am79c984a Advanced Micro Devices, am79c984a Datasheet - Page 15

• Manufacturer Part Number: am79c984a
• Description: Enhanced Integrated Multiport Repeater Eimr
• Manufacturer: Advanced Micro Devices

FUNCTIONAL DESCRIPTION
The Am79C984A 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 de-
vice is also expandable, enabling the implementation of
high port count repeaters based on several eIMR de-
vices.
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 repeaters functions are
summarized in the paragraphs below.
Basic Repeater Functions
The Am79C984A 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 retrans-
mit 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 multi-
ple eIMR device configurations, the data will also be re-
peated 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
P R E L I M I N A R Y
Am79C984A
times. This is referred to as MAU Jabber Lockup Pro-
tection (MJLP). The MJLP status for the eIMR device
can be read through the Control 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 re-
peated packet length to 96 bits by appending a Jam se-
quence 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 reconnec-
tion are specified by the IEEE 802.3 standard. In addi-
tion to the standard reconnection algorithm, the eIMR
device implements an alternative reconnection algo-
rithm, which provides a more robust partitioning func-
tion for the TP ports and/or AUI port. The eIMR device
partitions each TP port and the AUI port separately 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:
a. A collision condition exists continuously for more
b. 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
indicated when the port is simultaneously attempting to
transmit and receive.
Once a network port is partitioned, the eIMR device will
reconnect that port, according to the selected recon-
nection algorithm, as follows:
a. Standard reconnection algorithm—A data packet
than 2048 bit times. (AUI port—SQE signal active;
TP port—simultaneous transmit and receive).
secutive attempts to transmit to that port.
longer than 512-bit times (nominal) is transmitted or
received by the partitioned port without a collision.
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