XRT72L13 Datasheet, PDF (271/370 Pages) Exar Corporation – M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC

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PRELIMINARY
XRT72L13
M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC
REV. P1.0.6
Interfacing the "Transmit Overhead Data Input Interface
to the Terminal Equipment
Figure 78 illustrates how one should interface the
"Transmit Overhead Data Input Interface" to the Ter-
minal Equipment when using Method 2.
FIGURE 78. 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
Terminal Equipment
44.736 MHz Clock Source
TxInClk
TxOHEnable
44.736 MHz
Clock Source
TxOH
TxOHFrame
RxLineClk
TxOHIns
XRT72L13 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 XRT72L13) signal. If the
Terminal Equipment samples the "TxOHEnable"
signal "high"; then it knows that the XRT72L13 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 XRT72L13 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 XRT72L13).
Table 17 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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