dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 21

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

5 0 Functional Description
5 7 2 Beacon Process
Receive
When a Beacon frame is received its Frame Type is report-
ed (Beacon frame) along with the type of Beacon frame
There are two types of Beacon frames My Beacon and
Other Beacon
A Beacon frame is considered a My Beacon if its Source
Address matches this station’s address (long or short)
A Beacon frame is marked as Other Beacon if its Source
Address does not match this station’s address
Transmit
Beacon frames are transmitted continuously while in the
Beacon state
Beacon frames are generated by the Ring Engine unless an
Immediate Beacon Request is present at the MAC Request
Interface Even if an Immediate Beacon Request is present
at the MAC Request Interface at least one Beacon frame
must be generated by the Ring Engine before Beacon
frames from the Interface are transmitted
For internally generated Beacon frames the Ring Engine
uses the TBT in the Information field
Beacon Protocol
Entry to the Beacon state occurs under two conditions
Beacon frames are then transmitted until the Beacon pro-
cess is completed
If an Other Beacon frame is received this station exits the
Beacon state stops sending its own Beacon frames and
repeats the incoming Beacon frames
If a My Beacon frame is received the station has received
back its own Beacon frame thus successfully completing
the Beacon process The station then enters the Claim Pro-
cess
5 7 3 Handling Reserved MAC Frames
A Reserved MAC frame is any MAC frame aside from the
Claim and Beacon frame Tokens are not considered MAC
frames even though Format bit (FC FF) are the same as for
MAC frames
When a Reserved MAC frame (Other MAC) is received it
is treated as a Higher Claim If the Transmitter is in the
Claim state when a Reserved MAC frame is received the
Transmitter returns to the Idle state and then repeats the
next Reserved MAC frame received If the Transmitter is in
the Beacon state and a Reserved MAC frame is received
the Transmitter continues to transmit Beacon frames If the
Transmitter is in the Idle state the Reserved MAC frame is
repeated


A failed Claim Process (TRT expires during the Claim
process)
An SM MA Control request (Beacon)
(Set Function BCN to 1)
(Continued)
21
5 8 RECEIVE BATCHING SUPPORT
The Ring Engine stores each received SA and compares
the incoming SA with the previous SA This may be used to
batch status on frames received from the same station
The SameSA signal is asserted when
1 The curent and previous non-Void frames were not MAC
2 The size of the address of the current frame is the same
3 The SA of the current frame is the same as the SA of the
On MAC frames the Information fields are compared This
information may be useful to inhibit copying MAC frames
with identical information This is particularly useful for copy-
ing Claim and Beacon frames when new information is pres-
ent
The Same INFO signal is asserted when
1 The current and previous non-Void frames were both
2 The first four bytes of the INFO field of the current frame
The size of the address of MAC frames is not checked
5 9 IMMEDIATE FRAME TRANSMISSION
Immediate requests are used when it is desirable to send
frames without first capturing a token Immediate requests
are typically used as part of management processes for er-
ror isolation and recovery Immediate requests are also use-
ful in full duplex applications Immediate requests are serv-
iced only when the station’s Ring Operational flag is not
set (CTSR ROP
To transmit an Immediate request the request must first be
queued at the MA Request Interface If the Ring is not
operational (Ring Operational flag is not set) the request
will be serviced immediately If the Ring is operational
(Ring Operational flag is set) the request will be serviced
when the Ring becomes non-operational The Ring be-
comes non-operational as a result of a MAC Reset
(Function MCRST is set to One) or any of the conditions
causing the Reset or Recovery Actions are performed
In addition to servicing an Immediate request from the
Tx Data State it is also possible to service Immediate re-
quests from the Claim or Beacon State When transmitting
from the Claim or Beacon state in addition to requesting an
Immediate Transmission Service Class the RQCLM or
RQBCN signals (pins 15 and 16) must be asserted to indi-
cate an Immediate Claim or Immediate Beacon request
These requests will only be serviced when in the Claim or
Beacon state Entry to the Beacon State can be forced
frames
as the size of the address of the previous non-Void frame
previous non-Void frame
MAC frames (not necessarily the same FC value)
is the same as the first four bytes of the INFO field of the
previous non-Void frame
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