dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 15

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

5 0 Functional Description
If TRT expires while Late Flag is set TRT is loaded with
TMAX and the recovery process (Claim) is invoked When
TRT expires and the ring is not operational TRT is loaded
with TMAX TRT is also loaded with TMAX on a MAC Reset
5 1 2 Token Recovery
While the ring is operational every station in the ring uses
the Negotiated Target Token Rotation Time TNEG The
MAC implements the protocol for negotiation of this target
token rotation time (TTRT) through the Claim Process The
shortest requested Token Rotation Time is used by all of
the stations in the ring as the TNEG
If TRT expires with Late Flag set a token has not been
received within twice TTRT (Target Token Rotation Time) If
TVX (Valid Transmission Timer) expires the station has not
received a valid token within TVX Max Both these events
require token recovery and cause the Ring Engine to enter
the Claim Process
In the Claim Process a MAC continuously transmits Claim
frames containing TREQ Should the MAC receive a Claim
frame with a shorter TREQ (larger value Higher Claim) it
leaves the Claim State A station that receives its own Claim
frame gains the right to send the first token and make the
ring operational again If the Claim Process does not com-
plete successfully TRT will expire and the Beacon Process
is invoked
The Beacon Process is used for fault isolation A station
may invoke the Beacon Process through an SM Con-
trol request(Beacon) When a station enters the Beacon
Process it continuously sends out Beacon frames The
Beacon Process is complete when a station receives its
Asynchronous Priority
Service Class
Asynchronous
Asynchronous
Synchronous
Requested
TABLE 5-1 Beginning of Service Opportunity
(Continued)
Requested Token
FIGURE 5-1 Token Timing Logic
Capture Class
non-restricted
non-restricted
restricted
any
15
own Beacon frame That station then enters the Claim Pro-
cess to re-initialize the ring
5 2 SERVICING TRANSMISSION REQUESTS
A Request to transmit one or more frames is serviced by the
Ring Engine After a Request is submitted to the Ring En-
gine the Ring Engine awaits an appropriate Service Oppor-
tunity in which to service the Request Frames associated
with the Request are transmitted during the Service Oppor-
tunity The definition of a Service Opportunity is different
depending on the operational state of the ring
A Service Opportunity begins when the criteria presented to
the Ring Engine are met This criteria contains the request-
ed service class (synch asynch asynch 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
When data is not ready to be transmitted Void frames are
transmitted to reset the TVX timers in all stations During an
immediate request while in the Claim or Beacon States
when no Claim or Beacon frames are ready to be transmit-
ted the internally generated Claim or Beacon frames are
transmitted
5 2 1 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 Opportu-
nity when the ring is operational
Late Flag
Late Flag
Late Flag
Ring Op
Ring Op
Ring Op
Ring Op
THT
Criteria
l
THSH
e
e
e
e
e
e
e
1
1
1
1
0
0
0
non-restricted
non-restricted
Token Class
TL F 10387 – 3
Received
restricted
any