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SECTION
6.
9 PIN SERIAL INPUT/OUTPUT
lf
the
computer
is
configured
as DCE equipment
(pin
2
is
an input for RD), a null modem cable
is
required. See the SC32A manualfor details.
6.5.3
COMMUNICATION PROTOCOL/TROUBLE
SHOOTING
The ASCII standard defines an alphabet
consisting
of
128
different characters where
each character corresponds
to a number, letter,
symbol, or control code.
An ASCII
character
is
a binary
digitalcode
composed of a combination of seven "bits",
each
bit having a binary state of
1
or
more.
For
example,
the binary equivalent for the ASCII
character "1"
is
01
10001 (decimal
49).
ASCII characters are transmitted one bit at
a
time, starting with the
first (least significant)
bit.
During data transmission
the marking condition
is used to denote
the binary state
1, and
the
spacing condition for
the binary state
0.
The
signal
is
considered marking when the voltage
is
more negative
than minus three volts with
respect to ground, and spacing when the voltage
is more positive
than plus three volts.
Most computers use
8-bits (1 byte) for data
communications. The eighth bit
is
sometimes
used for
a
type of error checking called parity-
checking.
Even parity binary numbers have
an
even number of
1's,
odd-parity
characters have
an odd number
of
1's. When
parity
checking
is
used, the eighth bit
is
set to either
a
1
or a
0
to
make
the parity of the character
correct.
The
21X ignores the eighth bit of
a
character
that
is
receives, and transmits the eighth bit as
a bina
0.
This method is generally described as
"no
parity".
To separate ASCII characters a Start bit is sent
before the first data bit, and
a Stop bit
is
sent
after the eighth data
bit.
The start bit is always
a.sBace, and
the stop bit
is
always a mark.
Between characters
the signal is
in
the marking
condition.
Figure 6.5-1 shows how the ASCII character
"1"
is
transmitted. The SC32A interface transmits
spacing and marking voltages which are
and negative, as
shown. Signalvoltages
at
the
21X
ll0
port are
5
volts
in
the spacing conditi
and
0
volts
in
the marking condition.
START
BIT
l
tLse)
7
DATA
BITS
2345
EIGHTH
STOP
7 Brr
Brr
SPACE
(POS|TTVE VOLTAGE)
BINARY
STATE
MARK
(NrCRlVr
VOLTAGE)
FIGURE
6.5.1.
BAUD RATE
BAUD RATE
is
the number of bits transmitted
per
second.
The 21X can communicate at 300,
1200, 9600, and 76,800
baud.
ln
the
Telecommunications State, the 21X
willset
its
baud
rate
to match the baud rate of the modem.
The
baud rate of the modem or computer
is
usually set with dip switches
or programmed
from
the keyboard. The instrument's instruction
manual should
explain how to set
it.
the ASCII Character
1
DUPLEX
Full duplex means that two devices can
communicate in both directions simultaneously.
Half duplex means that the two
devices must
send
and receive alternately. Full duplex
always
be
specified when communicating with
Campbell Scientific peripherals
and modems.
However,
communication between some
CampbellScientific modems (such as the RF95
RF modem)
is
carried
out
in
a half duplex
fashion.
This can affect the way commands
should be sent to
and received from such a
6-4
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