XRT7250 datasheet(398/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

379

sometimes referred to as Dual Rail because the data

pulses occur in two polarities: positive and negative.

The role of the TxPOS, TxNEG and TxLineClk output

pins, for this mode are discussed below.

TxPOS - Transmit Positive Polarity Pulse: The

Transmit E3 LIU Interface block will assert this output

to the LIU IC when it desires for the LIU to generate

and transmit a positive polarity pulse to the remote

terminal equipment.

TxNEG - Transmit Negative Polarity Pulse: The

Transmit E3 LIU Interface block will assert this output

to the LIU IC when it desires for the LIU to generate

and transmit a negative polarity pulse to the remote

terminal equipment.

TxLineClk - Transmit Line Clock: The LIU IC uses

this signal from the Transmit E3 LIU Interface block to

sample the state of its TxPOS and TxNEG inputs.

The results of this sampling dictates the type of pulse

(positive polarity, zero, or negative polarity) that it will

generate and transmit to the remote Receive E3

Framer.

6.2.5.1

Selecting the various Line Codes

The user can select either the Unipolar Mode or Bipo-

lar Mode by writing the appropriate value to Bit 3 of

the I/O Control Register (Address = 0x01), as shown

below.

Table 80 relates the value of this bit field to the Trans-

mit E3 LIU Interface Output Mode.

NOTES:

1. The default condition is the Bipolar Mode.

2. This selection also effects the operation of the

Receive E3 LIU Interface block

6.2.5.1.1

The Bipolar Mode Line Codes

If the Framer is selected to operate in the Bipolar

Mode, then the DS3 data-stream can be transmitted

via the AMI (Alternate Mark Inversion) or the HDB3

Line Codes. The definition of AMI and HDB3 line

codes follow.

6.2.5.1.1.1

The AMI Line Code

AMI or Alternate Mark Inversion, means that consec-

utive "one's" pulses (or marks) will be of opposite po-

larity with respect to each other. The line code in-

volves the use of three different amplitude levels: +1,

0, and -1. +1 and -1 amplitude signals are used to

represent one's (or mark) pulses and the "0" ampli-

tude pulses (or the absence of a pulse) are used to

represent zeros (or space) pulses. The general rule

for AMI is: if a given mark pulse is of positive polarity,

then the very next mark pulse will be of negative po-

larity and vice versa. This alternating-polarity rela-

tionship exists between two consecutive mark pulses,

independent of the number of 'zeros' that may exist

between these two pulses. Figure 181 presents an il-

lustration of the AMI Line Code as would appear at

the TxPOS and TxNEG pins of the Framer, as well as

the output signal on the line.

I/O CONTROL REGISTER (ADDRESS = 0X01)

BIT 7BIT 6BIT 5BIT 4

BIT 3BIT2BIT 1BIT 0

Disable TxLOC

LOC

Disable

RxLOC

AMI/ZeroSup*

Unipolar/

Bipolar*

TxLine CLK

Invert

RxLine CLK

Invert

Reframe

R/W

010

000

TABLE 80: THE RELATIONSHIP BETWEEN THE CONTENT OF BIT 3 (UNIPOLAR/BIPOLAR*) WITHIN THE UNI I/O

CONTROL REGISTER AND THE TRANSMIT E3 FRAMER LINE INTERFACE OUTPUT MODE

BIT 3TRANSMIT E3 FRAMER LIU INTERFACE OUTPUT MODE

Bipolar Mode: AMI or HDB3 Line Codes are Transmitted and Received

Unipolar (Single Rail) Mode of transmission and reception of E3 data is selected.

XRT7250 Datasheet(PDF) 398 Page - Exar Corporation

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