XRT72L56 Datasheet, PDF (343/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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PRELIMINARY
XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
The LAPD Receiver will indicate the results of this
ror) within the Rx E3 LAPD Status Register, to the ap-
FCS Verification process by setting Bit 2 (RxFCS Er- propriate value as tabulated below.
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
RxABORT
BIT 5
BIT 4
RxLAPDType[1:0]
RO
RO
RO
RO
0
0
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 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.
RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)
BIT 7
Not Used
BIT 6
RxABORT
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 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 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
0
BIT 1
End of
Message
RO
0
BIT 0
Flag
Present
RO
0
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