Campbell 21X Manuale dell'Operatore Scaricare il pdf (Pagina 62)

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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