AM79C981 datasheet(39/40 Pages) AMD

39 / 40 page

Am79C981

1–109

Glossary

APPENDIX B

Active Status

In a non-collision state, an IMR+ chip is considered ac-

tive if it is receiving data on any one of its network ports,

or is in the process of broadcasting (repeating) FIFO

data from a recently completed data reception. In a colli-

sion state (the IMR+ device is generating Jam Se-

quence), an IMR+ device is considered active if any one

or more network ports is receiving data. The IMR+ de-

vice asserts the

REQ line to indicate that it is active.

Collision

In a carrier sense multiple access/collision detection

(CSMA/CD) network such as Ethernet, only one node

can successfully transfer data at any one time. When

two or more separate nodes (DTEs or repeaters) are si-

multaneously transmitting data onto the network, a Col-

lision state exists. In a repeater using one or more IMR+

devices, a Collision state exists when more than one

network port is receiving data at any instant, or when

any one or more network ports receives data while the

IMR+ device is transmitting (repeating) data, or when

the CI+/- pins become active (nominal 10 MHz signal)

on the AUI port.

Jam Sequence

A signal consisting of alternating 1s and 0s that is gener-

ated by the IMR+ device when a Collision state is de-

tected. This signal is transmitted by the IMR+ device to

indicate to the network that one or more network ports in

the repeater is involved in a collision.

Network Port

Any of the eight 10BASE-T ports or the AUI port present

in the IMR+ device (i.e. not the Expansion Port or the

Management Port).

Partitioning

A network port on a repeater has been partitioned if the

repeater has internally ‘disconnected’ it from the repeat-

er due to localized faults that would otherwise bring the

entire network down. These faults are generally cable

shorts and opens that tend to cause excessive collisions

at the network ports. The partitioned network port will be

internally re-connected if the network port starts behav-

ing correctly again, usually when successful ‘collision-

less’ transmissions and/or receptions resume.

Receive Collision

A network port is in a Receive Collision state when it de-

tects collision and is not one of the colliding network

‘nodes’. This applies mainly to a non-transmitting AUI

port because a remote collision is clearly identified by

the presence of a nominal 10 MHz signal on the CI+/-

pins. However, any repeater port would be considered

to be in a receive collision state if the repeater unit is re-

ceiving data from that port as the ‘one-port-left’ in the

collision sequence.

Transmit Collision

A network port is in a Transmit Collision state when colli-

sion occurs while that port is transmitting. On the AUI

port, Transmit Collision is indicated by the presence of a

nominal 10 MHz signal on the CI+/- pins while the AUI

port is transmitting on the DO+/- pins. On a 10BASE-T

port, Transmit Collision occurs when incoming data ap-

pears on the RXD+/- pins while the 10BASE-T port is

transmitting on the TXD+/- and TXP+/- pins.

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