DP83266 datasheet(12/152 Pages) NSC

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30 Architectural Description

(Continued)

Each Channel will only process one object type at a time

The BIU arbitrates between the Channels and issues a Bus

Request when any Channel requests service The priority of

Channel bus requests is as follows from highest priority to

lowest priority

1 Indicate Data Unit writes (highest priority when not trans-

mitting)

2 Output Data Unit fetches (highest priority when transmit-

ting)

3 Request Descriptor and Output Data Unit Descriptor

fetches

4 Input Data Unit Descriptor writes

5 Confirmation Message Descriptor writes

6 Next Pool Space Descriptor transfer to Current Pool

Space Descriptor (internal operation)

7 Pool Space Descriptor fetches

8 Limit RAM Operations (internal operation)

9 Pointer RAM Operations (lowest priority)

Addresses for Channel accesses are contained in the Point-

er RAM Registers

345 Pointer RAM

The Pointer RAM Block is used by both the Indicate and

Request Machines It contains pointers to all Data Units and

Descriptors manipulated by the MACSI device namely In-

put and Output Data Units Input and Output Data Unit De-

scriptors Request Descriptors Confirmation Message De-

scriptors and Pool Space Descriptors

The Pointer RAM Block is accessed by clearing the PTOP

(Pointer RAM Operation) bit in the Service Attention Regis-

ter which causes the transfer of data between the Pointer

RAM Register and a mailbox location in memory

346 Limit RAM

The Limit RAM Block is used by both the Indicate and Re-

quest Machines It contains data values that define the lim-

its of the ten Queues maintained by the MACSI device

Limit RAM Registers are accessed by clearing the LMOP

(Limit RAM Operation) bit in the Service Attention Register

which causes the transfer of data between the Limit RAM

40 FDDI MAC Facilities

41 SYMBOL SET

The Ring Engine (MAC) recognizes and generates a set of

symbols These symbols are used to convey Line States

(such as the Idle Line State) Control Sequences (such as

the Starting and Ending Delimiters) and Data

Additional information regarding the symbol set can be

found in the ANSI X3T95 PHY standard

The Ring Engine expects that the Starting Delimiter will al-

ways be conveyed on an even symbol pair boundary (ie

the JK symbol will always arrive as a byte not split across

two bytes) Following the starting delimiter data symbols

should always come in matched pairs Similarly the Ending

Delimiter should always come in one or more matched sym-

bol pairs

The symbol pairs conveyed at the PHY Interface are shown

in Table 4-1

42 PROTOCOL DATA UNITS

The Ring Engine recognizes and generates Tokens and

Frames

The Token is used to control access to the ring Only the

station that has captured the token has the right to transmit

new information The format of a token is shown in

Figure

4-1

TABLE 4-1 Symbol Pair Set

Type

Symbols

Starting Delimiter

JK or IL

Ending Delimiter

TT or TR or TS or TI

Frame Status

RR or RS or SR or SS

Idle

II or nI

Data Pair

n represents any data symbol (0–F)

Symbol pairs other than the defined symbols are treated as code violations

Additional information on the symbol pairs generated and interpreted by the

Ring Engine can be found in Section 821

TABLE 4-2 Frame Fields

Name

Description

Size

SFS

Start of Frame

Sequence

Preamble

8 or more Idle

symbol pairs

Starting Delimiter

JK symbol pair

Frame Control Field

1 data symbol pair

Destination Address

2 or 6 symbol pairs

Source Address

2 or 6 symbol pairs

INFO

Information Field

FCS

Frame Check

4 symbol pairs

Sequence

EFS

End of Frame

Sequence

Ending Delimiter

at least 1 T symbol

for Frames

at least 2 T symbols

for Tokens

Frame Status

3 or more R or S

symbols

SFS

EFS

FIGURE 4-1 Token Format

Frames are used to pass information between stations The

format of a frame is shown in

Figure 4-2 with the field defini-

tions in Table 4-2

SFS

Protected by FCS

EFS

INFO

FCS

FIGURE 4-2 Frame Format

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