XRT72L50 Datasheet, PDF (283/471 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
TABLE 49: 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 the user wishes to transmit a message via the LAPD Transmitter, the information portion (or the body) of the
message must be written into the Transmit LAPD Message Buffer, which is located at 0x8A through 0xDB in
on-chip RAM via the Microprocessor Interface. Afterwards, the user must do five things:
1. Configure the source of the N bit (within each outbound E3 frame, to be the LAPD Transmitter.
2. Specify the length of LAPD message to be transmitted.
3. Specify whether the LAPD Transmitter should transmit this LAPD Message frame only once, or an indefi-
nite number of times at One-Second intervals.
4. Enable the LAPD Transmitter.
5. Initiate the Transmission of the PMDL Message.
Each of these steps will be discussed in detail.
STEP 1 - Configure the source of the N bit (within each outbound E3 frame, to be the LAPD Transmitter.
This is accomplished by writing the appropriate data into the TxNSourceSel[1:0] bit-fields, within the TxE3
Configuration Register, as illustrated below.
TxE3 Configuration Register (Address = 0x30)
BIT 7
Tx
BIP-4
Enable
R/W
0
BIT 6
BIT 5
TxASourceSel[1:0]
R/W
R/W
0
0
BIT 4
BIT 3
TxNSourceSel[1:0]
R/W
R/W
0
0
BIT 2
Tx AIS
Enable
R/W
0
BIT 1
Tx LOS
Enable
R/W
0
BIT 0
Tx FAS
Source
Select
R/W
0
Setting TxNSourceSel[1:0] to “10” configures the Transmit E3 Framer block to use the LAPD Transmitter as the
data source for the N bits. Hence, the N bit, (within each outbound E3 frame) is now carrying LAPD Messages
to the remote terminal equipment.
STEP 2 - Specify the type of LAPD Message frame to be Transmitted (within the Transmit LAPD
Message Buffer)
The user must write in a specific octet value into the first octet position within the Transmit LAPD Buffer (e.g., at
Address Location 0x8A). This octet is referred to as the LAPD Message Frame ID octet. The value of this
octet must correspond to the type of LAPD Message frame that is desired to be transmitted. This octet will
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