XRT72L50 Datasheet, PDF (391/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
• Serialize the composite LAPD message. Between the two 0x7E flags, ZeroStuff any consecutive five “Ones”
by inserting an extra “0”. This insures that any occurrence of 0x7E in the payload does not serve as a
terminating flag sequence.
• Insert the Zero Stuffed LAPD message into the GC or NR byte-fields within the Outbound E3 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 transmission of the LAPD Message frame, the Framer will
generate an Interrupt to the MIcroprocessor/Microcontroller (if enabled). Afterwards, the LAPD Transmitter will
either halt its transmission of LAPD Message frames or will proceed to retransmit the LAPD Message frame,
repeatedly at One-Second intervals. In between these transmissions of the LAPD Message frames, 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 “1” into bit 3 (No Data Link) within
the Tx E3 Configuration register.
NOTE: In order to prevent the user’s data (e.g., the PMDL Message within the LAPD Message frame) from mimicking the
Flag Sequence byte or an ABORT Sequence, the LAPD Transmitter will parse through the PMDL Message data
and insert a “0” into this data, immediately following the detection of five (5) consecutive “1’s” (this stuffing occurs
while the PMDL message data is being read in from the Transmit LAPD Message frame. The Remote LAPD
Receive (See Section 6.3.3) will have the responsibility of checking the newly received PMDL messages for a
string of five (5) consecutive “1’s” and removing the subsequent “0” from the payload portion of the incoming LAPD
Message.
Figure 163 is a flow chart that depicts the procedure (in white boxes) that the user should use in order to
transmit a PMDL message via the LAPD Transmitter, when the LAPD Transmitter is configured to retransmit
the LAPD Message frame, repeatedly at One-Second intervals. This figure also indicates (via the Shaded
boxes) what the LAPD Transmitter circuitry will do before and during message transmission.
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