Digital Input Controller - GE PAC 4020 System Manual

Signal conditioning filters and attenuates the
process instrument signals to one of the standard
voltage ranges. A wide range of standard signal
conditioning cards for attenuation, filtering, current
inputs, resistance inputs (RTD bridges) and slide
wires are available.
Thermocouple inputs require the use of
thermocouple cold junction references. The tem-
perature of the cold junction referenced is measured
(rather than controlled). The correction for the
cold junction temperature is made by the program.
Open thermocouple detection is optional.
The RTOS software that drives the AIS has two
features that increase its accuracy. The first is
automatic (partial) compensation for thermal
drifts, accomplished by taking a rolling average of
the reading of a shorted input on each gain range
and subtracting it from each process variable
reading. The second feature, applicable to 50
point-per-second models only, is automatic gain
changing, assuring that each variable is "tracked"
and read at the highest possible gain and hence with
the highest possible accuracy. This software then
re-scales the reading just as if it were read at
the nominal gain specified in the control word, so
that calculation scaling is not affected.
The AIS hardware includes a voltage reference
card, allowing periodic on-line calibration for gain
(in addition to offset) by any functional programs
desiring ultimate accuracy.
In addition to the normal A/D overflow de-
tection, the GE/PAC 4020 computer's AIS also
detects, prevents and alarms attempts due to
hardware failures to address two input points at
once, preventing both erroneous readings and pos-
sible "bumps" to process sensors shared with other
control systems. This feature is also available
only in the 50 point per-second models.
Control iwords are transmitted from the
arithmetic unit to the analog input scanner, as
shown in Figure 8, by a TOM function. Converted
data words are transmitted from the scanner to
the arithmetic unit, as shown in Figure 8, by a
TIM function.
Solid-State
The high- speed analog input scanner accepts
low-level analog signals and stores the digital
representation of these signals in core memory.
It is a true differential, low-level system intended
for process applications, and employs one amplifier
per point.
The following operating modes are available:
Single Channel
In this mode the computer instruction causes
the system to enter the random scan mode, and will
place one data word in core memory.
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Sequential Scan
In this mode the computer instruction causes
the system to enter the sequential scan mode, and
will output sequentially the specified number of
data words, commencing with the specified initial
address.
The standard high- speed analog scanner, oper-
ating in the sequential scan mode, will scan up to
256 points at the rate of 40, 000 analog points per
second, with a resolution of 12 bits plus sign.
DIGITAL INPUT CONTROLLER
The GE/PAC digital input controller senses
contact closures or logic level voltages. These are
read into the arithmetic unit in groups of 23 bits,
plus an additional validity bit taken directly from the
power supply circuit breaker for detection of power
supply failure and process wiring grounds, whether
momentary or continuing.
The bits may be individual status bits, or may
be coded in any manner, to be interpreted by the
program. The digital input controller is used to
read status information from devices such as limit
switches, hot metal detectors, over-temperature
switches, pressure switches, pushbuttons, alarm
contacts, and shaft position encoders, and to read
numerical information from devices such as
operator's decade switches and counters.
The standard digital input controller senses
sixteen groups of information and can be expanded
to 64 groups. A contact change detection option is
available which activates an interrupt, causing a
branch to a scanning program. Contact inputs are
sensed with 28 volts d-c (standard) or 125 volts d-c
(option).
The digital input controller operates directly
from the arithmetic unit. One IN instruction reads
a group's status into the arithmetic unit. Maximum
input speed is 883, 000 bits per second. (Figure 9.)
In the application, the contacts are continuously
energized, with a separate DIC point dedicated to
each one. For devices which are read infrequently
(such as operator's decade switches read on de-
mand), the power supply to the external contacts
may be energized by the multiple output controller
during the sensing period, in which case several
contacts that are energized only at different times
may share a single input point at the digital input
controller, reducing cost.
The digital input controller does not remember
transient signals. Intermittent digital signals and
digital information in the form of pulses are sensed
by interrupt hardware rather than by the DIC.