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.
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 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 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 39Ω 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 360Ω pull down resistors when
terminated with a 78Ω load. However on a DTE card, CI
and DI do not need 78Ω terminating resistors. This also
means that the pull down resistors on CI and DI can be
1kΩ or greater depending upon the particular manchester
encoder/decoder chip used.
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 50Ω resistors
correctly terminate the receive pair and provide a
common mode for the Bias voltage connection point.
+5V
V
CC
TxIN+
TxIN–
D0
C1
D1
TxTP+
TxTP–
FILTER
39Ω
360Ω
360Ω
39Ω
COL+
COL–
RxTP+
BIAS
360Ω
360Ω
50Ω
FILTER
RxOUT+
RxOUT–
50Ω
RxTP–
GND
Figure 10. System Block Diagram
10
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
