v.23 Compatible Modem
11
MX604 PRELIMINARY INFORMATION
If the data retiming facility is not required, then the CLK input to the MX604 should be kept high at all times.
The asynchronous data to the FSK modulator will then be connected directly to the TXD input pin. This is
illustrated in Figure 10 and will also be the case when transmitting 75bps data which has no retime option.
TXD input :
N-2
N-2
N-1
N-1
N
N
N+1
N+1
N+2
N+2
FSK Modulator input :
Figure 10: FSK Operation without Tx Data Re-timing (CLK always high)
5.
Application
5.1
Line Interface
The signals on the telephone line are not suitable for direct connection to the MX604. A Line Interface circuit
is necessary to:
ꢁ
ꢁ
ꢁ
ꢁ
Provide high voltage and dc isolation
Attenuate the Tx signal present at the Rx input
Provide the low impedance drive necessary for the line
Filter the Tx and Rx signals
C
Z
+
C5
R2
RXIN
A1
LINE
R7
C6
1:1
0V
R4
R5
C7
RXAMPOUT
R6
A
B
R3
TXOUT
A2
V
BIAS
R2
R3
See Notes
See Notes
100kꢃ
C5
C6
C7
ꢄ1%,
ꢄ1%,
ꢄ1%,
22ꢂF
100pF
330pF
ꢄ20%
ꢄ10%
ꢄ10%
R4-R7
Figure 11: Line Interface Circuit
ꢀ1998 MXꢁCOM, INC.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
Tele: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480152.004
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