DP83251 datasheet(18/96 Pages) NSC

18 / 96 page

40 Modes of Operation (Continued)

44 CASCADE MODE

The PLAYER device can operate in the Cascade (parallel)

mode

Figure 4-4

which is used in high bandwidth point-

to-point data transfer applications This is a non-FDDI mode

of operation

CONCEPTS

In the Cascade mode multiple PLAYER devices are con-

nected together to provide data transfer at multiples of the

FDDI data rate Two cascaded PLAYER devices provide a

data rate twice the FDDI data rate three cascaded PLAYER

devices provide a data rate three times the FDDI data rate

etc

Multiple data streams are transmitted in parallel over each

pair of cascaded PLAYER devices All data streams start

simultaneously and begin with the JK symbol pair on each

PLAYER device

Data is synchronized at the receiver of each PLAYER de-

vice by the JK symbol pair Upon receiving a JK symbol pair

a PLAYER device asserts the Cascade Ready signal to indi-

cate the beginning of data reception

The Cascade Ready signals of all PLAYER devices are

open drain ANDed together to create the Cascade Start

signal The Cascade Start signal is used as the input to

indicate that all PLAYER devices have received the JK sym-

bol pair Data is now being received at every PLAYER de-

vice and can be transferred from the cascaded PLAYER

devices to the host system

See

Figure 4-5 for more information

OPERATING RULES

When the PLAYER device is operating in Cascade mode

the following rules apply

1 Data integrity can be guaranteed if the worst case fiber

optic transmission skew between parallel fiber cables is

less than 40 ns This amounts to about 785 meters of

fiber assuming a 1% worst case variance

2 Even though this is a non-FDDI application the general

rules for FDDI frames must be obeyed

Data frames must be a minimum of three bytes long (in-

cluding the JK symbol pair) Smaller frames will cause

Elasticity Buffer errors

Data frames must have a maximum size of 4500 bytes

with a JK starting delimiter and a (T or R or S)x or x(T or

R or S) ending delimiter byte

3 Due to the different clock rates the JK symbol pair may

arrive at different times at each PLAYER device The total

skew between the fastest and slowest cascaded PLAY-

ER devices receiving the JK starting delimiter must not

exceed 80 ns

4 The first PLAYER device to receive a JK symbol pair will

present it to the host system and assert the Cascade

Ready signal The PLAYER device will present one more

JK as it waits for the other PLAYER devices to recognize

their JK The maximum number of consecutive JKs that

can be presented to the host is 2

5 The Cascade Start signal is set to 1 when all the cascad-

ed PLAYER devices release their Cascade Ready sig-

nals

6 Bit 4 (CSE) of the Receive Condition Register B (RCRB)

is set to 1 if the Cascade Start signal (CS) is

not set

before the second falling edge of clock signal LBC from

when Cascade Ready (CR) was released CS has to be

set within approximately 80 ns of CR release This condi-

tion signifies that not all cascaded PLAYERs have re-

ceived their respective JK symbol pair within the allowed

skew range

7 If the JK symbols are corrupted in the point-to-point links

some PLAYER devices may not report a Cascaded Syn-

chronization Error

8 To guarantee integrity of the interframe information the

user must put at least 8 Idle symbol pairs between

frames The PLAYER device will function properly with

only 4 Idle symbol pairs however the interframe symbols

may be corrupted with random non-JK symbols

The BMAC device could be used to provide required fram-

ing and optical FCS support

DP83251 Datasheet(PDF) 18 Page - National Semiconductor (TI)

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