CMX624P4 View Datasheet(PDF) - MX-COM Inc
Part Name
Description
MFG CO.
'CMX624P4' PDF : 25 Pages View PDF
Bell 202 and V.23 Compatible Modem
16
CMX624 Preliminary Information
5.1.1.2 Transmit Gain
In the transmit direction, the level on the 4-wire line is determined by the level at the TX pin, the gain of the
Output Buffer Amplifier, a loss of nominally 6dB due to the line termination resistor R15, and the loss in the
transformer.
The TX pin signal level is proportional to VDD and is also affected by the setting of the Tx output level control
Bit (Bit 2) of the FSK Mode Register.
Assuming that the Tx output level control bit is set to ‘1’ (giving a FSK signal level of -3dB with respect to
775mVRMS at the TX pin when VDD = 5.0V) and that there is 1dB loss in the transformer, then:
Tx
FSK
four
wi re
line
level
=
(−
3
−
6
−
1) +
20
×
log10
2 × R14
R13
+
20
×
log10
VDD
5.0
dBm
For example, to generate a nominal Tx FSK line level of -9dBm, R13 should be 180kΩ when VDD = 5.0V,
falling to 120kΩ at 3.3V.
5.1.2 2-Wire Line Interface
Figure 13 shows an interface circuit suitable for connection to a 600Ω 2-wire line. The circuit also shows how
a relay may be driven from the RLYDRV pin.
Note: When the CMX624 is powered from less than 5.0V, buffer circuitry may be required to drive a 5V
relay.
2-Wire
Line
To Ring
Detect
+ve
Circuit
See 5.2
RXAMPOUT
R12
R11
C11
RX- -
RX+ +
RLYDRV
C14
TX
R16 R17
R13
CMX624
TXIN-
C13
1:1
R15
R14
C12
TX+
VBIAS
TX-
VBIAS
Figure 13: 2-Wire Line Interface Circuit
R11
See text ±1%
R17
100kΩ ±1%
R12
100kΩ ±1%
C11
220pF ±20%
R13
See text ±1%
C12
330pF ±20%
R14
100kΩ ±1%
C13
10nF ±20%
R15
600Ω ±1%
C14
100nF ±20%
R16
120kΩ ±1%
Tolerances for Resistors and Capacitors are as indicated unless otherwise stated.
Table 11: 2-Wire Line Interface Circuit components
Note: The relay circuit, AC and DC loads and line protection are not shown for clarity.
This circuit includes a 2-wire to 4-wire hybrid circuit, formed by R11, R15, R16, R17, C13 and the impedance
of the line itself, which ensures that the modem receive input and transmit output paths are both coupled
efficiently to the line, while minimizing coupling from the modem’s transmit signal into the receive input.
The values of R11 and R13 should be calculated in the same way as for the 4-wire interface circuit of
Figure 12.
©1999 MX-COM, Inc.
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480180.108
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
All trademarks and service marks are held by their respective companies.
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