XRT72L50 Datasheet, PDF (387/471 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 73: 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 then, 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 three things:
1. Specify the length of LAPD message to be transmitted.
2. Specify which bit-field (within the E3 frame) that the LAPD Message frame is to be transported on (e.g.,
either the GC or the NR byte).
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 - 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 to be transmitted. This octet will ultimately
be used by the Remote Terminal Equipment in order to help it identify the type of LAPD message frame that it
is receiving. Table 73 lists these octets and the corresponding LAPD Message types.
STEP 2 - Write the PMDL Message into the remaining part of the Transmit LAPD Message Buffer.
The user must now write in his/her PMDL Message into the remaining portion of the Transmit LAPD Message
buffer (e.g., addresses 0x8B through 0xDB).
STEP 3 - Specifying the Length of the LAPD Message
One of two different sizes of LAPD Messages can be transmitted, by writing the appropriate data to bit 1 within
the Tx E3 LAPD Configuration Register. The bit-format of this register is presented below.
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