AM79C940KCSLASHW datasheet(36/144 Pages) AMD

36 / 144 page

Am79C940

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

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:”

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