ML4658

11

The AUI drivers are capable of driving the full 50 meters

of cable length and have a rise and fall time of typically

3ns. The rise and fall times match to within 1ns. 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 transmit twisted pair (TxTP+, TxTP–).

A positive signal on the Tx+ lead relative to the Tx– lead

of the DO circuit will result in a positive signal on the

TxTP+ lead of the chip with respect to the TxTP– lead.

Before data will be transmitted onto the twisted pair 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 twisted pair. This circuit rejects

signals with pulse widths less than typically 20ns and

voltage levels more positive than –175mV. Once the 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± that is more positive than –175mV for more

than approximately 180ns.

At the start of a packet transmission, no more than 2 bits

are received from the DO circuit and not transmitted onto

the twisted pair. The difference between start-up delays

(bit loss plus steady-state propagation delay) for any two

packets that are separated by 9.6µs or less will not

exceed 200ns.

The output stage of the transmitter is a current mode

switch which develops the output voltage by driving

current through the terminating resistor and the output

filter. The transmitter employs a center tap 2:1 transformer

the transmit pair (TxTP+, TxTP–) is pulled low, the other

pin floats. The output pins to the twisted pair wires, TxTP+

and TxTP–, can drive a 100

Ω, 150Ωload, or a variety of

impedances that are characteristic of the twisted pair

wire. RTSET selects the current into the TxTP+, TxTP–

pins. This current along with the characteristic impedance

of the cable determines the output voltage.

Once the characteristic impedance of the twisted pair is

determined, one must select the appropriate RTSET

resistor as well as match the terminating impedances of

the transmit and receive filter. The RTSET resistor can be

selected as follows:

RTSET

=×

Ω

(/

)

100

220

pair cable.

Figure 11. Transmit Pre-Equalization Waveform

BINARY

TxTP+

TxTP–

OUTPUT AFTER

TRANSMIT FILTER

INPUT INTO

RECEIVER

0

1

1

100

1

1

0