dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 12

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

4 0 FDDI MAC Facilities
Beacon Frames
Beacon frames are transmitted continuously with minimum
preamble when the Ring Engine is in the Transmit Beacon
state The format of Beacon frames generated by the Ring
Engine is shown in Table 4-8 When long addressing is en-
abled frames with the long address are transmitted other-
wise frames with the short address are transmitted
When the Transmit Beacon State is entered from the Trans-
mit Claim State the first byte of the 4 byte TBT Field is
transmitted as Zero
Beacon frames that require alternative formats such as Di-
rected Beacons must be generated externally
The Ring Engine detects reception of valid Beacon frames
and distinguishes between Beacon frames transmitted by
this MAC (My Beacon) and Beacon frames transmitted by
other stations (Other Beacon) Details are given in Appen-
dix A
4 3 FRAME COUNTS
To aid in fault isolation and to enhance the management
capabilities of a ring the Ring Engine maintains several
frame counts The Error and Isolated frame counts incre-
ment when a frame is received with one or more errors that
were previously undetected The Ring Engine then corrects
the error such that a downstream station will not increment
its count
The size of the counters has been chosen such that minimal
software intervention is required even under marginal oper-
ating conditions
The following counts are maintained by the Ring Engine
4 3 1 Frame Received Count
The Frame Received Count (FRCT) is specified in the FDDI
MAC Standard and is the count of all complete frames re-
ceived This count includes frames stripped by this station
4 3 2 Error Isolated Count
The Error Isolated Count (EICT) is specified in the FDDI
MAC Standard and is the count of error frames detected by
this station and no previous station It increments when
1 An FCS error is detected and the received Error Indicator
2 A frame of invalid length (i e off boundary T) is received
3 Er is not R or S
My Beacon
My Beacon
(Er) is not equal to S
and Er is not equal to S
Type
FRCT
EICT
LFCT
FCCT
FNCT
FTCT
Not ELA
Enable
ELA
Frame Received
Error Isolated
Lost Frame
Frames Copied
Frames Not Copied
Frames Transmitted
Short
Long
Size
(Continued)
PA
PA
TABLE 4-8 Beacon Frames
SFS
SD
SD
12
FC
82
C2
4 3 3 Lost Frame Count
The Lost Frame Count (LFCT) is specified in the FDDI MAC
Standard and is the count of all instances where a format
error is detected in a frame or token such that the credibility
of PDU reception is placed in doubt The Lost Frame Count
is incremented when any symbol other than data or Idle
symbols are received between the Starting and Ending Del-
imiters of a PDU (this includes parity errors)
4 3 4 Frame Copied Count
The Frames Copied Count (FCCT) is specified in the FDDI
MAC-2 Standard and is the count of the number of frames
copied by this station The count is incremented when an
internal or external match occurs (when Option EMIND is
enabled) on the Destination Address no errors were detect-
ed in the frame and the frame was successfully copied
(VCOPY
formance statistics Frames copied promiscuously MAC
frames Void frames and NSA frames received with the A
indicator set are not included in this count
4 3 5 Frames Not Copied Count
The Frames Not Copied Count (FNCT) is specified in the
FDDI MAC-2 Standard and is the count of frames intended
for this station that were not successfully copied by this
station The count is incremented when an internal or exter-
nal (when Option EMIND is enabled) Destination Address
match occurs no errors were detected in the frame and the
frame was not successfully copied (VCOPY
count is an indication of insufficient buffering or frame pro-
cessing capability for frames addressed to the station MAC
frames Void frames and NSA frames received with the A
indicator set are not included in this count
4 3 6 Frames Transmitted Count
The Frames Transmitted Count (FTCT) is specified in the
FDDI MAC-2 Standard and is incremented every time a
complete frame is transmitted from the MAC Request Inter-
face The count is provided as an aid to accumulate station
performance statistics Void and MAC frames generated by
the Ring Engine are not included in the count
4 4 TIMERS
4 4 1 Token Rotation Timer
The Token Rotation Timer (TRT) times token rotations from
arrival to arrival TRT is used to control ring scheduling dur-
ing normal operation and to detect and recover from serious
ring error situations
TRT is loaded with the maximum token rotation time TMAX
when the ring is not operational TRT is loaded with the
negotiated Target Token Rotation Time TNEG when the
ring is operational
Null
Null
DA
e
1) This can be used to accumulate station per-
MSA
MLA
SA
INFO
TBT
TBT
FCS
FCS
FCS
e
0) This
TRRR
TRRR
EFS