dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 18

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

5 0 Functional Description
SA Transparency also overrides the Long and Short Ad-
dressing enables For example if Long Addressing is not
enabled it is still possible to transmit frames with Long Ad-
dresses Similarly if Short Addressing is not enabled it is
still possible to transmit Frames with Short Addresses This
may be useful in full duplex point to point applications and
for diagnostic purposes
Source Address Most Significant Bit Transparency
With the Source Address MSB Transparency option the
MSB of the SA is sourced from the data stream as opposed
to being transmitted as Zero The SA MSB Transparency
option is selected by asserting signal SAIGT (Pin 11)
Unless the Source Address Transparency option is also se-
lected the rest of the SA is generated by the Ring Engine
The MSB of the SA is used to denote the presence of the
Routing Information Field used in Source Routing algo-
rithms (as in the IEEE 802 5 protocol) This option is useful
for stations that utilize Source Routing In these applica-
tions the SA can still be generated by the Ring Engine
even when routing information is inserted into the data
stream
Void Stripping
This option is useful for removing bridged and ownerless
frames and remnants (fragments) from the ring
In the Void Stripping protocol two My Void frames are
transmitted at the end of a service opportunity Stripping
continues until one of the following conditions occur
If any frame of a Service Opportunity requests this option
then all frames on that service opportunity will be stripped
using this method Void Stripping is invoked upon the asser-
tion of the STRIP signal (Pin 13) at the beginning of a frame
transmission





One My Void frames returns (The Second My Void
will be stripped on the basis of the SA)
A Token is received
An Other Void is received
A MAC frame other than My Claim is received
A MAC Reset occurs
Frame Types
Non-MAC
MAC
Void
Short Address
13
9
9
(Continued)
TABLE 5-4 Valid Frame Length
(Minimum Number of Bytes)
18
Void Stripping is also automatically invoked by this station if
it wins the Claim Process before the initial token is issued
This removes all fragments and ownerless frames from the
ring when the ring becomes operational
FCS Transparency
Normally the BMAC device generates and transmits the
FCS When the Frame Check Sequence Transparency op-
tion is selected the Ring Engine device does not append
the FCS to the end of the Information field This option is
selected by asserting signal FCST (Pin 14)
The receiving stations treat the last four bytes of the data
stream as the FCS
This option may be useful for end to end FCS coverage
when crossing FDDI bridges for diagnostic purposes or in
Implementer frames
5 4 FRAME VALIDITY PROCESSING
A valid frame is a frame that meets the minimum length
criteria and contains an integral number of data symbol
pairs between the Starting and Ending Delimiters as shown
in Table 5-4
On the Transmit side frames are checked to see that they
are of a minimum length If the end of a frame is reached
before a valid length is transmitted the frame will be abort-
ed and a Void frame will be transmitted (as with all aborted
frames) A MAC frame with a zero length INFO field will not
be aborted even though the Receiver will not recognize it as
a valid frame Frame lengths are not checked for the maxi-
mum allowable length (4500 bytes)
Also on the Transmit side the L bit in the FC field is
checked against the ESA and ELA bits in the Option Regis-
ter (if the SA Transparency option is not selected) to insure
that a frame of that address length can be transmitted If the
selected address length is not enabled the frame is aborted
at the beginning of the SA field If SA Transparency is se-
lected the frame is not aborted
Long Address
17
21
17
Including a 4 Byte
Including a 0 Byte
INFO Field
INFO Field
Notes