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PROCESS INPUT -OUTPUT SUBSYSTEMS
ANALOG INPUT SCANNERS
Relay
The analog input scanner (AIS) accepts analog
signals representing process variables and provides
the computer with a digital representation of them
:
A useful concept is to think of the scanner as an
automated, high-quality' high-speed
digital
volt-
meter.
The AIS receives instructions from the central
processor on which analog point to read, which
gain and mode of operation to use. The scanner
conditions the analog signal, selects the point by
switching, amplifies the signal and converts the
signal to a binary number. Figure 8 illustrates the
configurations and major components of a typical
system.
The basic analog scanner cycle requires
20
milliseconds, which includes relay switching time,
amplifier
settling time, and conversion time. The
basic scan rate is therefore
50
points per second.
Scan rates up to
165
points per second can be
obtained by using the group scan mode of operation.
..
During
the group mode of operation, two, four
or eight points are
selected
simultaneously. A
high-speed relay (group switch)
then
applies one
of
these signals at a time to the amplifier input. After
-sufficient settling time an analog-to-digital con-
vel'.sion
is made, and the next point is selected. In
the group mode of operation, all points within a
group
will be digitized using the same gain range.
Both two-wire and three-wire switching of
analog input points are available. Three-wire
switching permits the
use
of a guard system,
allowing more accurate measurements where
common-mode noise voltages are significant.
The amplifier performs the functions of isola-
tion, common mode rejection, and amplification to
a level suitable for the
A/
D
converter. The ampli-
fier has six input ranges;
±10, ±2
0,
±40,
±80, ±160
millivolts and
±10
volts, selectable under program
control. Maximum allowable input voltage
is
500
volts. Maximum common mode voltage is
250
volts
peak
(100
volts for multigroup systems). Common
mode rejection ratio is
120
db, or
106,
when
using the optional three-wire switching.
CONTROL
UNIT
Instructions From
Central
Processor
Signals
From
Process
I
\
Signal
Con-
ditioning
and
Relay
Switch
I
I
I
r - - -
- - - 1 I
I
Signal
Con-
I I
ditioning
and
I-
_J
~ Relay
Switch
I
L---- _
_J
Amplifier
System
Digitzer
or
Analog to
Digital
Con-
verter
Amplifier
and Digitzer are
shared
by many
points.
Figure 8
GE/PAC Analog Sc1111r System
To
Central
Processor
31
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