DP83950BNU NSC [National Semiconductor], DP83950BNU Datasheet - Page 32

• Manufacturer Part Number: DP83950BNU
• Manufacturer: NSC [National Semiconductor]

PORT M arbitration to begin The repeater moves to the
5 0 Functional Description
Receive Collisions
A receive collision is a collision which occurs on the network
segment attached to PORT N i e
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
preamble jam 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 preamble jam 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
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 PORT N
The functional timing diagram Figure 5 6 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 5 4
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
ly
the collision signal
the collision is ‘‘re-
(Continued)
32
TRANSMIT COLLISION state when the port which has
SION state all ports of the repeater must transmit the 1010
LISION state has been entered This transmit activity is en-
TRANSMIT COLLISION state The most straight forward is
Figure 5 7 shows a multi-RIC system operating under trans-
M to transmit to their segments
been PORT N starts to transmit a Manchester encoded 1 on
to its network segment Whilst in the TRANSMIT COLLI-
RIC is obliged by the IEEE specification to ensure all of its
ports transmit for at least 96 bits once the TRANSMIT COL-
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
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 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
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
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 i e 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
B1
Eventually the collision on port B1 ends and the ANYXN
extension by the MSMs expires There is only one collision
jam pattern and PORT M arbitration is performed Each