AM79C960KCW datasheet(38/127 Pages) AMD

38 / 127 page

P R E L I M I N A R Y

AMD

1-380

Am79C960

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:

s The exact number of transmission retry attempts

(ONE, MORE, or RTRY)

s Whether the MAC engine had to Defer (DEF)

due to channel activity

s Loss of Carrier, indicating that there was an

interruption in the ability of the MAC engine to

monitor its own transmission. Repeated LCAR

errors indicate a potentially faulty transceiver

or network connection.

s Late Collision (LCOL) 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 a normal

operating network.

s Collision Error (CERR) indicates that the

transceiver 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 trans-

ceiver drop cable, or the fact the transceiver does

not support this feature (or the feature is disabled).

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 un-

derflow, 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 er-

rors (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 reception of eight ad-

ditional 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 controller can handle up to 7 dribbling

bits when a received packet terminates. During the re-

ception, the CRC is generated on every serial bit

(including the dribbling bits) coming from the cable, al-

though 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:

If the number of the dribbling bits are 1 to 7 and there

is no CRC error, then there is no Framing error

(FRAM = 0).

If the number of the dribbling bits are less than 8 and

there is a CRC error, then there is also a Framing

error (FRAM = 1).

If the number of dribbling bits = 0, then there is no

Framing error. There may or may not be a CRC

(FCS) error.

Counters are provided to report the Receive Collision

Count and Runt Packet Count and used for network sta-

tistics and utilization calculations.

Note that if the MAC engine 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 MAC engine will wait for the network to go

inactive before attempting to receive the next packet.

Media Access Management

The basic requirement for all stations on the network is

provide

fairness

channel

allocation.

The

802.3/Ethernet protocol defines 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

Packet Gap interval) after the last activity, before trans-

mitting on the medium. The channel is a multidrop

communications medium (with various topological con-

figurations 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) re-

quires that the CSMA/CD MAC monitor the medium

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

tected the medium 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 interpacket 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

measures, as specified in 4.2.8, are recom-

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