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AM79C982-4JC Datasheet(PDF) 10 Page - Advanced Micro Devices |
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AM79C982-4JC Datasheet(HTML) 10 Page - Advanced Micro Devices |
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10 / 26 page  AMD PRELIMINARY 1–12 Am79C982 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 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 between 1024- to 2048-bit times (AUI port—SQE signal active; TP port—simultaneous transmit and receive) b. A collision condition occurs during each of 32 con- secutive attempts to transmit to that port. 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 longer than 512-bit times (nominal) is transmitted or received by the partitioned port without a collision. b. Alternate reconnection algorithm—A data packet longer than 512-bit times (nominal) is transmitted by the partitioned port without a collision. 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 |
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