
á
XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
274
N
OTE
:
In
Figure 106
, the unshaded boxes depict the tasks that the user must perform. The shaded boxes present the
resulting tasks that the Transmit HDLC Controller block will perform.
The Mechanics of Transmitting a New LAPD Message frame, if the LAPD Transmitter has been
configured to re-transmit the LAPD Message frame, repeatedly, at One-Second intervals.
If the LAPD Transmitter has been configured to retransmit the LAPD Message frame repeatedly at one-second
intervals, then it will repeatedly transmit the LAPD Message frame to the Remote Terminal Equipment at one
second intervals.
If another (e.g., a different) PMDL Message is to be transmitted to the Remote Terminal Equipment, this new
message will have to be written into the Transmit LAPD Message buffer, via the Microprocessor Interface block
of the Framer IC. However, care must be taken when writing this new PMDL message. If this message is
written into the Transmit LAPD Message buffer at the wrong time (with respect to these One-second LAPD
Message frame transmissions), the user’s action could interfere with these transmissions, thereby causing the
F
IGURE
106. F
LOW
C
HART
D
EPICTING
HOW
TO
USE
THE
LAPD T
RANSMITTER
Enable the LAPD Transmitter for
Transmission
This is done by writing the value 00000xx1b
into the Tx DS3 LAPD Configuration Register.
(xx dictates the LAPD Message Length)
LAPD Transmitter inserts Frame Header
octets in front of the user payload.
Is
Message
Transmission
Complete
Are 5
Consecutive
"Ones" detected
between the
start and end flag
sequence
START
NO
YES
NO
W rite in Data Link Information
This is accomplished by writing the information
to be transmitted via the LAPD Transmitter to
locarions, 0x8A thru 0xDB in the Framer
address space.
W rite the Header in 0x86 to 0x89
Insert a "0" after
the string of 5
consecutive "1's"
LAPD Transmitter appends a Flag Sequence
Trailer octet to the end of the LAPD
Message after the 16 bit FCS.
LAPD Transmitter computes the 16 bit FCS
(a CRC-16 value) over the Header and
Payload bytes and inserts it into the LAPD
Message, following the user payload.
END
Generate Interrupt LAPD Transmitter will
continue to transmit Flag Sequence octets.
YES
Initiate Transmission of LAPD Message
This is done by writing the value 000010x0b
into the Tx DS3 LAPD Status/Interrupt
Register.
(x indictates that the user can choose to
enable/disable the LAPD Message Transfer
Complete Interrupt)