AM79C985JC datasheet(15/48 Pages) AMD

15 / 48 page

Am79C985

PRELIMINARY

FUNCTIONAL DESCRIPTION

The Am79C985 eIMR+ device is a single-chip imple-

mentation of an IEEE 802.3/Ethernet repeater (or hub).

It is offered with four integral 10BASE-T ports plus one

RAUI port comprising the basic repeater. The eIMR+

device is also expandable, enabling the implementation

of high port count repeaters based on several eIMR+

devices.

The eIMR+ device interfaces directly with AMD’s

Am79C987 HIMIB device. This allows hardware de-

signers to implement a fully managed multiport re-

peater, as speciﬁed by the IEEE 802.3 standard,

Section 19,

Layer Management for 10 Mbps Baseband

Repeaters. When the eIMR+ and HIMIB devices are

used as a chip set, the HIMIB device maintains com-

plete repeater and per-port statistics, which can be ac-

cessed on demand through an 8-bit parallel interface.

The eIMR+ chip complies with the full set of repeater

basic functions as deﬁned in Section 9 of ISO 8802.3

(ANSI/IEEE 802.3c). The basic repeater functions are

summarized in the paragraphs below.

Basic Repeater Functions

The Am79C985 chip implements the basic repeater

functions as deﬁned by Section 9.5 of the ANSI/IEEE

802.3 speciﬁcation.

Repeater Function

If any single network port senses the start of a valid

packet on its receive lines, the eIMR+ device will re-

transmit the received data to all other enabled network

ports (except when contention exists among any of the

ports or when the receive port is partitioned). To allow

multiple eIMR+ device conﬁgurations, the data will also

be repeated on the expansion bus data line (DAT).

Signal Regeneration

When retransmitting a packet, the eIMR+ device en-

sures that the outgoing packet complies with the IEEE

802.3 speciﬁcation in terms of preamble structure and

timing characteristics. Speciﬁcally, data packets re-

peated by the eIMR+ device will contain a minimum of

56 preamble bits before the Start-of-Frame Delimiter. In

addition, the eIMR+ restores the voltage amplitude of

the repeated waveform to levels speciﬁed in the IEEE

802.3 speciﬁcation. Finally, the eIMR+ device restores

signal symmetry to repeated data packets, removing jit-

ter and distortion caused by the network cabling. Jitter

present at the output of the AUI port will be better than

0.5 ns; jitter at the TP outputs will be better than 1.5 ns.

The start-of-packet propagation delay for a repeater set

is the time delay between the ﬁrst edge transition of a

data packet on its input port to the ﬁrst edge transition

of the repeated packet on its output ports. The start-of-

packet propagation delay for the eIMR+ is within the

speciﬁcation given in Section 9.5.5.1 of the IEEE 802.3

standard.

Jabber Lockup Protection

The eIMR+ device implements a built-in jabber protec-

tion scheme to ensure that the network is not disabled

by the transmission of excessively long data packets.

This protection scheme causes the eIMR+ device to in-

terrupt transmission for 96 bit-times if the device has

been transmitting continuously for more than 65,536 bit

times. This is referred to as MAU Jabber Lockup Pro-

tection (MJLP). The MJLP status for the eIMR+ device

can be read through the Management Port, using the

Get MJLP Status command.

Collision Handling

The eIMR+ device will detect and respond to collision

conditions as speciﬁed in the IEEE 802.3 speciﬁcation.

Repeater conﬁgurations consisting of multiple eIMR+

devices also comply with the IEEE 802.3 speciﬁcation,

using status signals provided by the expansion bus. In

particular, a repeater based on one or more eIMR+ de-

vices will handle the transmit collision and one-port-left

collision conditions correctly, as speciﬁed in Section 9

of the IEEE 802.3 speciﬁcation.

Fragment Extension

If the total packet length received is less than 96 bits,

including preamble, the eIMR+ device will extend the

repeated packet length to 96 bits by appending a Jam

sequence to the original fragment.

Auto Partitioning/Reconnection

Any of the TP ports or the AUI port can be partitioned if

the duration or frequency of collisions becomes exces-

sive. The eIMR+ device will continue to transmit data

packets to a partitioned port, but will not respond, as a

repeater, to activity on the partitioned port’s receiver.

The eIMR+ device will monitor the port and reconnect

it once certain criteria are met. The criteria for recon-

nection are speciﬁed by the IEEE 802.3 standard. In

addition to the standard reconnection algorithm, the

eIMR+ device implements an alternative reconnection

algorithm, which provides a more robust partitioning

function for the TP ports and/or AUI port. The eIMR+

device partitions each TP port and the AUI port sepa-

rately and independently of other network ports.

The eIMR+ device will partition an enabled network

port if either of the following conditions occurs at that

port:

1. A collision condition exists continuously for more

than 2048 bit times. (AUI port—SQE signal active;

TP port—simultaneous transmit and receive).

2. A collision condition occurs during each of 32 con-

secutive attempts to transmit to that port.

In the AUI port, a collision condition is indicated by an

active SQE signal. In a TP port, a collision condition is

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