dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 96

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

7 0 Signal Descriptions
State Descriptions
MR0 Not Ready
In this state the Ring Engine does not have a Service Op-
portunity If RQRCLS is not zero the Ring Engine is trying to
secure a Service Opportunity meeting the requested service
parameters
On a valid Service Opportunity the MR(01) transition is tak-
en The status signals TXED and TXABORT are cleared and
TXRDY is set to indicate that the Transmitter is ready to
service a request
MR1 Ready
In this state the Ring Engine has secured a Service Oppor-
tunity and is ready to service the current request The Trans-
mitter is sourcing Preamble Fill or internally generated Void
(from the Data state) Claim (from the Claim state) or Bea-
con (from the Beacon state) frames
The Service Opportunity is governed by the requested serv-
ice parameters If an unserviceable request is presented for
one or more cycles the Service Opportunity ends If THT
expires or a priority threshold is reached the Service Oppor-
tunity will end immediately or after the next frame depend-
ing on the state of the send window
The send window is an opportunity to send a frame without
being interrupted by time thresholds The Service Opportu-
nity may end during a send window if the service parame-
ters change
The send window opens each time TXRDY is asserted (en-
try to MR1) It remains open for a minimum of 6 byte times
The send window also opens if RQRDY is asserted while
TXRDY is asserted and if TXPASS is not asserted within
one byte time after RQRDY is asserted
(Continued)
FIGURE 7-12 MR1 Substate Diagram
96
The send window is held open until
At any time after RQRDY has been asserted and the final
frame of the request has been sent RQFINAL may be as-
serted to indicate that a token of the Issue Token Class
should be transmitted at the end of the current Service Op-
portunity If the MR(10) transition occurs while RQFINAL is
asserted and all the other conditions for accepting RQFI-
NAL hold the transmitted token will be of the Issue Token
Class
After RQFINAL has been asserted no more frames can be
sent until RQRDY has been deasserted and then reassert-
ed RQRDY should be deasserted and the service parame-
ters updated to reflect the next request (if any) as soon as
possible to allow the Ring Engine to make better ring
scheduling decisions If RQRDY is not deasserted by the
end of the last frame of a Service Opportunity a Void frame
will be transmitted before the token
When the Service Opportunity ends the MR(10) transition is
taken and TXPASS is asserted to indicate the end of the
current Service Opportunity RQFINAL (and RQRDY) must
be asserted not later than one byte time after TXPASS to
insure that a token of the appropriate issue class is issued
When a frame can be sent from the interface the MR(12)
transition is taken and TXACK is set to indicate that the
Transmitter is sending the frame
The state diagram for the internal substates within state
MR1 is shown in Figure 7-12
1) RQSEND or RQFINAL is asserted or
2) until L Max has expired (3 20 ms) a Void frame has
been sent (0 96 ms or 1 60 ms) and 7 more bytes of
preamble have been sent (0 56 ms) (When Op-
tion IRPT
e
1 this condition does not apply )
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