DA9191A.000
July 31, 1997
FUNCTIONS
◆ Data Reception in Narrow Band mode
Forward voice channel data format for narrow band.
DSAT
24
Sync word
30
Data
40
DSAT
24
Sync word = 011001010110101001100110100110
DSAT = Digital Supervisory Audio Tone is one of seven 24-bit digital sequences added to the voice transmission.
DSAT is transmitted at 200 NRZ bits/second. The following is a list of the seven DSAT sequences.
DSAT sequence
0
1
2
3
4
5
6
2556CBHEX
255B2BHEX
256A9BHEX
25AD4DHEX
26AB2BHEX
26B2ADHEX
2669ABHEX
The 40-bit long data sequence is generated at a 100 Manchester bits/second rate. The data sequence contains 28
bits of data and 12 parity bits.
The incoming data is captured by two 8-bit shift
registers. When one shift register is full the RXWRD
flag is set and an interrupt is generated. The
captured data must be read by the micro controller
within 20ms after the interrupt. Meanwhile, the other
shift register is being filled and when it is full a new
interrupt is generated. The shift registers are clocked
in at 400 Hz. Two samples of each state of both the
200 NRZ bits/second data and the 100 Manchester
bits/second data are loaded into the registers. Note
that there are as many transitions in the 200 NRZ
bits/second data as in the 100 Manchester
bits/second data. The micro controller will then be
used to filter the digital bit sequence and detect the
DSAT, SYNC WORD and DATA out of the bit
stream. The DATA must be checked with following
algorithm:
G(x) = x12 + x10 + x8 + x5 + x4 + x3 + x0
400 Hz
8-bit
SCL
shift register
C1
STxD
Rx
SCL
8-bit
400 Hz
shift register
14
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