AM79C940JC/W AMD [Advanced Micro Devices], AM79C940JC/W Datasheet - Page 36

• Manufacturer Part Number: AM79C940JC/W
• Manufacturer: AMD [Advanced Micro Devices]

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 MACE device 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 MACE
device activity.
On completion of transmission, the MACE device will
report the Transmit Frame Status for the frame. The
exact number of transmission retry attempts is reported
(ONE, MORE used with XMTRC, or RTRY), and
whether the MACE device had to Defer (DEFER) due
to channel activity. In addition, Loss of Carrier is
reported, indicating that there was an interruption in the
ability of the MACE device to monitor its own transmis-
sion. Repeated LCAR errors indicate a potentially
faulty transceiver or network connection. Excessive
Defer (EXDEF) will be reported in the Transmit Retry
Count register if the transmit frame had to wait for an
abnormally long period before transmission.
Additional transmit error conditions are reported
through the Interrupt Register.
The Late Collision (LCOL) error indicates that the
transmission suffered a collision after the slot time.
This is indicative of a badly configured network. Late
collisions should not occur in normal operating net-
work.
The 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 it
is disabled).
In addition to the reporting of network errors, the MACE
device will also attempt to prevent the creation of any
network error caused by inability of the host to service
the MACE device. During transmission, if the host fails
to keep the Transmit FIFO filled sufficiently, causing an
underflow, the MACE device 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 allow the receiving station to reject the
message).
The status of each receive message is passed via the
Receive Frame Status bytes. 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 bytes in the message. The MACE
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Am79C940
device will ignore up to seven additional bits at the end
of a message (dribbling bits), which can occur under
normal network operating conditions. The reception of
eight additional bits will cause the MACE device to
de-serialize the entire byte, and will result in the
received message and FCS being modified.
Received messages which suffer a collision after
64-byte times (after SFD) will be marked to indicate
they have suffered a late collision (CLSN). Additional
counters are provided to report the Receive Collision
Count and Runt Packet Count to be used for network
statistics and utilization calculations.
Note that if the MACE device detects a received packet
which has a 00b pattern in the preamble (after the first
8-bits which are ignored), the entire packet will be
ignored. The MACE device will wait for the network to
go inactive before attempting to receive additional
frames.
Media Access Management
The basic requirement for all stations on the network
is to provide fairness of channel allocation. The 802.3/
Ethernet protocols define a media access mechanism
which permits all stations to access the channel with
equality. Any node can attempt to contend for the chan-
nel by waiting for a predetermined time (Inter Packet
Gap interval) after the last activity, before transmitting
on the media. The channel is a bus or multidrop com-
munications medium (with various topological configu-
rations permitted) which allows a single station to
transmit and all other stations to receive. If two nodes
simultaneously contend for the channel, their signals
will interact causing loss of data, defined as a collision.
It is the responsibility of the MAC to attempt to avoid
and recover from a collision, to guarantee data integrity
for the end-to-end transmission to the receiving station.
Medium Allocation (Collision Avoidance)
The IEEE 802.3 Standard (ISO/IEC 8802-3 1990)
requires that the CSMA/CD MAC monitors the medium
for traffic by watching for carrier activity. When carrier
is detected, the media is considered busy, and the
MAC should defer to the existing message.
The IEEE 802.3 Standard also allows optional two part
deferral after a receive message.
See ANSI/IEEE Std 802.3-1990 Edition, 4.2.3.2.1:
Note: “ It is possible for the PLS carrier sense indica-
tion to fail to be asserted during a collision on the
media. If the deference process simply times the inter-
Frame gap based on this indication it is possible for a
short interFrame gap to be generated, leading to a
potential reception failure of a subsequent frame. To
enhance system robustness the following optional
measures, as specified in 4.2.8, are recommended
when interFrameSpacing Part1 is other than zero:”