XRT72L50 Datasheet, PDF (179/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
Information Payload
The Information Payload is the 76 bytes or 82 bytes of data (e.g., the PMDL Message) that the has been
written into the on-chip Transmit LAPD Message buffer (located at addresses 0x8A through 0xDB).
It is important to write in a specific octet value into the first byte position within the Transmit LAPD Message
buffer (located at Address = 0x8A). The value of this octet depends upon the type of LAPD Message frame/
PMDL Message that the user wishes to transmit. Table 21 lists the various types of LAPD Message frames/
PMDL Messages that are supported by the XRT72L50 Framer and the corresponding octet value that must be
written into the first octet position within the Transmit LAPD Message buffer.
TABLE 21: THE LAPD MESSAGE TYPE AND THE CORRESPONDING VALUE OF THE FIRST BYTE WITHIN THE
INFORMATION PAYLOAD
LAPD MESSAGE TYPE
CL Path Identification
IDLE Signal Identification
Test Signal Identification
ITU-T Path Identification
VALUE OF FIRST BYTE, WITHIN
INFORMATION PAYLOAD OF MESSAGE
0x38
0x34
0x32
0x3F
MESSAGE SIZE
76 bytes
76 bytes
76 bytes
82 bytes
Frame Check Sequence Bytes
The 16 bit FCS (Frame Check Sequence) is calculated over the LAPD Message Header and Information
Payload bytes by using the CRC-16 polynomial, x¹⁶ + x¹² + x⁵ + 1.
NOTE: For FCS calculation, Header also includes the starting Flag Sequence byte (0x7E).
Operation of the LAPD Transmitter
If a message is to be transmitted via the LAPD Transmitter, the information portion (or the body) of the
message must be written into the Transmit LAPD Message Buffer located at 0x8A through 0xDB in on-chip
RAM via the Microprocessor Interface. Afterwards, three things must be done:
STEP 1 - Specifying the Length of the LAPD Message
One of two different sizes of LAPD Messages can be transmitted. This is accomplish by writing the appropriate
data to bit 1 within the Tx DS3 LAPD Configuration Register. The bit-format of this register is presented below.
Transmit DS3 LAPD Configuration Register (Address = 0x33)
BIT 7
RO
0
BIT 6
BIT 5
Not Used
RO
RO
0
0
BIT 4
RO
0
BIT 3
Auto
Retransmit
R/W
X
BIT2
Not Used
RO
0
BIT 1
TxLAPD
Msg Length
R/W
X
BIT 0
TxLAPD
Enable
R/W
X
The relationship between the contents of bit-fields 1 and the LAPD Message size is given in Table 22.
TABLE 22: RELATIONSHIP BETWEEN TXLAPD MSG LENGTH AND THE LAPD MESSAGE SIZE
TXLAPD MSG LENGTH
LAPD MESSAGE LENGTH
0
LAPD Message size is 76 bytes
1
LAPD Message size is 82 bytes
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