XRT7250 datasheet(391/463 Pages) EXAR

391 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

372

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-

FIGURE 176. 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

Enable the LAPD Transmitter

Initiate the LAPD Message frame transmission.

Specify whether the “outbound” LAPD Message

frame is to be transported via the GC or the NR

byte-fields, within each “outbound” E3 Frame

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).

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.

Complete transmission of LAPD Message frame.

Generate “Completion of Transmission of LAPD

Message frame Interrupt.

Halt Transmission for an “indefinite period.

Wait until the user initiates “LAPD Message

Frame Transmission” again.

Configure the LAPD Transmitter to transmit

LAPD Message Frame Only Once

XRT7250 Datasheet(PDF) 391 Page - Exar Corporation

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