Service manual
UART
R
ee,iv,
d.
"
RS·232C Interflce
At the same time the
RS
·
23
2C interface
is
se
lected with a
h
ig
h PRIME state, V
OO
is
supplied from the
high
side of
the
RS
·232C interface.
Th
e S
IO
interface
is
selected with a
lo
w
PRIME
state and
V
DD
is turned off.
DUring
the system on and reset,
PRIME
is
at low.
PRIM
E:-~~;C
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______
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III
OTR
101
VOO
U h
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• _
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T)(O
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RXO
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UIJIllIID
ill
Whe
n PRIME
is
at a low state, the
RS
·232C interface
outputs either
in
a h
ig
h impedance or a
low
state (non·
active).
®
When
PRIME
is
at a low state, the SIO interface I
/O
signal
s,
SOF and
ROF
, are
in
an opposite polarity with
the UART input/o
utput
signa
ls,
TXO
and RXO.
Th
e
start bit is h
ig
h and the stop bit is low. So, both are
in
a
lo
w state when no data are sent or recei
ve
d (UART
TXO and RXO are at a high level).
®
When
PRIME
is
at a high state, both SOF and
RO
F are
at a
low
level
and non·active (stop bit
I.
<f)
When
PRIME
is
at a h
ig
h state,
TXO
and R
XO
of the
RS-232C interface are opposite
in
their p
ol
arity as are
those of TXO and R
XO
of the UART.
®
When
PRIME goes h
ig
h, V
OD
is
activated (RS-232C
interface
high
side voltage),
(
1)
Input signa ls a
re
received
by
the transistor a
nd
are
output
through the open collector and pulled up
to
VCC
using a resistor,
as
shown
in
the hybrid
Ie BX726
9W
. A diode
is
inserted across the base and
emitter of the
in
put wh
ic
h will bring the signal below
the
GNO
level
(stop bit, etc.) and make
it
assume to
be at the
GNO
le
ve
l.
There
fo
r
e,
the input signal is
con
ve
rted
in
the
hyb
rid Ie
to
be handled
as
a
logiC
signa
l.
-----,
r--
- - -
Vee
---
~
--------
G
ND
(2)
On
t
he
other hand, the output signal
is
output
through the circu it shown below (hybr
id
I
e).
"
C 1 ( l
SOPF
)
Vo>
.4
N45S4(C) ( 2.7KO)
C2 (O.
i1f)
Wh
ile
the input
level
is
CMOS
compatible (G-4.7
V)
,
the
output
is
co
nv
ert
ed
to
the V
DO
-V
EE
le
ve
l .
The figure below illustrates this.
'"
r-----
-,
GND
----.J
'--
_______________
__
'''j\
Va:
.--
.-. --··----------·-
---V11lH
CND
-
.--
.
..
---------u
v.
--u
-u-u--
..
------------------
--VT
H~
'"
'·'
l
'"
GND
'"
'--
__________
..J
CD
When
the input
(a)
is
low (GNOI.
the
level
(b)
is
below
GN
D and
is
assu
med by the MN4584 to be at a l
ow
l
eve
l.
The MN4584
IC
is
a Schmi
tt
inverter
to
which V
OO
and V
EE
is
supplied. Th
is
Ie
has
a hysteres
is
against
input.
® The R5-232C
in
terface inpu
t/
output
signals-CTS,
DSR, CD, and CI- are input to the UART (opposite
polarity), regardless of the state of PRIME
(high
or
l
ow)
.
OutpUt
7·7·1.
RS
-232C Interf.ce
Signa
l
Although si
gn
al
s of this interfa
ce
conform
to
the
EIA
standards, they are used for controls
that
differ
in
so
me
ways from the RS·232C interface in general.
- 1
4-
V[
[ VTHL
GND
VTHH VOD I
nput
,
..
'---------------------------------------------------------------------------------
------------------
~