![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT79L71IB-F_datasheet_100145/XRT79L71IB-F_240.png)
XRT79L71
PRELIMINARY
225
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
The Receive LAPD Controller block consists of the following sections.
The Receive LAPD Message Buffer
The Receive LAPD Controller
The Receive LAPD Message Buffer
The purpose of the Receive LAPD Message Buffer is to store the contents of LAPD/PMDL Messages that have
been received by the Receive LAPD Controller block. The Receive LAPD Message Buffer will serve as a
temporary storage location of these incoming LAPD/PMDL Messages until they can be read out by the
Microprocessor. the Receive LAPD Message Buffer is actually a 90 byte FIFO that is located at Address
Location 0x11C0 within the XRT79L71 address space.
The Receive LAPD Controller
The Receive LAPD Controller permits the user to receive path maintenance data link (PMDL) message from
the remote terminal equipment via the inbound DS3 frames. In this case, the PMDL Message is extracted out
of the 3 DL bit-fields of F-Frame # 5 within each incoming DS3 M-frame. The on-chip Receive LAPD Controller
is capable of receiving both standard and non-standard PMDL Messages of any length up to 82 bytes. The
XRT79L71 allocates a block of 90 bytes of on-chip RAM (e.g., the Receive LAPD Message buffer), to store the
contents of newly received PMDL Messages.
The message format complies with ITU-T Q.921 (LAPD)
protocol with different addresses and is presented below in Figure 96.
FIGURE 95. 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 LAPD CONTROLLER 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