2070-9x User’s Manual
Page 12
THEORY OF OPERATION
Dialup Modem:
The 2070-9xx provides a Hayes
compatible dialup smart modem
interface to the 2070 controller.
Power to the board can be switched
on and off from a front panel SPST
switch. This switch, when on,
provides a current path for the quad
SPST relay that connects/disconnects
the controller power supply to the
2070-9.
Isolation from the controller’s power
supply is provided by a encapsulated
transformer coupled DC to DC
converter which provides the
necessary power for both the smart
modem and the FSK Expansion Board.
Isolation from the 2070 controllers I/O
bus is provided by the use of opto-
isolaters that link the signals from the
controller to the smart modem while
isolating power and ground.
The RS-485 signals are first converted
to TTL using single supply line
drivers. The signals are then fed to
the opto-isolators.
The ouputs of the opto-isolaters are
fed to a PLD.
The functionality of the “Programable
Logic Device” is “created” by the GDI
engineering staff not by the
manufacturer of the part. This makes
it possible to make the modem
configurable for different modes of
operation without the need for dozens
of costly descrete components,
switches, and jumpers. It also
provides a means for making rapid
changes to the functionality without
increasing the cost to the user.
There are four switches connected to
the PLD which configures all modes
for the board. This includes RTS/CTS
handshaking modes, RS-485
inversion, and DB9 configuration.
The PLD then routes the signals to the
correct outputs. The signals from SP1
are always routed to the smart modem
module. The signals from SP2 can be
routed to the front panel DB9. But,
normally, the DB9 is connected to the
smart modem which provides a means
to manually configure it using the AT
command set.
However, the smart modem is
typically configured “on the fly” by the
controller via SP1 using an AT
command string.
The signals are then modulated/De-
Modulated by the smart modem and
transmitted/Received out/in the RJ11
connecter after first passing through
the line filters. The line filters filter out
and noise from the telephone system
that might create signal degradation
and cause errors.
The logic state of the signals are
displayed on the front panel useing
LEDs. When the LED is lit, the signal
is active. Each status signal is
buffered by an N-Channel MOSFET
transistior which provides the
necessary current for the LED.
The speaker is driven by a monolithic
700-mW low-voltage power amplifier
which is connected directly to the
smart modem.
PRELIMINARY
