DP83950B datasheet(32/80 Pages) NSC

32 / 80 page

50 Functional Description (Continued)

Receive Collisions

A receive collision is a collision which occurs on the network

segment attached to

PORT N ie

the collision is ‘‘re-

ceived’’ in a similar manner as a data packet is received and

then repeated to the other network segments Not surpris-

ingly receive collision propagation follows a similar se-

quence of operations as is found with data repetition

An arbitration process is performed to find

PORT N and a

preamblejam pattern is transmitted by the repeater’s other

ports When

PORT N detects a collision on its segment the

COLN Inter-RIC bus signal is asserted This forces all the

RICs in the system to transmit a preamblejam pattern to

their segments This is important since they may be already

transmitting data from their elasticity buffers The repeater

moves to the

RECEIVE COLLISION state when the RICs

begin to transmit the jam pattern The repeater remains in

this state until both the following conditions have been ful-

filled

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 simultaneous-

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

PORT N

The functional timing diagram

Figure 56 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 54

The RICs perform the same

PORT N arbitration and data

repetition operations as previously described The system is

notified of the receive collision on port B1 by the COLN bus

signal going active This is the signal which informs the main

state machines to output the jam pattern rather than the

data held in the elasticity buffers Once a collision has oc-

curred the IRC IRD AND IRE bus signals may become un-

defined When the collision has ended and the

Tw2 opera-

tion 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 has

been

PORT N starts to transmit a Manchester encoded 1 on

to its network segment Whilst in the

TRANSMIT COLLI-

SION state all ports of the repeater must transmit the 1010

jam pattern and

PORT M arbitration is performed Each

RIC is obliged by the IEEE specification to ensure all of its

ports transmit for at least 96 bits once the

TRANSMIT COL-

LISION state has been entered This transmit activity is en-

forced by the ANYXN bus signal Whilst ANYXN is active all

RIC ports will transmit jam To ensure this situation lasts for

at least 96 bits the MSMs inside the RICs 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 ie 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 deasserted and

PORT M stops transmitting to its

network segment In this circumstance the Inter-RIC bus

transitions to the RECEIVE COLLISION state The repeater

will remain in this state whilst

PORT M’s collision Tw2 colli-

sion extension and any receive signals are present When

these conditions are not true packet repetition finishes and

the repeater enters the

WAIT state

Figure 57 shows a multi-RIC system operating under trans-

mit collision conditions There are many different scenarios

which may occur during a transmit collision this figure illus-

trates one of these The diagram begins with packet recep-

tion by port A1 Port B1 experiences a collision 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 assertion

forces A1 to relinquish its

PORT N status start transmitting

stop asserting ACTN and release its hold on the PSM arbi-

tration signals (ACKO A and ACKI B) The first bit it transmit

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 (forcesACKO 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 Two packets on one segment

results in a collision

PORT M now moves from B1 to A1

Port A1 fulfills the same criteria as B1 ie it has an active

collision on its segment but in addition it is higher in the

arbitration chain This priority yields no benefits for port A1

since the ANYXN signal is still active There are now two

sources driving ANYXN the MSMs and the collision on port

Eventually the collision on port B1 ends and the ANYXN

extension by the MSMs expires There is only one collision

DP83950B Datasheet(PDF) 32 Page - National Semiconductor (TI)

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