5-10
Functional Description
Portable
PLUS Computer
Modem Power
Supply. The
modem
PCA contains its
own
power supply which is isolated from the
computer's power supply (because
the
modem's power can be off while the computer's power is on).
The
modem
power supply is
turned
on
and
off by the MODEMON
input
from the computer. The
modern power supply voltages
(+5V
and
-5
Vdc) must track one
another
closely: the power supply
is designed so
that
the
-5V
supply tracks within 2% of the
+5V
supply. The positive supply is a
standard dc-to-dc converter using voltage regulator
A3VR1
to provide the reference voltage. The nega-
tive supply is a switching regulator controlled by microprocessor A3U16.
System
Interface
Buffer Logic. A3U1, A3Q1, A3Q2,
and
A3Q11 interface the modem to the com-
puter.
When
the
modem's
power supply is
turned
off, these components protect both the computer
and
the
modem
from damage
due
to component latch-up,
and
any adverse affects of drawing power
through the
input
protection structures of the lCs on the modem PCA. When the modem's power is
on, these components overcome any mismatch between the power supplies of the two systems.
Controller. A3U14
is
the controlling microprocessor for the modem. Its on-board ROM contains the
code that governs the
modem
operation. The main system clock
A3Ull
drives a counter A3U13 which
provides
the
other
two frequencies needed by the system.
The
modem's
operation modes
(FSK,
DPSK, originate, answer, command, data,
and
dial) are con-
trolled by a combination of outputs of the microprocessor. These outputs drive analog switches A3U6
and
A3U9, inputs to the filter lC A3U12
and
the hook-switch A3U17,
and
software-controlled regis-
ters in the modulator
and
demodulator ICs A3U2
and
A3U3.
Transmit and Receive Fiters. The transmit
and
receive filters provide post-modulation
and
pre-demodulation filtering required for the modern to function properly. Both filters make use of a
switched-capacitor filter
IC
A3U12 as well as some discrete active
and
passive filters.
The Transmit Path. The normal transmit
path
is as follows:
TC1
(A3U3, pin 10) is the transmit data
output
of the modulator. It is passed through
an
active filter composed of A3U8A, A3R37,
and
A3C20;
then
ac coupled to the transmit
path
by A3C19. That signal
is
fed to the switched-capacitor
filter "transmit
in'
input
TI
(A3U12, pin
1).
The filtered signal is output from the 'transmit
out'
pin
A3U12, pin 8, fed through an analog switch (A3U9B), which selects between normal data mode
and
tone dial
mode
through the final amplifier/filter stage (A3U8B, A3R7,
and
A3C12),
and
on
out
through the
phone
line interface transformer A3T1.
Local analog loopback capability is provided by the two resistors (A3R17
and
A3R19), which feed the
transmit signal into the recieve
path
via the analog switch A3U9A.
Note: The values
of
A3R17
and
A3R19 have been chosen to simulate a receive signal level of
- 42 dBm, optimizing the testing condition for the demodulation circuits during local analog
loopback.
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