AM79C90 データシートの表示（PDF） - Advanced Micro Devices

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AM79C90

CMOS Local Area Network Controller for Ethernet (C-LANCE)

Advanced Micro Devices 

AMD

PRELIMINARY

under “Logical Address Filter.” The second logical ad-

dress is a broadcast address where all nodes on the net-

work receive the packet. The last receive mode of

operation is referred to as “promiscuous mode” in which

a node will accept all packets on the medium regardless

of their destination address.

Collision Detection and Implementation

The Ethernet and IEEE 802.3 CSMA/CD network ac-

cess algorithms are implemented completely within the

C-LANCE. In addition to listening for a clear medium be-

fore transmitting, Ethernet handles collisions in a prede-

termined way. Should two transmitters attempt to seize

the medium at the same time, they will collide and the

data on the medium will be garbled. The transmitting

nodes listen while they transmit, detect the collision,

then continue to transmit for a predetermined length of

time to “jam” the network and ensure that all nodes have

recognized the collision. The transmitting nodes then

delay a random amount of time according to the Ether-

net “truncated binary backoff” algorithm in order that the

colliding nodes do not try to repeatedly access the net-

work at the same time. The C-LANCE also offers a se-

lectable Modified Backoff Algorithm for better

performance on busy networks. Up to 16 attempts to ac-

cess the network are made by the C-LANCE before re-

porting an error due to excessive collisions.

Error Reporting and Diagnostics

Extensive error reporting is provided by the C-LANCE.

Error conditions reported relate either to the network as

a whole or to individual data packets. Network-related

errors are recorded as flags in the CSRs and are exam-

ined by the CPU following interrupt. Packet-related er-

rors are written into descriptor entries corresponding to

the packet.

System errors include:

Babbling Transmitter

— Transmitter attempting to transmit more than

1518 bytes, excluding preamble and start frame

delimiter

Collision

— Collision detection circuitry nonfunctional

Missed Packet

— Insufficient buffer space

Memory timeout

— Memory response failure

Packet-related errors:

CRC

— Invalid data

Framing

— Packet did not end on a byte boundary

Overflow/Underflow

— Indicates abnormal latency in servicing a DMA

request

Buffer

— Insufficient buffer space available

The C-LANCE performs several diagnostic routines

which enhance the reliability and integrity of the system.

These include a CRC check and two loop back modes

(internal/external). Errors may be introduced into the

system to check error detection logic. A Time Domain

Reflectometer is incorporated into the C-LANCE to aid

system designers in locating faults in the Ethernet physi-

cal medium. Shorts and opens manifest themselves in

reflections which are sensed by the TDR.

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Am79C90

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