VISUAL
TECHNOLOGY
INCORPORATED,
RAILROAD
AVENUE,
DUNDEE
PARK,
ANDOVER,
MA
01810
5. INTERFACES
5.1 CODE
The
VISUAL
200
terminal communicates using the ASCII 7
bit
code format. Communica-
tions
is
in the start-stop asynchronous serial mode.
5.2
ASYNCHRONOUS
DATA
,
Transmitted and received data
is
formatted
into
a 10
or
an
11
bit
word
containing a start
bit,
7 ASCII data bits, a
parity
bit
and one
or
two
stop bits. The start
bit
is
always a spacing
(zero)
bit,
followed
by
the 7 data bits
with
the least significant
bit
first. The single
parity
bit
is
next
and
can
be
odd, even, mark (always a one)
or
space
(always a zero).
Each
charac-
ter
ends
with
a stop
bit
which
has
a mark
polarity.
At
110 baud the terminal selects
two
stop bits, and
at
all
other
speeds
one stop
bit
is
used.
Received data
is
monitored
to
detect
parity
errors and framing errors.
If
the terminal
has
Even
parity
selected and the received character
parity
is
odd,
or
when Odd
parity
is
selected
and the received character
is
even
a
parity
error symbol (p
E)
is
displayed in place
of
the
received character. Framing errors
are
detected
by
determining the number
of
stop bits
received
as
compared
to
the number selected
by
the baud rate. Framing errors display
as
the
parity
error
symbol.
5.3
FULL/HALF
DUPLEX
5.3.1 Full Duplex
Full duplex communications
imply
that
data
can
be
transmitted and received in
both
direc-
tions simultaneously. The
VISUAL
200 requires
that
full
duplex operation
utilize
the
same
baud rate and
parity
in
both
directions.
5.3.2
Half
Duplex
The
VISUAL
200
operating in
half
duplex mode,
can
be
used
with
half
duplex modems
such
as
the 202. Communications
is
in one
direction
at a
time
and must
be
at the
same
baud rate and
parity
in
each
direction.
Upon
striking
the
first
key, the terminal
is
set
to
the
transmit
mode. Care should
be
exercised
at
this
point
as
some modems
can
take up
to
a
half
second before allowing the transmission
of
this character, resulting in lost data.
Waiting
until
the
first
character appears on the screen
will
eliminate the problem.
The end
of
transmission mode
is
determ ined
by
typing
the turn-around code
(ETX,
EOT
or
CR). The terminal
now
enters the receive mode
until
the
computer
sends
its
message
with
the
same
turn-around code.
When the
VISUAL
200 detects the computer's turn-around code, the terminal
is
auto-
matically set
to
the
transmit
mode, thereby reducing the
effect
of
the modem turn-around
time.
The
VISUAL
200
terminal
has
the capability
of
operating
with
a
half
duplex
modem
containing a secondary channel. When secondary channel
is
selected, turn-around
is
accom-
plished in the
same
manner in
addition
to
the capability
of
selecting the
direction.
If
the
terminal
is
in the
transmit
mode and the
computer
wants
to
transmit, normal
half
duplex
operation requires
that
the
computer
wait
until
the terminal
sends
the turn-around code
and
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