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PRELIMINARY
XRT79L71
240
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Some Background Information
In order to fully understand the role of the Receive Overhead Data Output Interface, some background
information needs to be discussed first.
As mentioned in
Section 4.1, the DS3 frame consists of 4760 bits. Of these bits, 4704 bits are payload bits and
the remaining 56 bits are overhead bits. The XRT79L71 has been designed to handle and process both the
payload type and overhead type of bits for each DS3 frame. Within the XRT79L71, the Receive Payload Data
Output Interface Block (which is discussed in considerable detail in
Section 4.3.6) has been designed to output
the payload data that has been extracted from the incoming DS3 data-stream to the System-Side Terminal
Equipment. Likewise, the Receive Overhead Data Output Interface block has been designed to handle and
process the overhead bits.
The Receive Overhead Data Output Interface block unconditionally outputs the contents of all overhead bits
within the incoming DS3 data-stream.
The XRT79L71 does not offer the user a means to shut off this
transmission of overhead data. However, the Receive Overhead Data Output Interface block does provide the
user with the appropriate output signals for an external Data-Link Layer or System-Side Terminal Equipment to
sample and process these overhead bits
In order to accomplish this, the Receive Overhead Data Output Interface block has numerous output pins.
Table 31 presents a list and a brief definition of each of these pins.
FIGURE 103. ILLUSTRATION OF THE FUNCTIONAL BLOCK DIAGRAM OF THE RECEIVE DIRECTION CIRCUITRY, WHEN-
EVER THE
XRT79L71 HAS BEEN CONFIGURED TO OPERATE IN THE DS3 CLEAR-CHANNEL FRAMER MODE (WITH THE
RECEIVE OVERHEAD DATA OUTPUT INTERFACE BLOCK HIGHLIGHTED)
Receive
Payload Data
Output
Interface
Block
Receive
Payload Data
Output
Interface
Block
Receive
DS3/E3
Framer
Block
Receive
DS3/E3
Framer
Block
Receive
DS3/E3
LIU Block
Receive
DS3/E3
LIU Block
RxSer
RxNib[3:0]
RxClk
RRING
RTIP
Receive
Overhead Data
Output Interface
Block
Receive
Overhead Data
Output Interface
Block
RxOHClk
RxOHInd
RxOH
RxOHEnable
RxOHFrame
RxNibClk
RxFrame
Rx LAPD
Controller
Block
Rx LAPD
Controller
Block
From Microprocessor
Interface Block
Rx LAPD
Buffer
(90 Bytes)
Rx LAPD
Buffer
(90 Bytes)
Rx FEAC
Processor
Block
Rx FEAC
Processor
Block