5·22
Functional
Description
Portable
PLUS
Computer
5.15.10
RS-232-C Supply Voltages
The RS-232-C interface
standard
requires two
output
voltage levels:
one
above
+5V
and
one below
-5V.
The LTl032 driver chip used in the Portable PLUS requires at least
+5.3V
and
-5.9V
supplies
at worst-case
load
currents to guarantee meeting the
standard's
voltage limits. The worst-case load
currents at
minimum
supply levels are 6 rnA for
both
the positive
and
negative supplies.
The
function
of
the
RS-232-C supplies circuitry
is
to convert the battery voltage
VBAT
to
+5.3
Vdc
minimum
at 6
mA
minimum
and
-5.9
Vdc maximum at 6 mA minimum. The 'conversion" process
for the positive supply is trivial, since
VBAT
itself meets the power requirements. This supply
output
is
called V + 232,
and
is
provided to the RS-232-C driver chip through a simple jumper wire from
VBAT.
The
negative supply
output
is
called
V-232,
and
it requires more circuitry.
SLEEP
and
RS2320N
are the two control inputs provided by the PPU for the RS-232 supplies.
When
SLEEP
is
set to logic 1 (sleep mode), the RS-232 supplies
draw
practically zero battery current;
if
SLEEP is set to logic 0 (awake mode), operation of the RS-232 supplies
depends
on
the
other
control
input,
RS2320N.
When
RS2320N
is set to logic 0, the RS-232 supplies
draw
zero battery current.
If
RS2320N
is
set to logic 1 at the same time that SLEEP set
is
to
logiC
0, the RS-232 supplies
turn
on,
supplying current to
the
RS-232-C driver chip
and
drawing current from
VBAT.
The
motherboard
PCA contains holes for optional circuitry within the RS-232 supplies. These compo-
nents
allow for substitution
of
a 1488-type RS-232-C driver for the LTl032. The 1488 driver requires
positive
and
negative supplies
of
higher voltage
than
those required by the LTl032;
the
positive
sup-
ply voltage required for the 1488 is greater
than
VBAT.
The optional components enable the RS-232
supplies circuitry to generate the higher positive voltage.
5.15.11
Voltage Doublers
A voltage doubler circuit is used to generate V - 232. When the optional components are loaded,
V + 232 is also generated
by
a voltage doubler circuit. (Normally, V + 232
is
provided
by
a jumper wire
to
VBAT.)
The
voltage doublers are a series
of
diodes
and
capacitors that convert the
output
of
an
11-
kHz
oscillator to the required supply voltages.
In
the following descriptions of the voltage doublers'
operations it
is
assumed
that
the forward voltage drop for standard diodes
is
O.7V
and
for schottky
diodes
0.
2V.
The
oscillator
output
is the tied-together drains
of
A2Q79
and
A2Q80. This oscillator swings between
VBAT
and
OV.
The oscillator output waveform
is
ac coupled through A2C67
and
clamped to
O.7V
maximum
by
A2CR58. The resulting waveform at A2CR58 anode swings between
O.7V
and
(-VBAT+0.7V).
A2CR59
and
and
A2C69 form a dc rectifier which produces a dc voltage
of
(-
VBAT
+ 0.9V) from the waveform
at
A2CR58 anode. A2C68 ac couples the waveform at A2CR58
anode
to
the
A2CR60 anode, which clamps the voltage to
(-
VBAT
+ 1.1
V)
maximum. Thus, the
waveform
at
A2CR60 anode swings between
(-VBAT+1.1V)
and
(-2VBAT+1.1V).
A2CR61
and
A2C70 form a dc rectifier that produces a dc voltage of
(-
2VBAT
+ 1.3V) from the A2CR60 anode
waveform. This dc rectified voltage is
V - 232.
The
voltage doubler for V + 232 operates in a similar fashion
when
it is loaded in
the
printed circuit
board. The
output
of
the oscillator is connected to a dc rectifier formed
by
A2CR55
and
A2C65,
producing a dc voltage
of
(VBAT
-0.7V)
. A2C64 ac couples the oscillator
output
waveform
to the
cathode
of
A2CR56, which clamps the voltage to (VBAT-1.4V) minimum. The resulting waveform at
the cathode
of
A2CR56 swings between
(2VBAT
-1.4V)
and
(VBAT
-1.4V).
A2CR57
and
A2C66 rec-
tify this
waveform
to produce
(2VBAT
- 2.1
V),
which
is
V + 232.
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