XRT72L13 Datasheet, PDF (320/370 Pages) Exar Corporation – M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC

XRT72L13
M13 MULTIPLEXER/CLEAR CHANNEL DS3 FRAMER IC
REV. P1.0.6
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PRELIMINARY
RX DS3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
RxAbort
BIT 5
BIT 4
RxLAPD Type[1, 0]
X
X
X
X
The receipt of the Flag Sequence octet can mean
one of two things.
1. The Flag Sequence byte marks the beginning or
end of an incoming LAPD Message.
2. The received Flag Sequence octet could be just
one of many Flag Sequence octets that are trans-
mitted via the DS3 Transport Medium, during idle
periods between the transmission of LAPD Mes-
sages.
The LAPD Receiver will clear the "Flag Present" bit
as soon as it has received an octet that is something
other than the "Flag Sequence" octet. At this point,
the LAPD Receiver should be receiving either octet
#2 of the incoming LAPD Message, or an Abort Se-
quence (e.g., a string of seven or more consecutive
"1s"). If this next set of data is an abort sequence,
then the LAPD Receiver will assert the RxAbort bit
(Bit 6) within the "Rx DS3 LAPD Status" Register.
However, if this next octet is Octet #2 of an incoming
BIT 3
BIT2
BIT 1
BIT 0
RxCR Type RxFCS Error End of
Message
Flag
Present
X
X
X
1
LAPD Message, then the "Rx DS3 LAPD Status"
Register will begin to present some additional status
information on this incoming message. Each of these
indicators is presented below in sequential order.
Bit 3 - RxCR Type - C/R (Command/Response)
Type
This bit-field reflects the contents of the C/R bit-field
within octet #2 of the LAPD Frame Header. When
this bit is "0" it means that this message is originating
from a customer installation. When this bit is "1" it
means that this message is originating from a net-
work terminal.
Bit 4,5 - RxLAPD Type[1, 0] - LAPD Message Type
The combination of these two bit fields indicate the
Message Type and the Message Size of the incoming
LAPD Message frame. Table 34 relates the values
of Bits 4 and 5 to the Incoming LAPD Message Type/
Size.
TABLE 34: THE RELATIONSHIP BETWEEN RXLAPDTYPE[1:0] AND THE RESULTING LAPD MESSAGE TYPE AND SIZE
RXLAPD TYPE[1, 0]
00
01
10
11
MESSAGE TYPE
Test Signal Identification
Idle Signal Identification
CL Path Identification
TU-T Path Identification
MESSAGE SIZE
76 bytes
76 bytes
76 bytes
82 bytes
NOTE: The Message Size pertains to the size of the "Infor-
mation portion" of the LAPD Message Frame (as presented
in Figure 104 ).
Bit 3 - Flag Present
The LAPD Receiver should receive another "Flag Se-
quence" octet, which marks the End of the Message.
Therefore, this bit field should be asserted once
again.
Bit 1 - EndOfMessage - End of LAPD Message
Frame
Upon receipt of the closing "Flag Sequence" octet,
this bit-field should be asserted. The assertion of this
bit-field indicates that a LAPD Message Frame has
been completely received. Additionally, if this newly
received LAPD Message is different from the previ-
ous message, then the LAPD Receiver will inform the
µC/µP of the "EndOfMessage" event by generating
an interrupt.
Bit 2 - RxFCSErr - Frame Check Sequence Error
Indicator
The LAPD Receiver will take the incoming LAPD
Message and compute its own version of the Frame
Check Sequence (FCS) word. Afterwards, the LAPD
Receiver will compare its computed value with that it
has received from the remote LAPD Transmitter. If
these two values match, then the LAPD Receiver will
presume that the LAPD Message has been properly
received; and the contents of the Received LAPD
Message (payload portion) will be retained at loca-
tions 0xDE through 0x135 in on-chip RAM. The
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