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PRELIMINARY
XRT79L71
132
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
The Mechanics of Transmitting a New LAPD Message, if the Transmit LAPD Controller has been
configured to retransmit the LAPD Message frame, repeatedly at one-second intervals
If the Transmit LAPD Controller has been configured to retransmit the LAPD Message repeatedly at one-
second intervals, then it will do the following at one-second intervals.
Parse through the contents of the Transmit LAPD Message buffer and Zero-Stuff the PMDL Message
Read in the stuffed PMDL Message from the Transmit LAPD Message buffer
Encapsulate this stuffed PMDL Message into a LAPD Message frame
Transmit this LAPD Message frame to the Remote Terminal.
To transmit another (e.g., different) PMDL message to the remote LAPD Receiver, the user will have to write
this new message into the Transmit LAPD Message buffer, via the Microprocessor Interface section of the
channel. However, the user must be careful when writing in this new 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, this action could interfere with these transmissions, thereby causing the Transmit LAPD
Controller to transmit a corrupted message to the remote LAPD Receiver. In order to avoid this problem, while
writing the new message into the Transmit LAPD Message buffer, do the following:
1.
Configure the DS3/E3 Framer Block to automatically reset activated interrupts
FIGURE 57. FLOW CHART DEPICTING HOW TO USE THE TRANSMIT LAPD CONTROLLER
START
WRITE IN DATA LINK INFORMATION
The user accomplishes this by writing the
information that he/she wishes to transmit
(via the LAPD Transmitter) to locations 86h
through DDh, within the UNI Address Space.
CONFIGURE THE LAPD
TRANSMITTER FOR TRANSMISSION
This is accomplished by writing 00000xx1b
to the Tx DS3 LAPD Configuration Register.
(where xx dictates LAPD Message Type)
START TRANSMISSION OF LAPD
MESSAGE
This is accomplished by writing 000010x0b
to the Tx DS3 LAPD Status/Interrupt
Register. (where x indicates the user’s choice
to enable/disable “LAPD Message Transfer
Complete” Interrupt
LAPD Transmitter inserts Frame Header
octets in front of the user payload.
LAPD Transmitter computes the 16 bit FCS
(a CRC-16 value) and inserts it into the LAPD
Message, following the user payload
LAPD Transmitter appends a Flag Sequence
Trailer octet to the end of the LAPD Message
(after the 16 bit FCS).
Is
5 consecutive
“1s” detected
?
Is
Message
Transmission
Complete
?
Insert a “0” after the
string of 5 consecutive
“1s”
END
Generate Interrupt
LAPD Transmitter will
continue to transmit
Flag Sequence octets.
Yes
No
Yes
No