dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 97

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

7 0 Signal Descriptions
SUBSTATE MRR0 Preamble
Upon entry to MR1 8 bytes of Preamble (Idles) are transmit-
ted in substate MRR0
After the Preamble if a frame can be sent from the inter-
face transition MR12 occurs The frame options are
latched TXED is cleared and TXACK is asserted
If a frame cannot be sent from the interface either Fill (addi-
tional Idles) or an internally generated frame (Void Claim or
Beacon) is transmitted
SUBSTATE MRR1 Fill
For requests if RQRDY is asserted (indicating that the cur-
rent request has been selected for service) or Option IRPT
is set (indicating that the ring is being interrupted) additional
fill bytes (Idles) are transmitted in substate MRR1
Fill continues until
If RQRDY is not asserted if RQCLM or RQBCN is asserted
or (unless Option IRPT is set) if the previous frame in the
current Service Opportunity was aborted an internal Void
Claim or Beacon frame is generated in substate MRR2
At the end of an internal frame TXED is set TXABORT is
cleared and another preamble is generated in substate
MRR0
MR2 Sending
In this state the Ring Engine is transmitting a frame from the
MAC Request Interface While the frame is being sent if an
unserviceable request is presented or any timer threshold is
reached the Service Opportunity will end after the current
frame
This implies that a Service Opportunity is never longer than
TMAX plus one maximum length frame interval for Immedi-
ate Requests (unless Option IRPT is set) or TNEG plus one
maximum length frame interval for Non-immediate Re-
quests The maximum length of the frame interval is the
maximum send window open time (4 64 ms – 5 28 ms) plus
F max F max is the maximum length of a frame includ-
ing 2 bytes of Preamble The default value of F max for
FDDI is 4500 bytes
1) a frame can be sent from the interface or
2) the Service Opportunity ends or
3) 32 bytes of Idles are transmitted or RQRDY is deas-
serted After that an internal Void frame is generated
in substate MRR2 (If Option IRPT is set Void frames
are not generated )
e
360 00 ms
(Continued)
97
On entry to MR2 TXACK is asserted It remains asserted
while data is being accepted from the interface RQSEND
must be deasserted within one byte time after entering
MR2
At any time after RQSEND is deasserted for the final frame
of a request RQFINAL may be asserted to indicate that a
token of the Issue Token Class should be transmitted at the
end of the current Service Opportunity If the MR(20) tran-
sition occurs while RQFINAL is asserted and all the other
conditions for accepting RQFINAL hold the transmitted to-
ken will be 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 requests (if any) as soon as
possible to allow the Ring Engine to make better ring
scheduling decisions
The last byte of data at the interface is indicated by RQEOF
which must be asserted with the last byte of data After the
Ending Delimiter of a frame is transmitted TXED is assert-
ed When not using FCS transparency TXED is asserted 7
byte times after RQEOF is asserted When using FCS trans-
parency it is asserted 3 byte times after RQEOF is assert-
ed TXACK is deasserted no later than one byte time after
RQEOF is asserted
At any time during frame transmission TXABORT may be
asserted This indicates that the frame was aborted due to
internal errors buffering errors parity errors RQABT MAC
reset reception of a MAC frame etc TXACK is deasserted
no later than TXABORT is asserted When a transmission is
aborted due to an error (and Option IRPT is not set) a Void
frame is transmitted to reset the TVX timers in all stations in
the ring
After a successful or unsuccessful frame transmission if the
current Service Opportunity can be continued transition
MR(21) occurs and TXRDY is asserted otherwise transition
MR(20) occurs and TXPASS is asserted
If at any time during a frame transmission the end of Serv-
ice Opportunity condition is detected transition MR(20) will
occur after the current frame transition