AM79C940 datasheet(35/122 Pages) AMD

35 / 122 page

AMD

Am79C940

inability of the host to keep pace with the MACE device

activity.

On completion of transmission, the MACE device will re-

port 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,

indicting that there was an interruption in the ability of

the MACE device to monitor its own transmission. Re-

peated LCAR errors indicate a potentially faulty trans-

ceiver

network

connection.

Excessive

Defer

(EXDEF) will be reported in the Transmit Retry Count

mally long period before transmission.

Additional transmit error conditions are reported

through the Interrupt Register.

The Late Collision (LCOL) error indicates that the trans-

mission suffered a collision after the slot time. This is

indicative of a badly configured network. Late collisions

should not occur in normal operating network.

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 mes-

sage (dribbling bits), which can occur under normal net-

work operating conditions. The reception of eight

additional bits will cause the MACE device to de-serial-

ize the entire byte, and will result in the received mes-

sage 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 count-

ers 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 ig-

nored. 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 mecha-

nism which permits all stations to access the channel

with equality. Any node can attempt to contend for the

channel by waiting for a predetermined time (Inter Pack-

et Gap interval) after the last activity, before transmitting

on the media. The channel is a bus or multidrop commu-

nications medium (with various topological configura-

tions permitted) which allows a single station to transmit

and all other stations to receive. If two nodes simultane-

ously contend for the channel, their signals will interact

causing loss of data, defined as a collision. It is the re-

sponsibility 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) re-

quires that the CSMA/CD MAC monitors the medium for

traffic by watching for carrier activity. When carrier is de-

tected, 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

indication to fail to be asserted during a collision

on the media. If the deference process simply

times the interFrame gap based on this indica-

tion it is possible for a short interFrame gap to

be generated, leading to a potential reception

failure of a subsequent frame. To enhance sys-

tem robustness the following optional meas-

ures, as specified in 4.2.8, are recommended

when interFrameSpacingPart1 is other than

zero:”

(1) Upon completing a transmission, start timing the

interpacket gap, as soon as transmitting and

carrierSense are both false.

(2) When timing an interFrame gap following reception,

reset the interFrame gap timing if carrierSense

becomes true during the first 2/3 of the interFrame gap

timing interval. During the final 1/3 of the interval the

timer shall not be reset to ensure fair access to the me-

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