PacComm TINY-2 - Page 131

HARDWARE
DESCRIPTION
Detailed
Circuit
Description
410
+
Serial
Port:
SIO
(U14)
channel
B
performs
asynchronous
communications
through
either
a
MAX
231
single
chip
CMOS
RS232
transceiver
(U15),
or
at
TTL
levels,
via
an
8-pin
connector
(P3).
RS232
signals
are
interfaced
via
an
“IBM
AT”
style
DE-9P
connector
with
a
DCE
pinout.
Baud
rates
of
300,
1200,
2400,
4800,
9600,
and
19.2k
are
supported.
HDLC:
The
HDLC
function
is
performed
by
SIO
(U14)
channel
A
connected
to
the
modem
disconnect
header
(J5),
and
to
the
onboard
integrated
circuit
modem.
Data
rates
of
300
bauds
to
19.2
kilobauds
are
supported
at
the
header.
The
SIO
HDLC
data
signal
is
converted
from
NRZ
to
NRZ1
in
two
logic
stages
enroute
to
the
modem.
*
Watchdog
Timer:
The
SIO
(U14)
RTSA
(pin
17)
provides
the
signal
that
ultimately
keys
the
radio
transmitter.
When
the
TINY-2/MICRO-2
is
transmitting,
RTSA
goes
tow,
which
is
fed
to
a
Schmidt
trigger
gate
(U6
pin
9)
which
turns
on
the
PTT
LED
and
returns
a
high
signal
to
the
gate
of
a
VN10
(FET)
transistor
(Q2).
This
high
state
on
the
VN10
gate
causes
it
to
conduct
to
ground,
keying
the
transmitter,
and
starts
C26
charging
through
R17.
If
the
RTSA
signal
remains
low
more
than
60
seconds,
C26
reaches
the
Schmict
trigger
voltage,
causing
the
gate
(U6)
to
turn
off
the
PTT
LED
and
the
VN10,
thus
unkeying
the
transmitter.
Modem:
The
TCM
3105
modem
(U16)
employsa
Bell
202
modulator
and
demodulator
in
a
single
CMOS
package.
The
modulator
transmits
either
1200
or
2200
Hz
depending
on
the
state
of
the
sio
TXD
line
(U14
pin
14).
The
modem
tone
output
(U
16
pin
11)
goes
through
a
10K
trimpot
to
allow
amplitude
adjustment
to
achieve
proper
transmitter
deviation.
Since
the
TCM
3105
always
produces
tones
when
powered,
there
is
an
audio
cutoff
circuit
employed
using
a
VN10
FET
(Q3)
and
a
2
2k
resistor
that
swamps
most
transmit
audio
to
ground
when
the
RTSA
line
is
high
(receive).
This
allows
normal
voice
use
ofthe
radio
while
the
TINY-2/MICRO-2
is
connected.
The
demodulator
contains
a
switched
capacitive
filtering
network,
and
is
sensitive
to
audio
levels
down
to
10
mv.
The
receive
bias
adjustment
(R9)
allows
optimizing
the
bias
level
for
a
particular
TCM3105
modem
IC
.
Received
data
is
output
on
U16
pin
8
and
is
routed
through
the
clock
recovery
circuit
before
the
SIO
(U14)
NOTE:
The
on-board
radio
modem
is
only
capable
o'
operation
as
installed.
For
higher
(or
lower)
radio
data
rates,
attach
an
extemal
modem
to
the
J5
modem
disconnect
header.
+
Battery
backed
RAM
(bbRAM):
The
entire
32K
of
RAM
(U8)
is
backed
up
with
a
3
volt
lithium
battery.
The
74HC132
IC
(U3)
is
also
battery
powered
keeping
pin
20
(CE)
of
the
RAM
disabled
(high)
during
periods
of
low
power
to
the
TINY-2/MICRO-2
(typically
7.5
volts
and
below).
FINY-2/MICRO-2
Technical
Ref.,
2
Ed
©
1989,
PacComm,
Inc.