V23 / Bell 202 Modem

CMX624

 2003 CML Microsystems Plc

9

D/624/7

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

x

-

-

1

0

150bps FSK

487Hz

387Hz

1

1

1200bps FSK

2200Hz

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’)

D3

D2

D1

D0

Lower

Frequency (Hz)

Upper

Frequency (Hz)

Keypad

Legend

Single Tone

Frequency (Hz)

0

0

0

0

941

1633

D

1633

0

0

0

1

697

1209

1

1209

0

0

1

0

697

1336

2

1336

0

0

1

1

697

1477

3

1477

0

1

0

0

770

1209

4

1209

0

1

0

1

770

1336

5

1336

0

1

1

0

770

1477

6

1477

0

1

1

1

852

1209

7

1209

1

0

0

0

852

1336

8

852

1

0

0

1

852

1477

9

852

1

0

1

0

941

1336

0

941

1

0

1

1

941

1209

*

941

1

1

0

0

941

1477

#

941

1

1

0

1

697

1633

A

697

1

1

1

0

770

1633

B

770

1

1

1

1

852

1633

C

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.

FSK MODE

Register bit 2

FSK Signal

DTMF Tone

(Low group)

DTMF Tone

(High group)

0 (low level)

-6dB

-5dB

-3dB

1 (high level)

-3dB

-2dB

0dB

0dB = 775mVrms

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.