XRT72L50
á
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
169
The Mechanics of Transmitting a New LAPD Message
As mentioned above, after the LAPD Transmitter has been enabled, and commanded to transmit the message,
residing in the Transmit LAPD Message buffer, it will continue to transmit this message at one-second intervals.
If another (e.g., different) PMDL message is to be transmitted to the Remote LAPD Receiver, the new message
will have to be written into the Transmit LAPD Message buffer, via the Microprocessor Interface section of the
Framer. However, care must be taken when writing in this new message. If this message is written into the
Transmit LAPD Message buffer at the wrong time (with respect to these one-second transmissions), the user's
action could interfere with these transmissions, thereby causing the LAPD Transmitter to transmit a corrupted
message to the Remote LAPD Receiver. In order to avoid this problem, while writing the new message into the
Transmit LAPD Message buffer, the following should be done:
1. Configure the Framer to automatically reset activated interrupts
This can be done by writing a "1" into Bit 3 of the Framer Operating Mode Register, as depicted below.
FIGURE 51. FLOW CHART DEPICT HOW TO USE THE LAPD TRANSMITTER
E n a b le th e L AP D T ra n s m itte r fo r
T ra n sm is sion
T his is do ne b y w riting the value 0 000 0xx 1b
in to th e T x D S 3 LA P D C onfigura tion R egis ter.
(xx dictates the L A P D M es sag e Le ngth )
LA P D T ran sm itter ins erts F ram e H e ade r
oc tets in front of th e us er pa yload.
Is
M e ss age
T ran sm iss ion
Com p lete
?
Are 5
C ons ec utive
"O nes " dete cted
be tw een the
start an d en d flag
se que nce
?
ST A R T
NO
YES
NO
W rite in D a ta L in k In fo rm a tio n
T his is ac co m plish ed b y w riting the info rm ation
to be tra nsm itte d via th e LA P D T ra nsm itte r to
lo carions , 0x 8A thru 0x D B in th e F ram er
ad dres s s pac e.
W rite th e H ead e r in 0x 86 to 0 x89
Ins ert a "0" after
the string of 5
co nse cutive "1's "
LA P D T ran sm itter a ppe nds a F lag S e quen ce
T railer o cte t to the e nd o f the LA P D
M e ss age after the 1 6 bit F C S .
LA P D T ran sm itter c om pute s th e 16 bit F C S
(a CRC-16 valu e) over th e Head er an d
P ayloa d bytes an d in serts it into the LA P D
M e ss age, follow in g the us er paylo ad.
EN D
G enera te In terrup t LA P D T ran sm itter w ill
co ntin ue to tran sm it F lag S e quen ce octe ts.
YES
In itia te T ra n s m is s io n o f L A P D M e s s a g e
T his is do ne b y w riting the value 0 000 10x 0b
in to th e T x D S 3 LA P D S tatu s/In terrup t
R egiste r.
(x indicta tes that the use r can ch oos e to
en able /disab le th e LA P D M ess age T rans fer
C om p lete Inte rrupt)