2070-9x User’s Manual
Page 13
The ouput signals from SP2 are
directly connected to a 50 pin socket
which the FSK Expansion Board plugs
in to. The Expansion Board provides
all signal isolation to both the
controller and the line.
FSK Option
The Space Frequency is used to
transmit a digital “1” and the Mark
frequency is used to transmit a digital
“0.” At the end of the data
transmission, the Soft Carrier
frequency is transmitted for 10 ms.
The Soft Carrier is to causes the
receiving modems receiver section to
shut down. This prevents noise from
getting though the Receiver.
The Receiver section of the modem
amplifies the signal it receives
through the coupling transformer and
filters it so that only frequencies in the
correct band will get passed on to the
demodulator. Signals as low as 6
milliVolts will produce RS-232 level
signals (
±
12V) at the receiver output.
Further noise prevention is
accomplished through the use of
special filter circuits which are used to
enable the Carrier Detect Signal.
There must be 8ms of the Mark
frequency present on the input for
Carrier Detect to activate.. When the
Carrier Detect signal goes active, the
Receiver will be enabled. That Mark
signal will also have to be above a
certain threshold before the Mark
detect circuit will be activated.
The CTS delay circuitry will cause the
Mark frequency to be present on the
receivers input for 12ms. When RTS
(Request to Send) is asserted, the
modem starts transmitting the Mark
frequency. The Host Computer or
Controller will not send data until CTS
is asserted.
Data Carrier Detect (DCD) controls the
output of the Receiver and can be
used to key the Controller or Host
Computer. The no-signal state of the
RXD signal is -12vdc or “Mark.”
At the end of a data transmission the
Controller or Host Computer de-
asserts RTS which causes the modem
to transmit the Soft Carrier frequency
for 10ms.
At the receiving end, the “in-band”,
“out-of-band”, and “soft carrier” filters
all cause the receiver to shut down.
Without these special filters, the
receiver could respond to the
transition from soft carrier to no signal
and could create a false start bit.
The Block diagram of the FSK modem
shows how the different sections of
the modem are interconnected.
PRELIMINARY
