Rockwell SoniCrafter BT8960 - Manual (Page 27)

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2.0 Functional Description
2.1 Transmit Section
Bt8960
Single-Chip 2B1Q Transceiver
N8960DSB
The bit stream is converted into symbols for the four-level cases as shown in
Table 2-2.
In two-level mode, the magnitude bit is forced to a zero. This forces the sym-
bols to be +3 and –3, as shown in Table 2-3.
The scrambler is essentially a 23-bit-long Linear Feedback Shift Register
(LFSR). The feedback points are programmable for central office and remote ter-
minal applications using the htur_lfsr bit of the Transmitter Modes Register. The
LFSR polynomials for local (HTU-C/LTU) and remote (HTU-R/NTU) unit oper-
ations are:
The scrambler operates differently depending on whether a two-level or four-
level mode is specified. In 2-level scrambled-ones mode, the LFSR is clocked
once-per-symbol; in 4-level mode, the LFSR is clocked twice-per-symbol.
The Transmitter Modes Register can also be used to zero the output of the
transmitter using the transmitter_off control bit.
The Bt8960 can generate isolated pulses to support the testing of pulse tem-
plates. When in the isolated pulse mode, the output consists of a single pulse sur-
rounded by zeros.
NOTE: Zero is not a valid 2B1Q level and only occurs in this special mode or
when the transmitter is off. The repetition rate of the pulses is controlled
by the meter timer. Any of the four 2B1Q levels may be chosen via the
Transmitter Modes Register’s isolated_pulse[1,0] control bits.
Table 2-2. Four-Level Bit-to-Symbol Conversions
First Input Bit
(sign)
Second Input Bit
(magnitude)
Output Symbol
0 0 –3
0 1 –1
1 1 +1
1 0 +3
Table 2-3. Two-Level Bit-to-Symbol Conversions
First Input Bit
(sign)
Second Input Bit
(magnitude)
Output Symbol
0 don’t care –3
1 don’t care +3
local x 23 – x 5 – 1 ⊕ ⊕ ⇒
remote x 23 – x 18 – 1
⊕ ⊕ ⇒