Rockwell-automation MHB44IND Pulse Input and Analogue Output Module Manuale Utente Scaricare il pdf (Pagina 12)

12

MHB44IND

October

200

5

–

I

ssue 2

Triguard

SC300E

Analogue outputs

Five

test

-

sockets

are

provided

for

monitoring

the

output

lines:

one

common

terminal

via

2.2k

ohm

to

the

+24V

field

supply,

and

the

other

four

via

10k

ohm

to

the

current

-

determining

resistors

on

the

termination

card.

These

resistors

are

250

ohm,

so

20mA

of

loop

-

current

corresponds

to

5V

across

them.

The

four

analogue

voltage

outputs

use

12

-

bit

DACs

to

produce

+0.4V

to

–

6V

with

respect

to

field

+24V

common,

applied

across

resistors

on

the

termination

car

d.

The

outputs

from

the

three

channels

go

through

a voting

scheme

using

FETs

to

select

the

middle

-

value

output,

so

that

if

one

channel

fails

high

or

low,

one

of

the

other

two

outputs

will

be

selected

(see

Figure

2

-

3).

The

resultant

current

of

0 to

22mA

determined

by

the

voltage

and

resistor

is

fed

to

the

field

device,

then

returns

to

the

supply

negative

via

the

termination

card.

Figure

2

-

3 Analogue

output

section

-

Schematic

block

diagram

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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