am79c982 Advanced Micro Devices, am79c982 Datasheet - Page 10

• Manufacturer Part Number: am79c982
• Description: Basic Integrated Multiport Repeater Bimr
• Manufacturer: Advanced Micro Devices

FUNCTIONAL DESCRIPTION
The Am79C982 Basic Integrated Multiport Repeater
device is a single chip implementation of an IEEE
802.3/Ethernet repeater (or hub). It is offered either with
four or eight integral 10BASE-T ports plus one AUI port
comprising the basic repeater. The bIMR device is also
expandable, enabling the implementation of high port
count repeaters based on several bIMR devices.
The bIMR chip complies with the full set of repeater ba-
sic functions as defined in section 9 of ISO 8802.3
(ANSI/IEEE 802.3c). These functions are summarized
below.
Repeater Function
If any single network port senses the start of a valid
packet on its receive lines, then the bIMR device will re-
transmit the received data to all other enabled network
ports. The repeated data will also be presented on the
DAT line to facilitate multiple-bIMR device repeater
applications.
Signal Regeneration
When re-transmitting a packet, the bIMR device en-
sures that the outgoing packet complies with the 802.3
specification in terms of preamble structure, voltage am-
plitude, and timing characteristics. Specifically, data
packets repeated by the bIMR chip will contain a mini-
mum of 56 preamble bits before the Start of Frame De-
limiter. In addition, the voltage amplitude of the repeated
packet waveform will be restored to levels specified in
the 802.3 specification. Finally, signal symmetry is re-
stored to data packets repeated by the bIMR device, re-
moving jitter and distortion caused by the network
cabling.
Jabber Lockup Protection
The bIMR chip implements a built-in jabber protection
scheme to ensure that the network is not disabled due to
transmission of excessively long data packets. This pro-
tection scheme will automatically interrupt the transmit-
ter circuits of the bIMR device for 96-bit times if the bIMR
device has been transmitting continuously for more than
65,536-bit times. This is referred to as MAU Jabber
Lockup Protection (MJLP). The MJLP status for the
bIMR chip can be read through the Management Port
using the Get MJLP Status command (M bit
returned).
Collision Handling
The bIMR chip will detect and respond to collision condi-
tions as specified in 802.3. A multiple-bIMR device re-
peater implementation also complies with the 802.3
specification due to the inter-bIMR chip status commu-
nication provided by the expansion port. Specifically, a
repeater based on one or more bIMR devices will
handle the transmit collision and one-port-left collision
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AMD
PRELIMINARY
Am79C982
conditions correctly as specified in Section 9 of the
802.3 specification.
Fragment Extension
If the total packet length received by the bIMR device is
less than 96 bits, including preamble, the bIMR chip will
extend the repeated packet length to 96 bits by append-
ing a Jam sequence to the original fragment.
Auto Partitioning/Reconnection
Any of the integral TP ports and AUI port can be parti-
tioned under excessive duration or frequency of colli-
sion conditions. Once partitioned, the bIMR device will
continue to transmit data packets to a partitioned port,
but will not respond (as a repeater) to activity on the par-
titioned port’s receiver. The bIMR chip will monitor the
port and reconnect it once certain criteria indicating port
‘wellness’ are met. The criteria for reconnection are
specified by the 802.3 standard. In addition to the stan-
dard reconnection algorithm, the bIMR device imple-
ments an alternative reconnection algorithm which
provides a more robust partitioning function for the TP
ports and/or the AUI port. Each TP port and the AUI port
are partitioned and/or reconnected separately and inde-
pendently of other network ports.
Either one of the following conditions occuring on any
enabled bIMR device network port will cause the port to
partition:
a. A collision condition exists continuously for a time
b. A collision condition occurs during each of 32 con-
Once a network port is partitioned, the bIMR device will
reconnect that port if the following is met:
a. Standard reconnection algorithm—A data packet
b. Alternate reconnection algorithm—A data packet
The reconnection algorithm option (standard or alter-
nate) is a global function for the TP ports, i.e. all TP ports
use the same reconnection algorithm. The AUI recon-
nection algorithm option is programmed independently
of the TP port reconnection option.
Link Test
The integral TP ports implement the Link Test function
as specified in the 802.3 10BASE-T standard. The bIMR
device will transmit Link Test pulses to any TP port after
between 1024- to 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.
longer than 512-bit times (nominal) is transmitted by
the partitioned port without a collision.