V.23 Compatible Modem

FX604

© 1996 Consumer Microcircuits Limited

6

D/604/3

1.5.2

Mode Control Logic

The FX604's operating mode is determined by the logic levels applied to the M0 and M1 input pins:

M1

M0

Rx Mode

Tx Mode

Data Retime[1]

0

0

1200bits/sec

75bits/sec

Rx

0

1

off

1200bits/sec

Tx

1

0

1200bits/sec

off

Rx

1

1

'Zero-Power'

-

[1] If enabled.

In the 'Zero-Power' mode, power is removed from all internal circuitry. When leaving 'Zero-Power' mode there

must be a delay of 20ms before any Tx data is passed to, or Rx data read from, the device to allow the bias

level, filters and oscillator to stabilise. On applying power to the device the mode must be set to 'ZP', i.e.

1.5.3

Rx Input Amplifier

This amplifier is used to adjust the signal received to the correct amplitude for the FSK receiver and Energy

Detect circuits (see section 1.6.1).

1.5.4

Receive Filter and Equaliser

Is used to attenuate out of band noise and interfering signals, especially the locally generated 75bits/sec

transmit tones which might otherwise reach the 1200bits/sec FSK Demodulator and Energy Detector circuits.

This block also includes a switchable equaliser section. When the RXEQ pin is low the overall group delay of

the receive filter is flat over the 1200bits/sec frequency range. If the RXEQ pin is high the receive filter's typical

overall group delay will be as shown in Figure 3.

Figure 3 Rx Equaliser Group Delay (RXEQ = '1') with respect to 1700Hz