CMX868 查看數據表（PDF） - MX-COM Inc
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CMX868 Datasheet PDF : 43 Pages
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Low Power V.22bis Modem
11
CMX868 Advance Information
In FSK Start-Stop modes, data written to $E4 will be transmitted with a 12.5% reduction in the length of the
Stop bit at the end of that character.
In all Synchronous Data modes data written to $E4 will be treated as though it had been written to $E3.
The underspeed transmission requirement of V.14 is automatically met by the CMX868 as in Start-Stop mode
it automatically inserts extra Stop bit(s) if it has to wait for new data to be loaded into the C-BUS Tx Data
Register.
The optional V.22/V.22bis compatible data scrambler can be programmed to invert the next input bit in the
event of 64 consecutive ones appearing at its input. It uses the generating polynomial:
1 x 14 x 17
4.2 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.22bis 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:
600bps V.22 signals are transmitted as a +90q carrier phase change for a ‘0’ bit, +270q for ‘1’.
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 first of the pair)
00
01
11
10
Phase change
+90q
0q
+270q
+180q
For V.22bis 2400bps QAM the transmit data stream is divided into groups of 4 consecutive data bits. The first
two bits of each group are encoded as a phase quadrant change and the last two bits define one of four
elements within a quadrant:
First two bits of group
(left-hand bit is the first of the pair)
00
01
11
10
Phase quadrant change
+90q (e.g. quadrant 1 to 2)
0q (no change of quadrant)
+270q (e.g. quadrant 1 to 4)
+180q (e.g. quadrant 1 to 3)
Q
11
01
10
11
Phase Quadrant 2 Phase Quadrant 1
10
00
00
01
I
01
00
00
10
Phase Quadrant 3 Phase Quadrant 4
11
10
01
11
Figure 8: V.22 bis Signal Constellation
¤2001 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 20480205.007
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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