XR72L52 Datasheet, PDF (440/480 Pages) Exar Corporation – TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L52
REV. 1.0.1
xr
TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
If the LAPD Receiver detects an error in the FCS value, then it will set the RxFCS Error bit-field to “1”. Con-
versely, if the LAPD Receiver does not detect an error in the FCS value, the 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 oc-
tet # 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
Rx ABORT
BIT 5
BIT 4
RxLAPDType[1:0]
BIT 3
RxCR
Type
BIT 2
RxFCS
Error
BIT 1
End of
Message
BIT 0
Flag
Present
RO
RO
RO
RO
RO
RO
RO
RO
0
0
0
0
1
0
0
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. Recall that from Section 6.2.3.1, that when operating the LAPD Transmitter, the user is
required to write in a byte of a specific value at address 0x8A 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 Re-
ceiver, 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 infor-
mation 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 7
Not Used
BIT 6
Rx ABORT
BIT 5
BIT 4
RxLAPDType[1:0]
BIT 3
RxCR
Type
BIT 2
RxFCS
Error
BIT 1
End of
Message
BIT 0
Flag
Present
RO
RO
RO
RO
RO
RO
RO
RO
0
0
0
0
0
0
0
0
Table 83 presents the relationship between the contents of RxLAPDType[1:0] and the type of message re-
ceived by the LAPD Receiver.
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