User`s guide
Vector
ZCB
Single
Board COmputer
It
is
~rtant
to
note
here
that
in
the
RS-232C
protocol,
any
given
line
has
one
name,
regardless
of
your
point
of
view.
For
example,
although
a
m:rlem, which
is
a
kind
of
Data
Comnunications
Equipment
("
DCE"
),
rece
i
ves
its
data
on
line
2,
line
2
is
still
called
Transmitted
Data.
Notice
that
the
names
of
the
lines
are
oore
meaningful
if
you
look
at
them
from
the
point
of
view
of
a
terminal
at
the
other
end
of
the
line,
that
is
fran
the
point
of
view
of
the
Data
Terminal
Equiprent
("orE").
According
to
RS-232C,
Data
Terminal
Equi:!;XTent
transmits
on
the
Transmitted
Data
line
and
receives
on
the
Received
Data
line.
The
ZCB
board
is
wired
to
behave
as
if
it
were
Data
Communications
Equi:!;XTent.
'!his
is
reasonable
because
the
board
is
nost
CCIIllIOnly
connected
to
printers,
CRT's,
and
other
kinds
of
Data
Terminal
Equipment.
This
explains
why
RS-232C
line
2 -
Transmitted
Data
-
is
connected
to
the
8251
RxD
(Receive
Data)
line,
and
why
line
3 - Received Data -
is
connected
to
the
8251
TxD
(Transmit
Data)
line,
in
the
factory
configuration.
If
you
want
to
connect
the
board
to
a nodem,
that
is,
you want
the
board
to
behave
like
Data
Teoninal
Equiprent,
you
have
to
reverse
lines
2
and
3
somewhere.
The
result
would
be
that
RS-232C
Transmitted
Data
is
connected
to
the
8251
TxD
pin
and
RS-232C Received
Data
is
connected
to
the
8251
RxD
pin,
whidl
is
exactly
the
w~
Data
Terminal
Equi~t
should
behave.
- .
To
send
handshakiIJ9
signals:
rata
Ccmnunication
Equipment
connected
to
the
bOard
may
requlre
a +12
VOC
on
some
other
line
(I1l)St
often
line
4
or
line
20).
Similarly
I a
piece
of
rata
Tenninal
Equipment
connected
to
the
board
may
require
+12
VOC
on
some
line
other
than
5
or
6,
though
this
is
rare.
You
can
supply
+12
VOC
as
a
constant
enabling
signal,
by
connecting
the
desired
RS-232C
line(s)
to
+12
VOC
through
a
pull-up
resistor.
Alternately,
you
can
allow
your
software
to
control
the
+12
VOC
enabling
signal(s),
if
desired.
The
8251
has
two
pins
that
can
be
controlled
by
software.
'!hey
are
pin
23
(RI'S)
and
pin
24 (DTR).
Software
controls
the
status
of
these
pins
by
outputting
a canmand
instruction
byte
to
the
8251,
as
described
in
the
8251
references
mentioned
in
the
Perspective
section
of
this
manual.
Note
that
by
sending
a
binary
1
to
one
of
these
status
lines,
the
line
is
turned
ON,
which
is
converted
by
the
RS-232C
line
driver
into
+12
VOC.
To
receive
handshaking
signals:
rata
Ccrrmunications
Equipnent
connected
to
the
board
may
send
+12
VOC
handshaking
signals
on
one-or
nore
lines,
I1l)St
often
RS-232C
lines
5
and/or
6.
Similarly,
Data
Terminal
Equipment
connected
to
the
board
may
send +12
VDC
on
one
or
oore
lines,
most
often
RS-232C
lines
4
and/or
20.
You
do
not
have
to
monitor
these
lines,
necessarily.
If
you
chocse
to,
you
can
read
them
through
software
control.
The
hardwired
line
drivers
and
receivers
are
already
in
place.
T1)e
8251
has
two
pins
that
can
be
used
to
receive
handshaking
signals:
pin
17
(CTS)
and
pin
22 (DSR).
Software
can
monitor
the
status
of
DSR
by
monitoring
the
appropriate
bit
in
the
8251
status
byte.
The
RS-232C
line
receiver
causes
+12
VOC
to
make
this
bit
a
binary
1 (ON).
Software
can
also
nonitor
the
crs
status.
Rather,
if
the
input
to
CTS
is
OFF,
the
8251
will
not
transmi
t
Rev.
l-B
6/11/80
2-17