XRT72L50 Datasheet, PDF (326/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
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However, if the FCS value does not match the locally-computed CRC-16 value, then the LAPD Receiver will
conclude that this LAPD Message frame is erred.
The LAPD Receiver will indicate the results of this FCS Verification process by setting Bit 2 (RxFCS Error)
within the Rx E3 LAPD Status Register, to the appropriate value as tabulated below.
RxE3 LAPD Status Register (Address = 0x19)
BIT 7
Not Used
BIT 6
RxABORT
RO
RO
0
0
BIT 5
BIT 4
RxLAPDType[1:0]
RO
RO
0
0
BIT 3
RxCR
Type
RO
0
BIT 2
RxFCS
Error
RO
1
BIT 1
End of
Message
RO
0
BIT 0
Flag
Present
RO
0
If the LAPD Receiver detects an error in the FCS value, 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 independent
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.
RxE3 LAPD Status Register (Address = 0x19)
BIT 7
Not Used
BIT 6
RxABORT
RO
RO
0
0
BIT 5
BIT 4
RxLAPDType[1:0]
RO
RO
0
0
BIT 3
RxCR
Type
RO
1
BIT 2
RxFCS
Error
RO
0
BIT 1
End of
Message
RO
0
BIT 0
Flag
Present
RO
0
When this bit-field is “0”, it means that this LAPD Message frame is originating from a customer installation.
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 information payload field, of the
LAPD Message frame. When operating the LAPD Transmitter, the user is required to write in a byte of a
specific value at address ox8A 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 Identification octet is transported to the Remote LAPD Receiver, along with the rest of the
LAPD frame. From this Message Type Identification octet, the LAPD Receiver 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.
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