XRT7250 datasheet(294/463 Pages) EXAR

294 / 463 page

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

275

As the Transmit E3 Framer block formulates the out-

bound E3 frames, the contents of the BIP-4 bits are

automatically XORed with the contents of this regis-

ter. The results of this XOR operation is written back

into the corresponding bit-field within the outbound

E3 frame, and is transmitted to the Remote Terminal

Equipment. Therefore, if the user does not wish to

modify any of these bits, then this register must con-

tain all “0’s” (the default value).

NOTE: This register is only active if the XRT7250 Framer IC

has been configured to insert the BIP-4 nibble into each

outbound E3 frame.

5.2.5

The Transmit E3 Line Interface Block

The XRT7250 Framer IC is a digital device that takes

E3 payload and overhead bit information from some

terminal equipment, processes this data and ulti-

mately, multiplexes this information into a series of

outbound E3 frames. However, the XRT7250 Framer

IC lacks the current drive capability to be able to di-

rectly transmit this E3 data stream through some

transformer-coupled coax cable with enough signal

strength for it to be received by the remote receiver.

Therefore, in order to get around this problem, the

Framer IC requires the use of an LIU (Line Interface

Unit) IC. An LIU is a device that has sufficient drive

capability, along with the necessary pulse-shaping

circuitry to be able to transmit a signal through the

transmission medium in a manner that it can be reli-

ably received by the far-end receiver. Figure 120 pre-

sents a circuit drawing depicting the Framer IC inter-

facing to an LIU (XRT7300 DS3/E3/STS-1 Transmit

LIU).

TXE3 BIP-4 ERROR MASK REGISTER (ADDRESS = 0X4A)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

TxBIP-4 Mask[3:0]

R/W

0000

000

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