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ML4658 Datasheet(PDF) 10 Page - Micro Linear Corporation |
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ML4658 Datasheet(HTML) 10 Page - Micro Linear Corporation |
10 / 17 page ML4658 10 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 1k 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+ D1 360 Ω COL– COL+ C1 360 Ω TxIN– TxIN+ D0 39 Ω 360 Ω 360 Ω 39 Ω 50 Ω 50 Ω |
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