XRT7250 datasheet(174/463 Pages) EXAR

174 / 463 page

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

155

In Figure 58 the Terminal Equipment samples the Tx-

OHFrame signal being "High" at the rising clock edge

# 0. At this point, the Terminal Equipment knows that

the XRT7250 is just about to process the very first

overhead bit within a given outbound DS3 frame. Ad-

ditionally, according to Table 21, the very first over-

head bit to be processed is the first X bit. In order to

facilitate the transmission of the Yellow Alarm, the

Terminal Equipment must set this X bit to 0. Hence,

the Terminal Equipment starts this process by imple-

menting the following steps concurrently.

a. Assert the TxOHIns input pin by setting it "High".

b. Set the TxOH input pin to 0.

After the Terminal Equipment has applied these sig-

nals, the XRT7250 will sample the data on both the

TxOHIns and TxOH signals upon the very next falling

edge of TxOHClk (designated at 0- in Figure 58.

Once the XRT7250 has sampled this data, it will then

insert a "0" into the first X bit position, in the outbound

DS3 frame.

Upon detection of the very next rising edge of the Tx-

OHClk clock signal (designated as clock edge 1 in

Figure 58), the Terminal Equipment will negate the

TxOHIns signal (e.g., toggles it "Low") and will cease

inserting data into the Transmit Overhead Data Input

Interface, until rising clock edge # 8 (of the TxOHClk

signal). According to Table 21, rising clock edge # 8

indicates that the XRT7250 is just about ready to pro-

cess the second X bit within the outbound DS3 frame.

Once again, in order to facilitate the transmission of

the Yellow Alarm this X-Bit must also be set to 0.

Hence, the Terminal Equipment will (once again) im-

plement the following steps, concurrently.

a. Assert the TxOHIns input pin by setting it "High".

FIGURE 58. ILLUSTRATION OF THE SIGNAL THAT MUST OCCUR BETWEEN THE TERMINAL EQUIPMENT AND THE

XRT7250, IN ORDER TO CONFIGURE THE XRT7250 TO TRANSMIT A YELLOW ALARM TO THE REMOTE TERMINAL

EQUIPMENT

Terminal Equipment/XRT7250 Interface Signals

TxOHClk

TxOHIns

TxOHFrame

TxOH

Remaining Overhead Bits with DS3 Frame

X bit = 0

TxOHFrame is sample “high”

Terminal Equipment asserts

“TxOHIns” and data on “TxOH” line

XRT7250 device samples the TxOHIns and

TxOH signals.

TxOHFrame is sample “high”

Terminal Equipment asserts

“TxOHIns” and data on “TxOH” line

XRT7250 device samples the TxOHIns and

TxOH signals.

XRT7250 Datasheet(PDF) 174 Page - Exar Corporation

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