XRT72L50 Datasheet, PDF (275/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
FIGURE 101. THE TERMINAL EQUIPMENT BEING INTERFACED TO THE TRANSMIT OVERHEAD DATA INPUT INTERFACE
(METHOD 1)
E3_OH_Clock_In
E3_OH_Out
Tx_Start_of_Frame
Insert_OH
34.368MHz
Clock Source
TxInClk
TxOHClk
TxOH
RxLineClk
TxOHFrame
34.368MHz
Clock Source
TxOHIns
Terminal Equipment
E3 Framer
Method 1 Operation of the Terminal Equipment
If the Terminal Equipment intends to insert any overhead data into the outbound E3 data stream, (via the
Transmit Overhead Data Input Interface), then it is expected to do the following.
1. To sample the state of the TxOHFrame signal (e.g., the Tx_Start_of_Frame input signal) on the rising edge
of the TxOHClk (e.g., the E3_OH_Clock_In signal).
2. To keep track of the number of rising clock edges that have occurred, via the TxOHClk (e.g., the
E3_OH_Clock_In signal) since the last time the TxOHFrame signal was sampled "High". By doing this the
Terminal Equipment will be able to keep track of which overhead bit is being processed by the Transmit
Overhead Data Input Interface block at any given time. When the Terminal Equipment knows which over-
head bit is being processed, at a given TxOHClk period, it will know when to insert a desired overhead bit
value into the outbound E3 data stream. From this, the Terminal Equipment will know when it should
assert the TxOHIns input pin and place the appropriate value on the TxOH input pin (of the XRT72L50).
Table 46 relates the number of rising clock edges (in the TxOHClk signal, since TxOHFrame was sampled
"High") to the E3 Overhead Bit, that is being processed.
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