V23 / Bell 202 Modem
CMX624
Bell 202 mode (bit 7 of SETUP register = ‘1’):
FSK MODE Reg
FSK / DTMF Modulator block output
FSK Signal Frequency
Bit 1
Bit 0
(Bit 7 of TX TONES = ‘0’)
‘0’ (Space)
‘1’ (Mark)
0
1
1
x
0
1
Disabled (o/p held at V / 2)
-
-
DD
150bps FSK
1200bps FSK
487Hz
2200Hz
387Hz
1200Hz
When bit 7 of the TX TONES Register is set to ‘1’, the block generates DTMF tone pairs or single tones
from the DTMF range as shown in the table below. Bit 6 of the TX TONES Register is then used to enable
or disable the block’s output to the Tx filter.
TX DATA Register Bits
0 - 3
DTMF Tone Pairs
(TX TONES Register Bit 4 = ‘0’)
Single Tone
(Bit 4 = ‘1’)
Lower
Upper
Keypad
Single Tone
D3
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
D2
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
D1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
D0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
Frequency (Hz)
Frequency (Hz)
Legend
Frequency (Hz)
941
697
697
697
770
770
770
852
852
852
941
941
941
697
770
852
1633
1209
1336
1477
1209
1336
1477
1209
1336
1477
1336
1209
1477
1633
1633
1633
D
1
2
3
4
5
6
7
8
9
0
*
1633
1209
1336
1477
1209
1336
1477
1209
852
852
941
941
941
#
A
B
C
697
770
852
1.5.7 Transmit Filter
This stage attenuates out of band signals present at the output of the FSK/DTMF modulator and also
includes a programmable 3dB level switch, selected by bit 2 of the FSK MODE Register.
The nominal output levels at the TOP pin when V = 5.0V are as shown below.
DD
FSK MODE
Register bit 2
0 (low level)
1 (high level)
FSK Signal
DTMF Tone
(Low group)
-5dB
DTMF Tone
(High group)
-3dB
-6dB
-3dB
-2dB
0dB
0dB = 775mVrms
These levels are proportional to V , and the actual transmit signal levels present on the 2- or 4-wire line
DD
will depend on the external circuitry as described in Section 1.6.1. Using the external components
recommended in Section 1.6.1 for a nominal FSK transmit level of -9dBm, DTMF tone levels of -8dBm and
-6dBm, then the out of band energy sent to the line will be within the limits shown in Figure 5 for both FSK
and DTMF signals.
2003 CML Microsystems Plc
9
D/624/7
CMLMICRO [ CML MICROCIRCUITS ]
