V.22 bis Modem with AT Commands
CMX866
The optional V.22/V.22 bis compatible data scrambler is enabled in V.22 and V.22bis handshake and
data transfer modes. Its function is to invert the next input bit in the event of 64 consecutive ones
appearing at its input. The scrambler uses the generating polynomial:
-14
-17
1 + x
+ x
1.5.6 FSK and QAM/DPSK Modulators
Serial data from the USART is fed via the optional scrambler to the FSK modulator if V.21, V.23, Bell 103
or Bell 202 mode has been selected or to the QAM/DPSK modulator for V.22, V.22 bis and Bell 212A
modes.
The FSK modulator generates one of two frequencies according to the transmit mode and the value of
current transmit data bit.
The QAM/DPSK modulator generates a carrier of 1200Hz (Low Band, Calling modem) or 2400Hz (High
Band, Answering modem) which is modulated at 600 symbols/sec as described below:
For V.22 and Bell 212A 1200bps DPSK the transmit data stream is divided into groups of two
consecutive bits (dibits) which are encoded as a carrier phase change:
Dibit
(left-hand bit is the
Phase change
first of the pair)
00
01
11
10
+90°
0°
+270°
+180°
ã 2004 CML Microsystems Plc
31
D/866/4
CMLMICRO [ CML MICROCIRCUITS ]
