XRT7250 datasheet(194/463 Pages) EXAR

194 / 463 page

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

175

The Transmit DS3 LIU Interface block can transmit

data to the LIU IC or other external circuitry via two

different output modes: Unipolar or Bipolar. If the Un-

ipolar (or Single Rail) mode is selected, then the con-

tents of the DS3 Frame is output, in a binary (NRZ

manner) data stream via the TxPOS pin to the LIU IC.

The TxNEG pin will only be used to denote the frame

boundaries. TxNEG will pulse "High" for one bit peri-

od, at the start of each new DS3 frame, and will re-

main "Low" for the remainder of the frame. Figure 67

presents an illustration of the TxPOS and TxNEG sig-

nals during data transmission while the Transmit DS3

LIU Interface block is operating in the Unipolar mode.

This mode is sometimes referred to as Single Rail

mode because the data pulses only exist in one po-

larity: positive.

When the Transmit DS3 LIU Interface block is operat-

ing in the Bipolar (or Dual Rail) mode, then the con-

tents of the DS3 Frame is output via both the TxPOS

and TxNEG pins. If the Bipolar mode is chosen, then

the DS3 data to the LIU can be transmitted via one of

two different line codes: Alternate Mark Inversion

(AMI) or Binary - 3 Zero Substitution (B3ZS). Each

one of these line codes will be discussed below. Bi-

FIGURE 66. A SIMPLE ILLUSTRATION OF THE TRANSMIT DS3 LIU INTERFACE BLOCK

From Transmit DS3

Framer Block

TxPOS

TxNEG

TxLineClk

Transmit DS3

LIU Interface

Block

FIGURE 67. THE BEHAVIOR OF TXPOS AND TXNEG SIGNALS DURING DATA TRANSMISSION WHILE THE TRANSMIT

DS3 LIU INTERFACE IS OPERATING IN THE UNIPOLAR MODE

TxPOS

TxNEG

TxLineClk

Data

Frame Boundary

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