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PLC
vs “Process Quality”
I/O |
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| In
general, there are three classes of I/O. Each of which has particular
characteristics that suit the markets in which they are used. This
information is intended to help you compare the various types of
I/O so that you can apply the best solution to each of your application
needs. |
Process (SCS) I/O
PLC I/O
Laboratory I/O |
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| Process
(SCS) I/O: |
| Distributed
control systems (DCS) tend to be process oriented, which typically
means a significant number or majority of analog I/O. High performance
is expected of the I/O in terms
of accuracy, reliability, and “big system” features. These systems
are more likely to be
mission critical than PLC systems. Historically, DCS systems have
been high priced and
tend to be larger systems. The I/O on a legacy DCS is generally
quite expensive because
of the demanding requirements and quite frankly the DCS suppliers
as sole source vendors.
SIXNET
I/O is process quality I/O. Like the DCS of old, it is accurate
(actually more so because it is a newer design). It contains
features such as hot swap (change an I/O module and have it
instantly self-configure while the power remains on). It can
be connected in a redundant configuration and it contains multiple
levels of electrical isolation and other features to enhance
its reliability. It is supplied with complete asset management
software (the SIXNET I/O Tool Kit) which previously was only
available on large, expensive DCS systems. SIXNET
I/O has status reporting, on-line calibration capability, and
many features expected in the process control industries. Unlike
the DCS, however, SIXNET I/O in a Scalable Control System, or
SCS, is modular and may be flexibly configured. SIXNET I/O is
called “I/O for Windows” because it is designed to be used with
modern open systems, which in one way or another, are used with
Windows software, Ethernet networking, and other high performance
and highly open technologies. In summary, SIXNET I/O is high
reliability, high performance process quality I/O at “I/O for
Windows” pricing. |
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| PLC
I/O: |
PLC
I/O is optimized for machine control. There is typically more discrete
I/O than analog I/O. Emphasis is on high density (small footprint)
and low cost, with enhanced performance features taking a back seat.
The analog I/O in PLC systems is generally of lower performance
and accuracy than process I/O. The front-end optical isolators classically
found on discrete I/O channels is the only isolation in the path
from the input terminals to the processor. Typically, PLC I/O does
not include many diagnostic features. PLC I/O is not supplied with
the valuable asset management tools supplied with SIXNET process
I/O. This is because PLC I/O merely reports “register values” to
the PLC and there are few provisions to report other information
from the I/O modules to the PLC. PLC I/O networks such as Devicenet,
Profibus-DP and Interbus-S do not have the range of capabilities
made possible by the Ethernet networking. (By contrast, modern process
control systems are turning to Ethernet.) PLC systems tend to be
designed
as stand-alone controllers with loosely coupled supervisory networks
that exchange I/O
register data. Accordingly, the I/O vendors do not supply the integrated
system tools for
design and maintenance found in the DCS (process) world. In fact,
configuration tools are considered to be overhead to PLC manufacturers
and third party I/O suppliers.
They devote as little time and money to their configuration tools
as possible. |
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| Laboratory
I/O: |
| Open
(exposed) I/O boards are often referred to as laboratory style I/O
because of the type of applications it is best suited for. This
type of I/O typically plugs into a computer or specialized backplane,
or interconnects with ribbon cables. Laboratory I/O is used in situations
in which the rigors of industrial packaging and high reliability
are not required. This type of I/O is often used in situations in
which special functions such as exotic counter functions are required.
In
recent years laboratory I/O manufacturers such as Opto 22 and
National Instruments have improved their packaging by encasing
their laboratory I/O in attractive plastic that DIN rail mounts.
Their I/O circuitry, however, has not evolved very much. Typically,
these laboratory I/O systems pass fragile five-volt computer power
between I/O modules which greatly increases their vulnerability
to electrical noise and damage. (Fluctuations as small as a fraction
of a volt can disrupt operation and a power spike as small as
two volts can permanently damage their I/O modules.) These laboratory-styled
products contain limited or no other signal isolation, just like
their open board ancestors. When these products are used in the
electrically clean environments they were designed for, they tend
to work adequately. (This also pertains to the bench testing or
system simulations that often is used to qualify industrial systems,
but do not reflect the actual harsh conditions found on the plant
floor.) Although these repackaged products may have much the same
physical appearance as PLC or process I/O, the internal workings
of these laboratory devices is not as robust as the true industrial
I/O products supplied by PLC and process control vendors. |
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Certified
to Perform
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