ML4658CP datasheet(10/17 Pages) MICRO-LINEAR

10 / 17 page

ML4658

SYSTEM DESCRIPTION

Figure 10 shows a typical block diagram of an external

10BASE-T transceiver interface. On one side of the

transceiver is the AU interface and the other is the twisted

pair. The AU interface is AC coupled when used in an

external transceiver or can be AC or DC coupled when

used in an internal transceiver. The AU interface for an

external transceiver includes isolation transformers, some

biasing resistors, and a voltage converter for power.

The twisted pair side of the transceiver requires external

transmit and receive filters, isolation transformers, and

terminating resistors. These components can be obtained

in a single hybrid package from suppliers listed in Figure

12. The transmitter sends pre-equalized data through the

transmit filters onto the twisted pair. The pre-equalized

data uses a standard two step output waveform that lowers

the amplitude of the 5MHz component so that at the

receiving end both the 5MHz and 10MHz components

have the same amplitude. The external transmit filter

smooths the edges of the signal before passing it onto the

twisted pair.

The receive pair side of the transceiver accepts the data

after it passes through the isolation transformer and the

receive low pass filter. Since this is an AC coupled input,

the Bias pin is used to set the proper common mode

voltage for the receive inputs. A pair of 50

Ω resistors

correctly terminate the receive pair and provide a

common mode for the Bias voltage connection point.

AU INTERFACE

The AU interface consists of 3 pairs of signals, DO, CI and

DI, as shown in Figure 10. The DO pair contains transmit

data from the DTE which is received by the transceiver

and sent out onto the twisted pair. The DI pair contains

valid data that has been either received from the twisted

pair or looped back from the DO and output through the

Dl pair to the DTE. The CI pair indicates whether a

transmit based collision has occurred. It is an output that

oscillates at 10MHz. CI pair is also used for Jabber and

SQE Test.

The transceiver may be AC or DC coupled depending on

the application. For the AC coupled interface, the DO

input must be DC biased (shifted up in voltage) for the

proper common mode input voltage. The BIAS pin serves

this purpose. When DC coupled, the manchester encoder/

decoder transmit output pair provides this common mode

voltage and the Bias pin is not connected.

The two 39

Ω 1% resistors tied to the Tx+ and Tx– pins

serve two purposes. They provide a point to connect the

common mode bias voltage, and they provide the proper

matching termination for the AUI cable. The CI and DI

pair, which are output drivers 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

Ω or greater depending upon the particular manchester

encoder/decoder chip used.

Figure 10. System Block Diagram

FILTER

+5V

VCC

FILTER

TxTP+

TxTP–

RxTP+

BIAS

RxTP–

GND

RxOUT–

RxOUT+

360

Ω

COL–

COL+

360

Ω

TxIN–

TxIN+

Ω

360

Ω

360

Ω

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