Bell 202 Compatible Modem
FX614
1.5.2
Mode Control Logic
The FX614's operating mode is determined by the logic levels applied to the M0 and M1 input pins:
M1
0
0
1
1
M0
0
1
0
1
Rx Mode
Tx Mode
1200bits/sec
150bits/sec
Off
1200bits/sec
1200bits/sec
Off / 5bits/sec
'Zero-Power'
Data Retime
[1]
Rx
Tx
Rx
-
[1] If enabled.
In the 'Zero-Power' (ZP) mode, power is removed from all internal circuitry. When leaving the 'ZP' 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. M0 =
'1', M1 = '1', until V
DD
has stabilised.
1.5.3
Rx Input Amplifier
This amplifier is used to adjust the received signal 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 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') wrt 1700Hz
©
1997 Consumer Microcircuits Limited
6
D/614/4
CMLMICRO [ CML MICROCIRCUITS ]
