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XRT79L71
PRELIMINARY
103
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
Method 1 Operation of the Transmit Overhead Data Input Interface Block
To operate the Transmit Overhead Data Input Interface block, design/configure the System-Side Terminal
Equipment to continuously execute the following tasks.
TASK # 1: The System-Side Terminal Equipment must sample the state of the TxOHFrame output pin from
the XRT79L71 upon the rising edge of the TxOHClk clock signal which is also output from the XRT79L71.
Whenever the System-Side Terminal Equipment samples the TxOHFrame output pin "High" then it will know
that the Transmit Overhead Data Input Interface block is ready to accept overhead bits for a new DS3 frame.
TASK # 2: As the System-Side Terminal Equipment samples the TxOHFrame signal, it must also keep track of
the number of rising edges within the TxOHClk signal that have occurred since the last time TxOHFrame was
sampled "High". By doing this, the System-Side Terminal Equipment will be able to keep track of which
overhead bit is currently being processed by the Transmit Overhead Data Input Interface block at any given
TxOHClk clock period.
When the System-Side Terminal Equipment knows which overhead bit is being
processed within a given TxOHClk clock period, it can decide when to insert the appropriate bit-value into
Transmit Overhead Data Input Interface block and in-turn, force the Transmit DS3/E3 Framer block to insert
this bit into the appropriate overhead bit-position within the outbound DS3 data-stream. From all of this, the
System-Side Terminal Equipment will know when it should assert the TxOHIns input pin and place the
appropriate value on the TxOH input pin of the XRT79L71.
Table 20 relates the number of rising clock edges, within the TxOHClk output signal, since the TxOHFrame
output signal was sampled "High" to the DS3 Overhead Bit being processed by the Transmit Overhead Data
Input Interface block. The user can use this table as a guide for inserting the appropriate overhead bits, within
the outbound DS3 data-stream, for Method 1.
FIGURE 44. ILLUSTRATION ON HOW ONE SHOULD INTERFACE THE SYSTEM-SIDE TERMINAL EQUIPMENT TO THE
TRANSMIT OVERHEAD DATA INPUT INTERFACE BLOCK WHEN USING METHOD 1
System-Side Terminal
Equipment
XRT779L71 DS3/E3 Framer
DS3_OH_Out
DS3_OH_Clock_In
Tx_Start_of_Frame
TxOHClk
TxOHFrame
TxOHIns
44.736 MHz Clock Source
TxInClk
TxOH
Insert_OH