MC145442 Datasheet, PDF (6/12 Pages) Motorola, Inc – SINGLE-CHIP 300-BAUD MODEM

GENERAL DESCRIPTION
The MC145442 and MC145443 are full–duplex low–speed
modems. They provide a 300–baud FSK signal for bidirec-
tional data transmission over the telephone network. They
can be operated in one of four basic configurations as deter-
mined by the state of MODE (Pin 13) and LB (Pin 2). The
normal (non–loopback) and self test (loopback) modes in
both answer and originate modes will be discussed.
For an originate or channel 1 mode, a logic high level is
placed on MODE (Pin 13) and a logic low level is placed on
LB (Pin 2). In this mode, transmit data is input on TxD, where
it is converted to a FSK signal and routed through a low–
band band–pass filter. The filtered output signal is then buff-
ered by the Tx op–amp line driver, which is capable of driving
– 9 dBm onto a 600 Ω line. The receive signal is connected
through a hybrid duplexer circuit on Pins 15 and 16, RxA2
and RxA1. The signal then passes through the anti–aliasing
filter, the sample–and–hold circuit, is switched into the high–
band band–pass filter, and then switched into the ac amplifier
circuit. The output of the ac amplifier circuit is routed to the
demodulator circuit and demodulated. The resulting digital
data is then output through RxD (Pin 5). The carrier detect
circuit receives its signal from the output of the ac amplifier
circuit and goes low when the incoming signal is detected
(see Figure 4a).
In the answer or channel 2 mode, a logic low level is
placed on MODE (Pin 13) and on LB (Pin 2). In this mode,
the data follows the same path except the FSK signal is
routed to the high–band band–pass filter and the sample–
and–hold signal is routed through the low–band band–pass
filter. (See Figure 4b.)
In the analog loopback originate or channel 1 mode, a logic
high level is placed on MODE (Pin 13) and on LB (Pin 2).
This mode is used for a self check of the modulator, demodu-
lator, and low–band pass–band filter circuit. The modulator
side is configured exactly like the originate mode above ex-
cept the line driver output (TxA, Pin 17) is switched to the
negative input of the hybrid op–amp. The RxA2 input pin is
open in this mode and the non–inverting input of the hybrid
circuit is connected to VAG. The sample–and–hold output by-
passes the filter so that the demodulator receives the modu-
lated Tx data (see Figure 4c). This test checks all internal
device components except the high–band band–pass filter,
which can be checked in the answer or channel 2 mode test.
In the analog loopback or channel 2 mode, a logic low level
is placed on MODE (Pin 13) and a logic high level on LB
(Pin 2). This mode is used for a self check of the modulator,
demodulator, and high–band pass–band filter circuit. This
configuration is exactly like the originate loopback mode
above, except the signal is routed through the high–band
pass–band filter (see Figure 4d).
MC145442•MC145443
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MOTOROLA