82596DX Datasheet, PDF (21/77 Pages) Intel Corporation – HIGH-PERFORMANCE 32-BIT LOCAL AREA NETWORK COPROCESSOR

82596DX SX
PREAMBLE
START
FRAME
DELIMITER
DESTINATION SOURCE
ADDRESS
ADDRESS
LENGTH
FIELD
Figure 12 Frame Format
DATA
FIELD
FRAME
CHECK
SEQUENCE
END
FRAME
DELIMITER
RECEIVING FRAMES
To reduce CPU overhead the 82596 is designed to
receive frames without CPU supervision The host
CPU first sets aside an adequate receive buffer
space and then enables the 82596 Receive Unit
Once enabled the RU watches for arriving frames
and automatically stores them in the Receive Frame
Area (RFA) The RFA contains Receive Frame De-
scriptors Receive Buffer Descriptors and Data Buff-
ers (see Figure 13) The individual Receive Frame
Descriptors make up a Receive Descriptor List
(RDL) used by the 82596 to store the destination
and source addresses the length field and the
status of each frame received (see Figure 14)
Once enabled the 82596 checks each passing
frame for an address match The 82596 will recog-
nize its own unique address one or more multicast
addresses or the broadcast address If a match is
found the 82596 stores the destination and source
addresses and the length field in the next available
RFD It then begins filling the next available Data
Buffer on the FBL which is pointed to by the current
RFD with the data portion of the incoming frame As
one Data Buffer is filled the 82596 automatically
fetches the next DB on the FBL until the entire frame
is received This buffer chaining technique is particu-
larly memory efficient because it allows the system
designer to set aside buffers to fit frames much
shorter than the maximum allowable frame length If
AL-LOC ^e 1 or if the flexible memory structure is
used the addresses and length field can be placed
in the receive buffer
Once the entire frame is received without error the
82596 does the following housekeeping tasks
 The actual count field of the last Buffer Descrip-
tor used to hold the frame just received is updat-
ed with the number of bytes stored in the associ-
ated Data Buffer
 The next available Receive Frame Descriptor is
fetched
 The address of the next available Buffer Descrip-
tor is written to the next available Receive Frame
Descriptor
 A frame received interrupt status bit is posted in
the SCB
 An interrupt is sent to the CPU
If a frame error occurs for example a CRC error the
82596 automatically reinitializes its DMA pointers
and reclaims any data buffers containing the bad
frame The 82596 will continue to receive frames
without CPU help as long as Receive Frame De-
scriptors and Data Buffers are available
82596 NETWORK MANAGEMENT
AND DIAGNOSTICS
The behavior of data communication networks is
normally very complex because of their distributed
and asynchronous nature It is particularly difficult to
pinpoint a failure when it occurs The 82596 has ex-
tensive diagnostic and network management func-
tions that help improve reliability and testability The
82596 reports on the following events after each
frame is transmitted
 Transmission successful
 Transmission unsuccessful Lost Carrier Sense
 Transmission unsuccessful Lost Clear to Send
 Transmission unsuccessful A DMA underrun oc-
curred because the system bus did not keep up
with the transmission
 Transmission unsuccessful The number of colli-
sions exceeded the maximum allowed
 Number of Collisions The number of collisions
experienced during transmission of the frame
 Heartbeat Indicator This indicates the presence
of a heartbeat during the last Interframe Spacing
(IFS) after transmission
When configured to Save Bad Frames the 82596
checks each incoming frame and reports the follow-
ing errors
 CRC error Incorrect CRC in a properly aligned
frame
 Alignment error Incorrect CRC in a misaligned
frame
 Frame too short The frame is shorter than the
value configured for minimum frame length
 Overrun Part of the frame was not placed in
memory because the system bus did not keep up
with incoming data
 Out of buffer Part of the frame was discarded
because of insufficient memory storage space
 Receive collision A collision was detected during
reception and the destination address of the in-
coming frame passes 82596 address filtering
Collisions in the preamble are not counted
 Length error A frame not matching the frame
length parameter was detected
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