Campbell Scientific CR10 CR10 Measurement and Control - Page 81

SECTION

6.

g.PIN

SERIAL INPUT/OUTPUT

START

BIT

ltrsel

7

DATA

B

S

23456

EIGHTH

STOP

7 Btr

Brr

SPACE

(POSIIVE

VOLTAGE)

BINARY

STATE

MARK

(NTCRIVE

VOLTAGE)

lf

the

computer/terminal

is

configured as

DCE

equipment (pin 2 is

an

input

for

RD),

a

null

modem cable

is

required.

See

the

SC32A

manual for details.

6.7.3

COMMUNICATION PROTOCOUTROUBLE

SHOOTING

The

ASCII

standard defines an alphabet

consisting

of

128

ditferent characters where

each

chaiacter

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

(one) or

0

(zero).

For example, the binary equivalent for the ASCII

character

"1"

is

0110001

(decimal 49).

ASCII characters are

transmitted one

bit at

a

time,

starting with

the

1st (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 8th

bit is sometimes

used for

a

type

of error checking

called

parity-

checking.

Even parity binary

characters

have

an

even

number

of

1's, odd-parity characters

have an

odd

number

of

1's.

When

parity

checking

is used,

the 8th

bit

is

set

to

either

a

1

or

a

0

to

make the parity of the

character

correct.

The

CR10 ignores

the

8th bit of

a

character

that

is receives, and transmits the 8th

bit as a binary

0.

This method is generally

described

as

"no parity".

To separate ASCII characters

a

Start

bit

is

sent

before

the

1st bit, and a Stop

bit

is

sent after

the

8th

bit.

The

start bit is always a space, and

the

stop bit is always

a

mark.

Between characters

the

signal

is

in

the

marking condition.

FIGURE

6.7-1. Transmitting the

ASCII

Character

1

Figure 6.7-1 shows how the ASCII

character

"1"

is

transmitted.

When

transmitted

by

the

CR10

using

the SC32A

RS232 interface spacing and

marking voltages

are

positive

and

negative, as

shown.

Signalvoltages

at

the CR10

l/O port

are 5V in the spacing condition, and 0V

in

the

marking condition.

BAUD RATE

BAUD RATE

is

the

number

of

bits transmitted

per

second.

The

CR10

can communicate

at

300,

1200, 9600, and 76,800

baud.

In

the

Telecommunications

State,

the

CR10 will

set

its

baud rate

to

match

the baud

rate of

the

computer/terminal.

Typically

the

baud rate

of

the modem/computer/

terminal

is set either with dip switches,

or

programmed

from the keyboard. The

instrument's

instruction manual should

explain

how to set

it.

DUPLEX

Fullduplex

means that two devices can

communicate

in both

directions

simultaneously.

Half duplex means that the

two devices

must send

and receive

alternately.

Full duplex should always

be specified when

communicating

with Campbell

Scientific

peripherals

and

modems.

However,

communication

between

some

Campbell Scientific

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 modem, especially

when

implemented

by computer software.

To

overcome the

limitations of

half duplex,

some

communications

links

expect

a

terminal

sending data to also write

the

data to

the

screen.

This saves

the

remote

device

having

to

echo that data

back.

lf, when communicating

with

a Campbell Scientific device, characters

are displayed

twice

(in pairs), it is likely that

the

terminal

is set

to

half

duplex

rather than

the

correct

setting of

full

duplex.

6-7

Campbell Scientific CR10 CR10 Measurement and Control - Page 81

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