XRT74L73 Datasheet, PDF (319/488 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
)
TXE3 LAPD STATUS AND INTERRUPT REGISTER (ADDRESS = 0X34)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
Not Used
TxDL Start TxDL Busy
RO
RO
RO
RO
R/W
RO
0
0
0
0
X
X
BIT 1
TxLAPD
Interrupt
Enable
R/W
1
BIT 0
TxLAPD
Interrupt
Status
RUR
X
‘The purpose of t his interrupt is to let the Micropro-
cessor/Microcontroller know that the LAPD Transmit-
ter is available and ready to transmit a LAPD Mes-
sage frame (which contains a new PMDL Message)
to the remote terminal equipment. Bit 0 (Tx LAPD In-
terrupt Status) within the Tx E3 LAPD Status and In-
terrupt Register will reflect the status for the Transmit
LAPD Interrupt.
NOTE: This bit-field will be reset upon reading this register.
Summary of Operating the LAPD Transmitter
Once the user has invoked the TxDL Start command,
the LAPD Transmitter will do the following.
• Generate the four octets of the LAPD Message
frame header (e.g., the Flag Sequence, SAPI, TEI,
Control, etc.,) and insert them into the header byte
positions within the LAPD Message frame.
• It will read in the contents of the Transmit LAPD
Message buffer (e.g., the PMDL Message data)
and insert it into the Information Payload portion of
the LAPD Message frame.
• Compute the 16-bit Frame Check Sequence (FCS)
value of the LAPD Message frame (e.g, of the
LAPD Message header and Payload bytes) and
insert this value into the FCS value octet positions
within the LAPD Message frame.
• Append a trailer Flag Sequence octet to the end of
the LAPD Message frame (following the 16-bit FCS
octets).
• Fragment the resulting LAPD Message frame into
bits and begin inserting these bits into the “N” bit-
field within each outbound E3 frame.
• Complete the transmission of the overhead bytes,
information payload byte, FCS value, and the trail-
ing Flag Sequence octets via the Transmit E3
Framer block.
Once the LAPD Transmitter has completed its trans-
mission of the LAPD Message frame, the Framer will
generate an Interrupt to the MIcroprocessor/Micro-
controller (if enabled). Afterwards, the LAPD Trans-
mitter will either halt its transmission of LAPD Mes-
sage frames or will proceed to retransmit the LAPD
Message frame, repeatedly at one-second intervals.
In between these transmissions of the LAPD Mes-
sage 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 Configu-
ration 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 detec-
tion 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 4.3.5) will have the responsibility of checking the
newly received PMDL messages for a string of five (5) con-
secutive “1’s” and removing the subsequent “0” from the
payload portion of the incoming LAPD Message.
Figure 125 presents a flow chart diagram.
Figure 125 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 inter-
vals. This figure also indicates (via the Shaded box-
es) what the LAPD Transmitter circuitry will do before
and during message transmission.
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