DP83953 datasheet(23/90 Pages) NSC

23 / 90 page

4.0 Functional Description (Continued)

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Receive Collisions (AUI Port only)

A receive collision is a collision which occurs on the net-

work segment attached to the AUI port. The collision is

"received" in a similar manner as a data packet is received,

and then repeated to the other network segments. Not sur-

prisingly, the receive collision propagation follows a similar

sequence of operations as data repetition.

An arbitration process is performed to find

PORT N and a

preamble/jam pattern is transmitted by the repeater's other

ports. When the AUI port as

PORT N detects a collision on

its segment the COLN Inter-RIC bus signal is asserted.

This forces all the RIC2As in the system to transmit a pre-

amble/jam pattern to their segments. This is important

since they may be already transmitting data from their elas-

ticity buffers. The repeater moves to the

RECEIVE COLLI-

SION state and begins to transmit the jam pattern. The

repeater remains in this state until both the following condi-

tions have been fulfilled:

1. at least 96 bits have been transmitted onto the network,

2. the activity has ended.

Under close examination, the repeater specification reveals

that the actual end of activity has its own permutations of

conditions:

1. collision and receive data signals may end simulta-

neously,

2. receive data may appear to end before collision signals,

3. receive data may continue for some time after the end of

the collision signal.

Network segments using coaxial media may experience

spurious gaps in segment activity when the collision signal

goes inactive. This arises from the inter-action between the

receive and collision signal squelch circuits, implemented

in coaxial transceivers, and the properties of the coaxial

cable itself. The repeater specification avoids propagation

of these activity gaps by extending collision activity by the

Tw2 wait time. Jam pattern transmission must be sustained

throughout this period. After this, the repeater will move to

the

WAIT state unless there is a data signal being received

by the AUI port as

PORT N.

The functional timing diagram, Figure 7, shows the opera-

tion of a repeater system during a receive collision. The

system configuration is the same as earlier described and

is shown in Figure 6.

The RIC2As perform the same

PORT N arbitration and

data repetition operations described previously. The sys-

tem is notified of the receive collision on the AUI port by the

COLN bus signal going active. This signal informs the main

state machines to send out the jam pattern rather than

valid data stored in the elasticity buffers. Once a collision

has occurred, the IRC, IRD and IRE bus signals may

become undefined. When the collision has ended and the

Tw2 operation performed, the repeater moves to the WAIT

state.

Transmit Collisions

A transmit collision is a collision that is detected upon a

segment to which the repeater system is transmitting. The

port state machine monitoring the colliding segment

asserts the ANYXN bus signal. The assertion of ANYXN

causes

PORT M arbitration to begin. The repeater moves

to the

TRANSMIT COLLISION state when the port which

had been

PORT N starts to transmit a Manchester

encoded 1 on to its network segment. While in the

TRANS-

MIT COLLISION state, all ports of the repeater must trans-

mit the 1010... jam pattern, and

PORT M arbitration is

performed. Each RIC2A is obligated, by the IEEE specifica-

tion, to ensure all of its ports transmit for at least 96 bits

once the

TRANSMIT COLLISION state has been entered.

This transmit activity is enforced by the ANYXN bus signal.

While ANYXN is active, all RIC2A ports will transmit jam.

To ensure this situation lasts for at least 96 bits, the MSM

inside the RIC2As assert the ANYXN signal throughout this

period. After this period has elapsed, ANYXN will only be

asserted if there are multiple ports with active collisions on

their network segments.

There are two possible ways for a repeater to leave the

TRANSMIT COLLISION state. The most straight forward is

when network activity, i.e., collisions and their

Tw2 exten-

sions, end before the 96 bit enforced period expires. Under

these conditions the repeater system may move directly to

the

WAIT state when 96 bits have been transmitted to all

ports. If the MSM enforced period ends and there is still

one port experiencing a collision, the

ONE PORT LEFT

state is entered. This may be seen on the Inter-RIC bus

when ANYXN is de-asserted and

PORT M stops transmit-

ting to its network segment. In this circumstance the Inter-

RIC bus transitions to the RECEIVE COLLISION state.

The repeater will remain in this state while

PORT M's colli-

sion,

Tw2 collision extension and any receive signals are

present. When these conditions are not true, packet repeti-

tion finishes and the repeater enters the

WAIT state.

Figure 8 shows a multi-RIC2A system operating under

transmit collision conditions. There are many different sce-

narios which may occur during a transmit collision, this fig-

ure illustrates one of these. The diagram begins with

packet reception by port A1. Port B1 experiences a colli-

sion, since it is not

PORT N it asserts ANYXN. This alerts

the main state machines in the system to switch from data

to jam pattern transmission.

Port A1 is also monitoring the ANYXN bus line. Its asser-

tion forces A1 to relinquish its

PORT N status, start trans-

mitting, stop asserting ACTN and release its hold on the

PSM arbitration signals (ACKO A and ACKI B). The first bit

it transmits will be a Manchester encoded "1" in the jam

pattern. Since port B1 is the only port with a collision, it

attains

PORT M status and stops asserting ANYXN. It

does however assert ACTN, and exert its presence upon

the PSM arbitration chain (forces ACKO B low). The MSMs

ensure that ANYXN stays active and thus force all of the

ports, including

PORT M, to transmit to their segments.

After some time port A1 experiences a collision. This arises

from the presence of the packet being received from port

A1's segment and the jam signal the repeater is now trans-

mitting onto this segment. Simultaneous receive and trans-

mit activity on one segment results in a collision. Port A1

fulfills the same criteria as B1, i.e., it has an active collision

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