am79c961a Advanced Micro Devices, am79c961a Datasheet - Page 71

• Manufacturer Part Number: am79c961a
• Description: Pcnet?-isa Ii Jumperless, Full Duplex Single-chip Ethernet Controller For Isa
• Manufacturer: Advanced Micro Devices

including the destination address, source address,
length/type and packet data.
The receive section of the MAC engine will detect an
incoming preamble sequence and lock to the encoded
clock. The internal MENDEC will decode the serial bit
stream and present this to the MAC engine. The MAC
will discard the first 8 bits of information before search-
ing for the SFD sequence. Once the SFD is detected,
all subsequent bits are treated as part of the frame. The
MAC engine will inspect the length field to ensure min-
imum frame size, strip unnecessary pad characters (if
enabled), and pass the remaining bytes through the
Receive FIFO to the host. If pad stripping is performed,
the MAC engine will also strip the received FCS bytes,
although the normal FCS computation and checking
will occur. Note that apart from pad stripping, the frame
will be passed unmodified to the host. If the length field
has a value of 46 or greater, the MAC engine will not
attempt to validate the length against the number of
bytes contained in the message.
If the frame terminates or suffers a collision before
64 bytes of infor mation (after SFD) have been
received, the MAC engine will automatically delete the
f r a m e f r o m t h e R e c e i v e F I F O, w i t h o u t h o s t
intervention.
Addressing (source and destination address
handling)
The first 6 bytes of information after SFD will be inter-
preted as the destination address field. The MAC
engine provides facilities for physical, logical, and
broadcast address reception. In addition, multiple
physical addresses can be constructed (perfect
address filtering) using external logic in conjunction
with the EADI interface.
Error detection (physical medium transmission
errors)
The MAC engine provides several facilities which
report and recover from errors on the medium. In addi-
tion, the network is protected from gross errors due to
inability of the host to keep pace with the MAC engine
activity.
On completion of transmission, the following transmit
status is available in the appropriate TMD and CSR
areas:
The exact number of transmission retry attempts
(ONE, MORE, or RTRY).
Whether the MAC engine had to Defer (DEF) due to
channel activity.
Loss of Carrier, indicating that there was an inter-
ruption in the ability of the MAC engine to monitor its
own transmission. Repeated LCAR errors indicate
a p o ten ti a ll y fau l ty tra n sc e ive r or ne two r k
connection.
Am79C961A
In addition to the reporting of network errors, the MAC
engine will also attempt to prevent the creation of any
network error due to the inability of the host to service
the MAC engine. During transmission, if the host fails
to keep the Transmit FIFO filled sufficiently, causing an
underflow, the MAC engine will guarantee the message
is either sent as a runt packet (which will be deleted by
the receiving station) or has an invalid FCS (which will
also cause the receiver to reject the message).
The status of each receive message is available in the
appropriate RMD and CSR areas. FCS and Framing
errors (FRAM) are reported, although the received
frame is still passed to the host. The FRAM error will
only be reported if an FCS error is detected and there
are a non-integral number of bits in the message. The
MAC engine will ignore up to seven additional bits at
the end of a message (dribbling bits), which can occur
under normal network operating conditions. The recep-
tion of eight additional bits will cause the MAC engine
to de-serialize the entire byte, and will result in the
received message and FCS being modified.
The PCnet-ISA II controller can handle up to 7 dribbling
bits when a received packet terminates. During the
reception, the CRC is generated on every serial bit
(including the dribbling bits) coming from the cable,
although the internally saved CRC value is only
updated on the eighth bit (on each byte boundary). The
framing error is reported to the user as follows:
1. If the number of the dribbling bits are 1 to 7 and
2. If the number of the dribbling bits are less than 8
3. If the number of dribbling bits = 0, then there is no
Counters are provided to report the Receive Collision
Count and Runt Packet Count used for network statis-
tics and utilization calculations.
Note that if the MAC engine detects a received packet
which has a 00b pattern in the preamble (after the first
Late Collision (LCOL) indicates that the transmis-
sion suffered a collision after the slot time. This is
indicative of a badly configured network. Late colli-
sions should not occur in a normal operating net-
work.
Collision Error (CERR) indicates that the trans-
ceiver did not respond with an SQE Test message
within the predetermined time after a transmission
completed. This may be due to a failed transceiver,
disconnected or faulty transceiver drop cable, or the
fact the transceiver does not support this feature (or
the feature is disabled).
there is no CRC error, then there is no Framing error
(FRAM = 0).
and there is a CRC error, then there is also a
Framing error (FRAM = 1).
Framing error. There may or may not be a CRC
(FCS) error.
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