XRT7250 datasheet(176/463 Pages) EXAR

176 / 463 page

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

157

Method 2 Operation of the Terminal Equipment

If the Terminal Equipment intends to insert any over-

head data into the outbound DS3 data stream (via the

Transmit Overhead Data Input Interface), then it is ex-

pected to do the following.

1. To sample the state of both the TxOHFrame and

the TxOHEnable input signals, via the

DS3_Clock_In (e.g., either the TxInClk or the

RxOutClk signal of the XRT7250) signal. If the

Terminal Equipment samples the TxOHEnable

signal "High", then it knows that the XRT7250 is

about to process an overhead bit. Further, if the

Terminal Equipment samples both the TxO-

HFrame and the TxOHEnable pins "High" (at the

same time) then the Terminal Equipment knows

that the XRT7250 is about to process the first

overhead bit, within a new DS3 frame.

2. To keep track of the number of times that the

TxOHEnable signal has been sampled "High"

since the last time both the TxOHFrame and the

TxOHEnable signals were sampled "High". By

doing this, the Terminal Equipment will be able to

keep track of which overhead bit the Transmit

Overhead Data Input Interface is about ready to

process. From this, the Terminal Equipment will

know when it should assert the TxOHIns input pin

and place the appropriate value on the TxOH

input pins (of the XRT7250).

Table 23 also relates the number of TxOHEnable out-

put pulses (that have occurred since both the TxO-

HFrame and TxOHEnable pins were sampled "High")

to the DS3 overhead bit, that is being processed.

FIGURE 59. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT OVERHEAD DATA

INPUT INTERFACE (METHOD 2)

Terminal Equipment

XRT7250 DS3 Framer

DS3_OH_Out

DS3_OH_Enable

Tx_Start_of_Frame

TxOHEnable

TxOHFrame

TxOHIns

44.736 MHz Clock Source

TxInClk

TxOH

Insert_OH

RxLineClk

44.736 MHz

Clock Source

DS3_Clock_In

XRT7250 Datasheet(PDF) 176 Page - Exar Corporation

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