DP83266 datasheet(15/152 Pages) NSC

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40 FDDI MAC Facilities (Continued)

Frames Generated Externally

The Ring Engine transmits frames passed to it from the Sys-

tem Interface The data portion of the frame is created by

the System Interface The data portion begins with the FC

field and ends with the last byte of the INFO field The FC

field is passed transparently to the ring The length bit in the

FC field is used to determine the length of the transmitted

addresses The data is passed as a byte stream across the

MAC Request Interface as shown in Table 4-5

TABLE 4-5 Frame Formats

Field

Size

Request

(bytes)

8 s40

Idle Pairs

2 or 6

MSA MLA or SA

INFO

FCS

4 if present

FCS

Before the frame is transmitted the Ring Engine inserts the

Start of Frame Sequence with at least 8 bytes of Preamble

but no more than 40 bytes of Preamble The starting delimi-

ter is transmitted as a JK symbol pair The Source Address

is normally transmitted by the Ring Engine since it uses the

Source Address to determine when to strip a frame from the

ring This can be overridden by using the Source Address

transparency capability Similarly the Frame Check Se-

quence (4 bytes) is normally transmitted by the Ring Engine

This can be overridden with the FCS transparency capabili-

ty With FCS transparency the FCS is transmitted from the

data stream The End of Frame Sequence is always trans-

mitted by the Ring Engine as TR RR

Frames Generated by Ring Engine

The Ring Engine generates and detects several frames in

order to attain and maintain an operational ring

Void Frames

Void frames are used during normal operation The Ring

Engine generates two types of void frames regular Void

frames and My

Void frames

If Short addressing is enabled Void frames with the short

address (MSA) are transmitted Otherwise Void frames with

the long address (MLA) are transmitted Table 4-6 shows

the Void frame format

Void frames are transmitted in order to reset the Valid

Transmission timers (TVX) in other stations to eliminate un-

necessary entry to the Claim state Stations are not required

to copy Void frames Void frames are transmitted by the

Ring Engine in two situations

1 While holding a token when no data is ready to be trans-

mitted

2 After a frame transmission is aborted

Void frames are transmitted by the Ring Engine in

three situations

1 After a request to measure the Ring Latency has been

made and the next early token is captured

2 After this station wins the Claim process and before the

token is issued

3 After a frame has been transmitted with the STRIP op-

tion and before the token for that service opportunity is

issued

Void frames are also detected by the Ring Engine A Void

frame with a Source Address other than MSA or MLA is

considered an Other

Void frame

Claim Frames

Claim frames are generated continuously with minimum pre-

amble while the Ring Engine is in the Transmit Claim state

The format of Claim frames generated by the Ring Engine is

shown in Table 4-7 When long addressing is enabled

frames with the long address (MLA) are transmitted Other-

wise frames with the short address (MSA) are transmitted

The Ring Engine detects reception of valid Claim frames A

comparison is performed between the first four bytes of the

received INFO field and the value of TREQ programmed in

the parameter RAM in order to distinguish Higher

Claim

Lower

Claim Duplicate

Claim and My

Claim

Beacon Frames

Beacon frames are transmitted continuously with minimum

preamble when the Ring Engine is in the Transmit Beacon

state The format of Beacon frames generated by the Ring

Engine is shown in Table 4-8 When long addressing is en-

abled frames with the long address (MLA) are transmitted

Otherwise frames with the short address (MSA) are trans-

mitted

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