DP83241 datasheet(14/20 Pages) NSC

14 / 20 page

50 Detailed Information (Continued)

522 CDD Device Driving Multiple PLAYER Devices

In a FDDI concentrator or dual attach station it may be

necessary for a single DP83241 Clock Distribution Device

(CDD device) to drive multiple DP8325155 PLAYER devic-

es Since these PLAYER devices will be running synchro-

nously to each other they must have the same clocks The

easiest way to accomplish this is to have one CDD device

drive multiple PLAYER devices

We are only concerned with the ECL outputs being able to

drive multiple loads The conventional way of directly wiring

the one output to many inputs will not work If the ECL sig-

nals are split into multiple traces then reflections will result

which may ruin the signal’s integrity An appropriate method

where individual traces with a series resistor connected to

each load is used instead The series resistor should match

the line impedance and be placed as close to the CDD de-

vice as possible The resistor will act as a voltage divider

and cut the voltage level of the signal in half When this

modified signal reaches the input of the unterminated gate

reflections will cause the signal to double and the receiving

input will see the full voltage swing The reflection will then

travel back towards the CDD device but the series resist-

ance will stop this action An emitter pulldown resistor is

needed at the CDD device for this method of routing the

ECL signals The value of this resistor can be calculated

from the following equation

Re (Max) e

10 Zo b RS

Where

Re (Max)

Largest emitter pulldown resistor

that can be used

Number of parallel lines being driven

Trace impedance

Series damping resistor

Another method for sending the ECL signal to multiple play-

ers is to route the ECL signal as a bus line and have each

load connected to the bus The ECL bus line must be termi-

nated only at the very end with a matching impedance (eg

a50X line will be terminated with a 50X load to VCC b 2V)

It is preferred that the input pin be directly connected to the

bus and not have a signal tap connected to the bus Howev-

er if a tap off the bus is necessary the shortest possible tap

is recommended

TLF10385 – 12

FIGURE 5-7 CDD Device Driving Four PLAYER Devices in Parallel

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