AM79C981 datasheet(11/40 Pages) AMD

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PRELIMINARY

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Am79C981

Expansion Port

The IMR+ chip Expansion Port is comprised of five pins;

two are bi-directional signals (DAT and JAM), two are in-

put signals (

ACK and COL), and one is an output signal

(

REQ). These signals are used when a multiple-IMR+

device repeater application is employed. In this configu-

ration, all IMR+ chips must be clocked synchronously

with a common clock connected to the X1 inputs of all

IMR+ devices. Reset needs to be synchronized to

X1 clock.

The IMR+ device expansion scheme allows the use of

multiple IMR+ chips in a single board repeater or a

modular multiport repeater with a backplane architec-

ture. The DAT pin is a bidirectional I/O pin which can be

used to transfer data between the IMR+ devices in a

multiple-IMR+ chip design. The data sent over the DAT

line is in NRZ format and is synchronized to the common

clock. The JAM pin is another bidirectional I/O pin that is

used by the active IMR+ chip to communicate its internal

status to the remaining (inactive) IMR+ devices. When

JAM is asserted HIGH, it indicates that the active IMR+

device has detected a collision condition and is generat-

ing Jam Sequence. During this time when JAM is as-

serted HIGH, the DAT line is used to indicate whether

the active IMR+ chip is detecting collision on one port

only or on more than one port. When DAT is driven

HIGH by the IMR+ chip (while JAM is asserted by the

IMR+ chip), then the active IMR+ device is detecting a

collision condition on one port only. This ‘one-port-left’

signaling is necessary for a multiple-IMR+ device re-

peater to function correctly as a single multiport repeater

unit. The IMR+ chip also signals the ‘one port left’ colli-

sion condition in the event of a runt packet or collision

fragment; this signal will continue for one expansion port

bus cycle (100 ns) before deasserting

REQ.

The arbitration for access to the bussed bi-directional

signals (DAT and JAM) is provided by one output (

REQ)

and two inputs (

ACK and COL). The IMR+ chip asserts

the

REQ pin to indicate that it is active and wishes to

drive the DAT and JAM pins. An external arbiter senses

the

REQ lines from all the IMR+ devices and asserts the

ACK line when one and only one IMR+ chip is asserting

its

REQ line. If more than one IMR+ chip is asserting its

REQ line, the arbiter must assert the COL signal, indi-

cating that more than one IMR+ device is active. More

than one active IMR+ device at a time constitutes a colli-

sion condition, and all IMR+ devices are notified of this

occurence via the

COL line of the Expansion Port.

Note that a transition from multiple IMR+ devices arbi-

trating for the DAT and JAM pins (with

COL asserted,

ACK deasserted) to a condition when only one IMR+

chip is arbitrating for the DAT and JAM pins (with

ACK

asserted,

COL deasserted) involves one expansion port

bus cycle (100 ns). During this transitional bus cycle,

COL is deasserted, ACK is asserted, and the DAT and

JAM pins are not driven. However, each IMR+ device

will remain in the collision state (transmitting jam se-

quence) during this transitional bus cycle. In subse-

quent expansion port bus cycles (

REQ and ACK still

asserted), the IMR+ devices will return to the ‘master

and slaves’ condition where only one IMR+ device is ac-

tive (with collision) and is driving the DAT and JAM pins.

An understanding of this sequence is crucial if non-

IMR+ devices (such as an Ethernet controller) are con-

nected to the expansion bus. Specifically, the last

device to back off of the Expansion Port after a multi-

IMR+ chip collision must assert the JAM line until it too

drops its request for the Expansion Port.

External Arbiter

A simple arbitration scheme is required when multiple

IMR+ devices are connected together to increase the to-

tal number of repeater ports. The arbiter should have

one input (

REQ1...REQn) for each of the n IMR+ de-

vices to be used, and two global outputs (

COL and

ACK). This function is easily implemented in a PAL® de-

vice, with the following logic equations:

ACK =

REQ1 &

REQ2 & REQ3 & ....REQn

REQ1 & REQ2 & REQ3 & ....REQn

•

REQ1 & REQ2 & REQ3 & .... REQn

COL =

ACK & (REQ1 + REQ2 + REQ3 + ... REQn)

Above equations are in positive logic, i.e., a variable is

true when asserted.

A single PALCE16V8 will perform the arbitration func-

tion for a repeater based on several IMR+ devices.

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