9

ML4665

The comparator is a high-speed differential zero crossing

detector that slices and accurately digitizes the receive

signal. The output of the comparator is fed in parallel into

both the receive squelch circuit and the loopback MUX.

LINK DETECT CIRCUIT AND LOW LIGHT

The link detect circuit monitors the input signal and

determines when the input falls below a preset voltage

level. When the input falls below a preset voltage, the

ML4665 goes into the Low Light state. In the Low Light

state the transmitter is disabled, but continues sending the

1MHz idle signal, the loopback is disabled, the receiver is

disabled, and the

LMON LED pin goes to high shutting off

the

LMON LED. To return to the Link Pass state, the

optical receiver power must be 20% higher than the shut-

off state. This built-in hysteresis adds stability to the Link

Monitor circuit. Once the receiver power threshold is

exceeded, the ML4665 waits 250ms to 750ms, then

checks to see that Tx+. Tx– is idle and no data is being

received before re-enabling the transmitter, receiver,

loopback circuit, and lighting up the

LMON LED.

receiver. The ML4665 is capable of exceeding the

10BASE-FL specifications for sensitivity. The sensitivity is

dependent on the layout of the PC board. A good low

noise layout will exceed the 10BASE-FL specifications,

while a poor layout will fail to meet the sensitivity and

BER spec.

The response time of the Link Detect circuit is set by the

switched on if the input exceeds the set threshold for a

time given by:

T=

A

700

µ

(3)

To switch the link from on to off, the above time will be

doubled. A value of 0.05

µF will meet to 10BASE-FL

specifications.

REF

THRESH

GEN

VREF

VTHADJ

R1

R2

Figure 5.

DIFFERENCES BETWEEN ML4665 AND ML4663/ML4668

The ML4665 is a low cost, reduced pin count alternative

to the industry standard ML4663/ML4668. The following

itemizes the differences between the devices.

1. The SQEN pin found in the ML4663/ML4668, has

been removed. In the ML4665, jabber is always

enabled and SQE pulses are not sent on the AUI

collision pair following a transmission.

2. The

JAB, CLSN, RCV and XMT LED pins on the

ML4663/ML4668 have been removed. LEDs

showing transmit, receive and collision activity can

be added externally. See Figure 6.

ML4663/ML4668, have been removed. In the

ML4665, these pins are tied together internally,

threshold cannot be externally modified.

COL+, Rx+

Tx+

VBB

5k

0.01

µ

100k

510

74C04

(OR EQUIVALENT)

IN914 (OR EQUIVALENT)

MC10125

(OR EQUIVALENT)

Figure 6.