XRT7250 datasheet(325/463 Pages) EXAR

325 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

306

The LAPD Receiver will indicate the results of this

FCS Verification process by setting Bit 2 (RxFCS Er-

ror) within the Rx E3 LAPD Status Register, to the ap-

propriate value as tabulated below.

If the LAPD Receiver detects an error in the FCS val-

ue, then it will set the RxFCS Error bit-field to “1”.

Conversely, if the LAPD Receiver does not detect an

error in the FCS value, then it will clear the RxFCS

Error bit-field to “0”.

NOTE: The LAPD Receiver will extract and write the PMDL

Message into the Receive LAPD Message buffer indepen-

dent of the results of FCS Verification. Hence, the user is

urged to validate each PMDL Message that is read in from

the Receive LAPD Message buffer, by first checking the

state of this bit-field.

3. Check and Report the State of the “C/R” Bit-field

After receiving the LAPD Message frame, the LAPD

Receiver will check the state of the “C/R” bit-field,

within octet # 2 of the LAPD Message frame header

and will reflect this value in Bit 3 (Rx CR Type) within

the Rx E3 LAPD Status Register, as depicted below.

When this bit-field is “0”, it means that this LAPD

Message frame is originating from a customer instal-

lation. When this bit-field is “1”, it means that this

LAPD Message frame is originating from a network

terminal.

4. Identify the Type of LAPD Message Frame/PMDL

Message

Next, the LAPD Receiver will check the value of the

first octet within the PMDL Message field, of the

LAPD Message frame. When operating the LAPD

Transmitter, the user is required to write in a byte of a

specific value into the first octet position within the

Transmit LAPD Message buffer. The value of this

byte corresponds to the type of LAPD Message

frame/PMDL Message that is to be transmitted to the

Remote LAPD Receiver. This Message-Type Identifi-

cation octet is transported to the Remote LAPD Re-

ceiver, along with the rest of the LAPD frame. From

this Message Type Identification octet, the LAPD Re-

ceiver will know the type of size of the newly received

PMDL Message. The LAPD Receiver will then reflect

this information in Bits 4 and 5 (RxLAPDType[1:0])

within the Rx E3 LAPD Status Register, as depicted

below.

RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

RxABORT

RxLAPDType[1:0]

RxCR

Type

RxFCS

Error

End of

Message

Flag

Present

000

100

RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

RxABORT

RxLAPDType[1:0]

RxCR

Type

RxFCS

Error

End of

Message

Flag

Present

001

000

RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

RxABORT

RxLAPDType[1:0]

RxCR

Type

RxFCS

Error

End of

Message

Flag

Present

000

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