XRT74L73 Datasheet, PDF (207/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
FIGURE 66. ILLUSTRATION OF THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT OVERHEAD DATA
INPUT INTERFACE (METHOD 2)
DS3_Clock_In
DS3_OH_Enable
DS3_OH_Out
Tx_Start_of_Frame
Insert_OH
44.736 MHz Clock Source
TxInClk
TxOHEnable
44.736 MHz
Clock Source
TxOH
TxOHFrame
RxLineClk
TxOHIns
Terminal Equipment
XRT72L5x DS3 Framer
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
expected 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 XRT74L73) signal. If the
Terminal Equipment samples the TxOHEnable
signal "High", then it knows that the XRT74L73 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 XRT74L73 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 XRT74L73).
Table 39 also relates the number of TxOHEnable
output pulses (that have occurred since both the Tx-
OHFrame and TxOHEnable pins were sampled
"High") to the DS3 overhead bit, that is being pro-
cessed.
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