XRT72L13 Datasheet, PDF (280/370 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
• Compute the 16 bit "Frame Check Sum" (FCS) of
the LAPD Message Frame (e.g., of the LAPD Mes-
sage header and information payload) and append
this value to the LAPD Message.
• Append a "trailer" Flag Sequence octet to the end
of the message LAPD (following the 16 bit FCS
value).
• Serialize the composite LAPD message and begin
inserting the LAPD message into the "DL" bit fields
of each outgoing DS3 Frame.
• Complete the transmission of the frame overhead,
payload, FCS value, and trailer Flag Sequence
octet via the Transmit DS3 Framer.
Once the LAPD Transmitter has completed its trans-
mission of the LAPD Message, the Framer will gener-
ate an interrupt to the local µC/µP (if enabled). After-
wards, the LAPD Transmitter will proceed to retrans-
mit the LAPD Message, repeatedly at one second in-
tervals. During "Idle" periods (e.g., in between these
transmission of the LAPD Message), the LAPD
Transmitter will be sending a continuous stream of
"Flag Sequence Bytes". The LAPD Transmitter will
continue this behavior until the user has disabled the
LAPD Transmitter by writing a "0" to bit 0 (TxLAPD
Enable) within the Tx DS3 LAPD Configuration Reg-
ister. If the LAPD Transmitter is idle, then it will con-
tinuously send the Flag Sequence octets (via the
"DL" bits of each outbound DS3 Frame) to the remote
terminal equipment.
NOTE: In order to prevent the user's data (e.g., the payload
portion of the LAPD Message Frame) from mimicking the
"Flag Sequence" byte, the LAPD Transmitter will insert a "0"
into the LAPD data stream immediately following the detec-
tion of five (5) consecutive "1s" (this "stuffing" occurs only
while the information payload is being transmitted). The
'remote' LAPD Receiver (see Section 4.3.3.2) will have the
responsibility of "detecting the 5 consecutive "1s" and
removing the subsequent "0" from the payload portion of
the incoming LAPD message.
Figure 82 presents a flow chart depicting the proce-
dure (in 'white boxes') that the user should use in or-
der to transmit a LAPD message. This figure also in-
dicates (via the "shaded" boxes) what the LAPD
Transmitter circuitry will do before and during mes-
sage transmission.
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