XRT74L74 Datasheet, PDF (214/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L74
REV. P1.1.1
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
ADVANCED CONFIDENTIAL
FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT2
BIT 1
BIT 0
Local Loopback
R/W
0
DS3/E3*
R/W
0
Internal LOS
Enable
R/W
1
RESET
R/W
0
Interrupt
Enable Reset
R/W
1
Frame Format
R/W
0
TimRefSel[1:0]
R/W
R/W
1
x
3. Interface the XRT74L74, to the Terminal Equip-
ment, as illustrated in Figure 61.
5.2.2 The Transmit Overhead Data Input Inter-
face
Figure 63 presents a simple illustration of the Trans-
mit Overhead Data Input Interface block within the
XRT74L74.
FIGURE 63. SIMPLE ILLUSTRATION OF THE TRANSMIT OVERHEAD DATA INPUT INTERFACE BLOCK
TxOHFrame
TxOHEnable
TxOH
TxOHClk
TxOHIns
TTrraannsmsmitit
OOvveerhrheeaadd
DDaatataInInppuut _t
InInteterfrafacceeBBlolocckk
To Transmit DS3 Framer Block
The DS3 Frame consists of 4760 bits. Of these bits,
4704 bits are payload bits and the remaining 56 bits
are overhead bits. The XRT74L74 has been de-
signed to handle and process both the payload type
and overhead type bits for each DS3 frame. Within
the Transmit Section within the XRT74L74, the Trans-
mit Payload Data Input Interface has been designed
to handle the payload data. Likewise, the Transmit
Overhead Data Input Interface has been designed to
handle and process the overhead bits.
The Transmit Section of the XRT74L74 generates or
processes the various overhead bits within the DS3
frame, in the following manner.
The Frame Synchronization Overhead Bits (e.g.,
the F and M bits)
The F and M bits are always internally generated by
the Transmit Section of the XRT74L74. These over-
head bits are used (by the Remote Terminal Equip-
ment) for Frame Synchronization purposes. Hence,
user values cannot be inserted for the F and M bits in-
to the outbound DS3 data stream, via the Transmit
Overhead Data Input Interface. Any attempt to exter-
nally insert values for the “F” and “M” bits, will be ig-
nored by the Transmit Overhead Data Input Inter-
face"High" block.
212