HFBR-4663 Просмотр технического описания (PDF) - HP => Agilent Technologies

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HFBR-4663

Single Chip 10BASE-FL Transceiver

HP => Agilent Technologies 

System Description

Figure 1 shows a schematic

diagram of the HFBR-4663 in an

internal or external 10BASE-FL

MAU. On one side of the

transceiver is the AU interface

and the other is the fiber optic

interface. The AU interface is AC

coupled when used in an external

transceiver or an internal

transceiver. The AU interface for

an external transceiver includes

isolation transformers, some

biasing resistors, and a voltage

regulator for power.

The fiber optic side of the

transceiver requires an external

fiber optic transmitter and fiber

optic receiver. The transmitter

uses a current driven output that

directly drives the fiber optic

transmitter. The receive side of

the transceiver accepts the data

after passing through a fiber optic

receiver, which consists of a

module containing a pin diode

and a transimpedance amplifier.

AU Interface

The AU interface consists of 3

pairs of signals, DO, CI and DI as

shown in Figure 1. The DO pair

contains transmit data from the

DTE which is received by the

transceiver and sent out onto the

fiber optic cable. The DI pair

contains valid data that has been

either received from the fiber

optic cable or looped back from

the DO and output through the DI

pair to the DTE. The CI pair

indicates whether a collision has

occurred. It is an output that

oscillates at 10 MHz if a collision,

jabber or SQE Test has taken

place, otherwise it remains idle.

When the transceiver is external,

these three pairs are AC coupled

through isolation transformers,

while an internal transceiver may

be capacitively coupled. Tx+, Tx-

is internally DC biased (shifted up

in voltage) for the proper

common mode input voltage.

The two 39 Ω 1% resistors (or

one 78 Ω 1% resistor) tied to he

Tx+ and Tx- pins will provide the

proper termination. The CI and DI

pair, which are output from the

transceiver to the AUI cable,

require 360 Ω pull down resistors

when terminated with a 78 Ω

load. However on a DTE card, CI

and DI do not need 78 Ω

terminating resistors. This also

means that the pull down resistors

on CI and DI can be 1 kΩ or

greater depending upon the

particular Manchester encoder/

decoder chip used. Using higher

value pull down resistors as in a

DTE card will save power.

The AUI drivers are capable of

driving the full 50 meters of cable

length and have a rise and fall

time of typically 4 ns. In the idle

state, the outputs go to the same

voltage to prevent DC standing

current in the isolation

transformers.

Transmission

The transmit function consists of

detecting the presence of data

from the AUI DO input (Tx+, Tx-)

and driving that data onto the

fiber optic LED transmitter. A

positive signal on the Tx+ lead

relative to the Tx- lead of the DO

circuit will result in no current,

hence the fiber optic LED is in a

low light condition. When Tx+ is

more negative than Tx-, the

HFBR-4663 will sink current into

the chip and the fiber optic LED

will emit light.

Before data will be transmitted

onto the fiber optic cable from

the AU interface, it must exceed

the squelch requirements for the

DO pair. The Tx squelch circuit

serves the function of preventing

any noise from being transmitted

onto the fiber. This circuit rejects

signals with pulse widths less than

typically 20 ns (negative going),

or with levels less than -250 mV.

Once Tx squelch circuit has

unsquelched, it looks for the start

of idle signal to turn on the

squelch circuit again. The

transmitter turns on the squelch

again when it receives an input

signal at Tx+, Tx- that is more

positive than -250 mV for more

than approximately 180 ns.

At the start of a packet trans-

mission, no more than 2 bits are

received from the DO circuit and

not transmitted onto the fiber

optic cable. The difference

between start-up delays (bit loss

plus steady-state propagation

delay) for any two packets that

are separated by 9.6 µs or less

will not exceed 200 ns.

Fiber Optic LED Driver

The output stage of the trans-

mitter is a current mode switch

which controls the output light by

sinking current through the LED

into the TxOUT pin. Once the

current requirement for the LED

is determined, the RTSET resistor

is selected. The following

equation is used to select the

84

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