the frame the E Indicator is changed

to an S (Set) symbol If a station recognizes the DA of a

frame as its own address (Individual Group or Broadcast)

the A Indicator is changed to an S symbol If that station

then copies the frame the C Indicator is changed to an S

symbol

The received value of the Control Indicators for every frame

received is reported at the MAC Indicate Interface on sig-

nals MID(7 – 0) On a frame transmitted by this station the

returning Control Indicators give the transmission status

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