XRT7250 datasheet(195/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

176

polar mode is sometimes referred to as Dual Rail be-

cause 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 DS3 LIU Interface block will assert this out-

put to the LIU IC when it desires for the LIU to gener-

ate and transmit a positive polarity pulse to the re-

mote terminal equipment.

TxNEG - Transmit Negative Polarity Pulse: The

Transmit DS3 LIU Interface block will assert this out-

put to the LIU IC when it desires for the LIU to gener-

ate and transmit a negative polarity pulse to the re-

mote terminal equipment.

TxLineClk - Transmit Line Clock: The LIU IC uses

this signal from the Transmit DS3 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 DS3

Framer.

4.2.5.1

Selecting the various Line Codes

Either the Unipolar Mode or Bipolar Mode can be se-

lected by writing the appropriate value to Bit 3 of the I/

O Control Register (Address = 0x01), as shown be-

low.

Table 32 relates the value of this bit field to the Trans-

mit DS3 LIU Interface Output Mode.

NOTES:

1. The default condition is the Bipolar Mode.

2. This selection also effects the operation of the

Receive DS3 LIU Interface block

4.2.5.1.1

The Bipolar Mode Line Codes

If framer is to be operated in the Bipolar Mode, then

the DS3 data-stream can be transmitted via the AMI

(Alternate Mark Inversion) or the B3ZS Line Codes.

The definition of AMI and B3ZS line codes follow.

4.2.5.1.1.1

The AMI Line Code

AMI or Alternate Mark Inversion, means that consec-

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

ity with respect to each other. The line code involves

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" amplitude pulses

(or the absence of a pulse) are used to represent ze-

ros (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 polarity and vice

versa. This alternating-polarity relationship exists be-

tween two consecutive mark pulses, independent of

the number of 'zeros' that may exist between these

two pulses. Figure 68 presents an illustration of the

AMI Line Code as would appear at the TxPOS and

TxNEG pins of the Framer, as well as the output sig-

nal 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

000

TABLE 32: THE RELATIONSHIP BETWEEN THE CONTENT OF BIT 3 (UNIPOLAR/BIPOLAR*) WITHIN THE UNI I/O

CONTROL REGISTER AND THE TRANSMIT DS3 FRAMER LINE INTERFACE OUTPUT MODE

BIT 3TRANSMIT DS3 FRAMER LIU INTERFACE OUTPUT MODE

Bipolar Mode: AMI or B3ZS Line Codes are Transmitted and Received

Unipolar (Single Rail) Mode of transmission and reception of DS3 data is selected.

XRT7250 Datasheet(PDF) 195 Page - Exar Corporation

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