DP83266 datasheet(20/152 Pages) NSC

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50 Functional Description (Ring Engine) (Continued)

A Service Opportunity begins when the criteria presented to

the Ring Engine are met This criteria contains the request-

ed service class (sync async async priority immediate) and

the type of token to capture (restricted non-restricted any

none)

During a service opportunity the Ring Engine guarantees

that a valid frame is sent with at most 40 bytes of preamble

(unless OptionIRPT is set) When data is not ready to be

transmitted Void frames are transmitted to reset the TVX

timers in all stations During an immediate request from the

Claim or Beacon state if the data for external Claim or Bea-

con frame is not ready to be transmitted the Ring Engine

will transmit Claim or Beacon frames using the same inter-

nal data used for normal Claim and Beacon processing

521 Service Opportunity while Ring Operational

Beginning of Service Opportunity

Table 5-1 shows the conditions that must be true when a

valid token is received in order to begin a service opportuni-

ty when the ring is operational

In addition to the criteria mentioned above additional crite-

ria apply to the servicing of Synchronous and Restricted

Requests

Synchronous Requests are not serviced if

RELRBCNR e 1 (see Section 451)

Restricted requests are not serviced when

RELRMACR e 1 (see Section 453)

Restricted Dialogues may only begin when a non-restrict-

ed token has been received and transmitted (see Section

453)

End of Service Opportunity

The Service Opportunity continues until either a token is

issued or the ring becomes non-operational

A token is issued after the current frame if any is transmit-

ted when

1 It is no longer necessary to hold the token

All frames of all active requests have been transmitted

2 The token became unusable while servicing a request

Asynchronous Priority threshold reached (if an Async

Priority Request is being serviced)

THT expired (if enabled)

When the ring becomes non-operational the current frame

transmission is aborted The ring may go non-operational

while holding a token as a result of any one of the following

conditions being present

A MAC Reset

Reception of a valid MAC frame

TRT expiration (TRT was reset when the token was cap-

tured)

Issue Token Type

The criteria presented to the Ring Engine to begin a service

opportunity also contains the Issue Token Class The Issue

Token Class is used if servicing of that request was com-

pleted (the last frame of that request was transmitted) Oth-

erwise a token of the capture token class is issued

TABLE 5-1 Beginning of Service Opportunity

Requested

Received

Service

Token

Criteria

Token

Class

Capture

Class

Asynchronous non-restricted THT l THSH

non-restricted

Priority

Late

Flag e 0

Ring

Op e 1

Asynchronous non-restricted Late

Flag e 0 non-restricted

Ring

Op e 1

Asynchronous restricted

Late

Flag e 0 restricted

Ring

Op e 1

Synchronous

any

Ring

Op e 1

any

When servicing multiple requests on a single service oppor-

tunity the issue token class of the previous class becomes

the capture class for the next request for purposes of deter-

mining usability

The type of token issued depends on the service class and

the type of token captured as shown in Table 5-2

522 Service Opportunity While Ring Not Operational

While the ring is not operational a service opportunity oc-

curs if an immediate transmission is requested from the

transmitter Data Claim or Beacon state and the transmitter

is in the appropriate state

The service opportunity continues until any one of the fol-

lowing conditions exist

1 No (additional) frames are to be sent

2 Time TMAX elapses on this request

3 The transmitter exits the requested state

4 The ring becomes operational while servicing an immedi-

ate request

523 Frame Transmission

Frames associated with the current request may be trans-

mitted at any time during a Service Opportunity In many

applications data is ready to be transmitted when the Re-

quest is presented to the interface Soon after the Service

Opportunity begins frame transmission begins In other ap-

plications in order to minimize the effects of ring latency it

is desirable to capture the token when no data is ready to

be transmitted This capability results in wasted ring band-

width and should be used judiciously

During transmission a byte stream is passed from the Sys-

tem Interface to the MAC Request Interface The data is

passed through the Ring Engine and appears at the PHY

Request Interface two byte times later

While a frame is being transmitted the Request parameters

for the next Request (if different) may be presented to the

interface At the end of the current frame transmission a

decision is made to continue or cancel the current service

opportunity based on the new Request parameters

Several errors can occur during a transmission A transmis-

sion may be terminated unsuccessfully because of external

buffering or interface parity errors internal Ring Engine er-

rors a MAC reset or reception of a MAC frame When a

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