XRT74L73 Datasheet, PDF (419/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
FIGURE 183. FLOW CHART DEPICTING HOW TO USE THE LAPD TRANSMITTER (LAPD TRANSMITTER IS CONFIGURED
TO TRANSMIT A LAPD MESSAGE FRAME ONLY ONCE).
Start
Write the “LAPD Message frame identification”
octet into the first octet position within the
“Transmit LAPD Message” buffer (Address = 0x86).
Write the PMDL Message into the remaining
portion of the “Transmit LAPD Message” buffer
(from 0x87 to 0xDD).
Specify the type/size of the LAPD Message frame
to be transmitted.
Write in the appropriate value into bits 5 and 6
within the Tx E3 LAPD Configuration Register
Specify whether the “outbound” LAPD Message
frame is to be transported via the GC or the NR
byte-fields, within each “outbound” E3 Frame
Initiate the LAPD Message frame transmission.
LAPD Transmitter will “stuff” the contents of the
PMDL Message (residing within the “Transmit
LAPD Message” buffer).
LAPD Transmitter will read out “stuff” PMDL Message
and encapsulate it into a LAPD Message frame.
LAPD Transmitter will compute and insert the
FCS value, into the LAPD Message frame.
LAPD Transmitter will fragment LAPD Message
frame into “octets” and begin to insert these octets into
the GC or NR byte-field (depending upon user’s
selection) into each “outbound” E3 frame.
Configure the LAPD Transmitter to transmit
LAPD Message Frame Only Once
Complete transmission of LAPD Message frame.
Enable the LAPD Transmitter
Generate “Completion of Transmission of LAPD
Message frame Interrupt.
LAPD Transmitter will generate a continuous
string of “Flag Sequence” bytes. These bytes will
be transported via either the GC or the NR byte
field (depending upon user’s selection).
Halt Transmission for an “indefinite period.
Wait until the user initiates “LAPD Message
Frame Transmission” again.
The Mechanics of Transmitting a New LAPD Mes-
sage frame, if the LAPD Transmitter has been
configured to re-transmit the LAPD Message
frame, repeatedly, at One-Second intervals.
If the LAPD Transmitter has been configured to re-
transmit the LAPD Message frame repeatedly at
One-Second intervals, then it will do the following (at
One-Second intervals).
• Stuff the PMDL Message.
• Read in the stuffed PMDL Message from the Trans-
mit LAPD Message buffer.
• Encapsulate this stuffed PMDL Message into a
LAPD Message frame.
• Transmit this LAPD Message frame to the Remote
Terminal Equipment.
If another (e.g., a different) PMDL Message is to be
transmitted to the Remote Terminal Equipment this
new message will have to be written into the Transmit
LAPD Message buffer, via the Microprocessor Inter-
face block of the Framer IC. However, care must be
taken when writing this new PMDL message. If this
message is written into the Transmit LAPD Message
buffer at the wrong time (with respect to these One-
Second LAPD Message frame transmissions), the
user’s action could interfere with these transmissions,
thereby causing the LAPD Transmitter to transmit a
corrupted message to the Remote Terminal Equip-
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