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PRELIMINARY
XRT79L71
74
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Mode 1 Operation of the Transmit Payload Data Input Interface Block
Whenever the XRT79L71 has been configured to operate in this mode, it will function as the source of the
44.736MHz clock signal via the RxOutClk output signal.
NOTE: The "RxOutClk" signal is a buffered version of the "Recovered Line Clock" signal, from the Receive DS3/E3 LIU
Block).
This clock signal is used as the System-Side Terminal Equipment clock source by both the Transmit Payload
Data Input Interface block of the XRT79L71 and the System-Side Terminal Equipment device or circuitry.
The System-Side Terminal Equipment should serially output the payload data, that is to be transported via the
outbound DS3 data-stream, via its DS3_Data_Out output pin. The user is advised to design the System-Side
Terminal Equipment circuitry such that it will update the data via the DS3_Data_Out output pin upon the rising
edge of the 44.736MHz clock signal at its DS3_Clock_In input pin as depicted below in Figure 31.
The XRT79L71 will latch the contents of the TxSer input pin, upon the rising edge of the RxOutClk signal. The
XRT79L71 will indicate that it is processing the very last bit of a given DS3 frame by pulsing its TxFrame output
pin "High" for one bit-period. The TxFrame output pin will be held "Low" at all other times.
Whenever the
System-Side Terminal Equipment detects this pulse at its Tx_End_of_Frame input pin, then it is expected to
begin the transmission of the contents of the very next outbound DS3 frame, via the DS3_Data_Out output or
the TxSer input pin.
Finally, the Transmit Payload Data Input Interface block, within the XRT79L71, will indicate that it is about to
process an overhead bit by pulsing the TxOH_Ind output pin "High" for one period prior to its processing. In
Figure 30, the TxOH_Ind output pin of the XRT79L71 is connected to the DS3_Overhead_Ind input pin of the
System-Side Terminal Equipment circuitry. Whenever the DS3_Overhead_Ind input pin is pulsed "High" the
System-Side Terminal Equipment is expected to NOT transmit a DS3 payload bit upon the very next rising
edge of DS3_Clock_In. Instead, the System-Side Terminal Equipment is expected to delay its transmission of
the very next payload bit by one RxOutClk clock period.
NOTE: For information on operating the "Transmit Payloads Data Input Interface" block in the "Gapped-Clock" Mode, see
FIGURE 30. AN ILLUSTRATION OF HOW TO INTERFACE THE SYSTEM-SIDE TERMINAL EQUIPMENT TO THE TRANSMIT
PAYLOAD DATA INPUT INTERFACE BLOCK OF THE XRT79L71 FOR MODE 1 (SERIAL/LOOP-TIMING) OPERATION
System-Side Terminal
Equipment
XRT79L71 DS3/E3
Framer IC
DS3_Data_Out
DS3_Clock_In
Tx_Start_of_Frame
DS3_Overhead_Ind
TxSer
RxOutClk
TxFrame
TxOH_Ind
NibInt
44.736 MHz Clock Signal