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PRELIMINARY
XRT79L71
272
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
34.368MHz clock signal via the RxOutClk output signal. 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.
NOTE: The "RxOutClk" signal is a buffered version of the "Recovered Line Clock" signal, from the Receive DS3/E3 LIU
Block).
The System-Side Terminal Equipment should serially output the payload data that is to be transported via the
outbound E3 data-stream via its E3_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 E3_Data_Out output pin upon the rising
edge of the 34.368MHz clock signal at its E3_Clock_In input pin as depicted below in Figure 125.
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 E3 frame by pulsing its TxFrame output
pin "High" for one bit-period. The TxFrame output pin will have held "Low" at all other times. Whenever the
System-Side Terminal Equipment detects this pulse at its Tx_Start_of_Frame input pin, then it is expected to
begin the transmission of the contents of the very next outbound E3 frame, via the E3_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 124, the TxOH_Ind output pin of the XRT79L71 is connected to the E3_Overhead_Ind input pin of the
System-Side Terminal Equipment circuitry. Whenever the E3_Overhead_Ind input pin is pulsed "High" the
System-Side Terminal Equipment is expected to NOT transmit an E3 payload bit upon the very next rising
edge of E3_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.
NOTES:
1.
Since the E3, ITU-T G.751 Frame consists of twelve consecutive overhead bits, whenever the TxOH_Ind output
pin of the XRT79L71 pulses "High" it will do so for 12 consecutive bit-periods when processing the FAS, A and N
FIGURE 124. 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
E3_Data_Out
E3_Clock_In
Tx_End_of_Frame
E3_Overhead_Ind
TxSer
RxOutClk
TxFrame
TxOH_Ind
NibInt
34.368 MHz Clock Signal