8

ML4665

TxOUT

51

Ω

51

Ω

51

Ω

RTSET = 560

Ω

ECL

Figure 3. Converting Optical LED Driver Output to

Differential ECL.

RECEPTION

The input to the transceiver comes from a fiber optic

receiver as shown in figure 1. At the start of packet

reception no more than 2.7 bits are received from the

fiber cable, and are not transmitted onto the DI circuit.

The receive squelch will reject frequencies lower than

2.51MHz.

While in the unsquelch state, the receive squelch circuit

looks for the start of idle signal at the end of the packet.

Start of idle occurs when the input signal remains idle for

more than 160ns. When start of idle is detected, the

receive squelch circuit returns to the squelch state and the

start of idle signal is output on the DI circuit (Rx+, Rx–).

COLLISION

Whenever the receiver and the transmitter are active at

the same time the chip will activate the collision output.

The collision output is a differential square wave matching

the AUI specifications and capable of driving a 78

Ω load.

The frequency of the square wave is 10MHz

± 15% with a

60/40 to 40/60 duty cycle. The collision oscillator also is

activated Jabber.

LOOPBACK

The loopback function emulates a 10BASE-T transceiver

whereby the transmit data sent by the DTE is looped back

over the AUI receive pair. Some LAN controllers use this

loopback information to determine whether a MAU is

connected by monitoring the carrier sense while

transmitting. The software can use this loopback

information to determine whether a MAU is connected to

the DTE by checking the status of carrier sense after each

packet transmission.

When data is received by the chip while transmitting, a

collision condition exits. This will cause the collision

oscillator to turn on and the data on the DI pair will

collision oscillator will remain on until either DO or

this mode the chip operates as a full duplex transmitter

and receiver, and collision detection is disabled. A

loopback through the transceiver can be accomplished by

tying the fiber transmitter to the receiver.

JABBER FUNCTION REQUIREMENTS

The Jabber function prevents a babbling transmitter from

bringing down the network. Within the transceiver is a

Jabber timer that starts at the beginning of each

transmission and resets at the end of each transmission. If

the transmission last longer than 20ms the jabber logic

disables the transmitter, and turns on the collision signal

COL+, COL–. When Tx+ and Tx– finally go idle, a second

timer measures 0.5 seconds of idle time before the

transmitter is enabled and collision is turned off. Even

though the transmitter is disabled during jabber, the 1MHz

idle signal is still transmitted.

LOW LIGHT CONDITION

The

LMON LED output is used to indicate a low light

condition.

LMON is activated low when both the receive

power exceeds the Link Monitor threshold and there are

of these conditions do not exist,

LMON will go high.

INPUT AMPLIFIER

with a gain of about 100 and input resistance of 1.3k

Ω.

Maximum sensitivity is achieved through the use of a DC

restoration feedback loop and AC coupling the input.

When AC coupled, the input DC bias voltage is set by an

on-chip network at about 1.7V. These coupling capacitors,

in conjunction with the input impedance of the amplifier,

at

f =

1

2

300C

L

π1

(1)

Since the amplifier has a differential input, two capacitors

of equal value are required. If the signal driving the input

is single ended, one of the coupling capacitors can be tied

The internal amplifier has a lowpass filter built-in to band

limit the input signal which in turn will improve the signal

to noise ratio.

Although the input is AC coupled, the offset voltage

within

the amplifier will be present at the amplifier’s output. This

error a DC feedback loop is incorporated. This negative

pole it creates can effect the stability of the feedback loop.

To avoid stability problems, the value of this capacitor

should be at least 10 times larger than the input coupling

capacitors.

Figure 4.