AM79C985KCW AMD [Advanced Micro Devices], AM79C985KCW Datasheet - Page 15

• Manufacturer Part Number: AM79C985KCW
• Manufacturer: AMD [Advanced Micro Devices]

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
P R E L I M I N A R Y
Am79C985
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
2. A collision condition occurs during each of 32 con-
In the AUI port, a collision condition is indicated by an
active SQE signal. In a TP port, a collision condition is
than 2048 bit times. (AUI port—SQE signal active;
TP port—simultaneous transmit and receive).
secutive attempts to transmit to that port.
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