XRT7250 datasheet(286/463 Pages) EXAR

286 / 463 page

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

267

• 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 118 presents a flow chart diagram.

Figure 118 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 Mes-

sage 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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