dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 16

• Manufacturer Part Number: dp83261
• Description: Bmac Device Fddi Media Access Controller
• Manufacturer: National Semiconductor Corporation

5 0 Functional Description
In addition to the criteria mentioned above additional crite-
ria apply to the servicing of Synchronous and Restricted
Requests
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
2 The token became unusable while servicing a request
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
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
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
5 2 2 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









Synchronous Requests are not serviced if RELR BCNR
is set (See Section 4 5 1)
Restricted requests are not serviced when RELR BCNR
RELR CLMR or RELR OTRMAC are set (See Section
4 5 3)
Restricted Dialogues may only begin when a non-restrict-
ed token has been received and transmitted (See Sec-
tion 4 5 3)
All frames of all active requests have been transmitted
Asynchronous Priority threshold reached (If an Asynch
Priority Request is being serviced)
THT expired (if enabled)
A MAC Reset
Reception of a valid MAC frame
TRT expiration (TRT was reset when the token was cap-
tured)
Immediate Non-Restricted
Immediate Restricted
Continue Restricted
Begin Restricted
Non-Restricted
End Restricted
Service Class
Immediate
TABLE 5-2 Token Transmission Type
(Continued)
Token Captured
Non-Restricted
Non-Restricted
16
Restricted
Restricted
None
None
None
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 TMAX of time elapses on this request
3 The transmitter exits the requested state
4 The ring becomes operational while servicing an immedi-
5 2 3 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 bandwidth
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
During a transmission several errors can occur 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
transmission is aborted due to an external error (and Op-
tion IRPT is not set) a Void frame is transmitted to reset the
TVX timers in all stations in the ring When a frame is abort-
ed due to a transmission error the Service Opportunity is
not automatically ended
5 3 REQUEST SERVICE PARAMETERS
5 3 1 Request Service Class
The Request Service corresponds to the Request
Service Class and the token class parameters of the
(SM )MA DATA request and (SM )MA Token request
primitives as specified in the Standard
14 useful combinations of the Requested Service Class
(Non-Restricted Asynchronous Restricted Asynchronous
Synchronous Immediate) the Token Capture and Issue
Class and THT Enable are supported by the Ring Engine as
shown in Table 5-3
ate request
Non-Restricted
Non-Restricted
Non-Restricted
Token Issued
Restricted
Restricted
Restricted
None