There are many different signals in use today. The good news is that
there are fewer "standards" today then there were ten years ago.
The most important advice we can give you is -- make sure that
your I/O module matches the actual signal / device that you are connecting
to.
Classes of I/O
Typically there are three classes (types) of I/O:
Discrete I/O
Analog I/O
Specialty I/O
Discrete I/O is simply On / OFF, True / False or some other binary
designation of two states. Analog I/O are varying levels such as
temperatures, pressures, levels. Specialty I/O is everything else -- high
speed motion control, high speed counters, high speed interfaces to encoders,
etc. Note the word "high speed" in connection with specialty I/O.
Typically a specialty I/O module is a module with its own processor that allows
the module to operate faster than the PLC and regular I/O. More
information is provided in our PLC
tutorial.
Signal Characteristics
There are several factors that have to be considered for every input and
output:
Voltage level
Current level
Noise
Examples of things not to do are:
do not connect a 24VDC I/O module to a 120 VAC device
do not connect an output that sources 1/4 amp to a device that draws 1/2 amp
do not expect high accurate analog signals without having lots of isolation
and other noise reducing measures
Voltage
We categorize three main voltage groups:
Low voltage (typically 5 volts or less) such as TTL, thermocouples, strain
gauges, etc.
Medium DC voltage: 12 VDC, 24 VDC, 48 VDC
High voltage: 120 VAC or higher, most AC voltages
Low voltages are very susceptible to
receiving interference where high voltages are prone to causing
interference.
Medium DC voltage (namely 24 VDC) tends to be in the middle
of the two extremes. That (plus a few other reasons like current draw) is why everyone has
been "standardizing" on 24 VDC for industrial applications.
You can not do without the low voltage devices. However,
you can try to contain them in a shielded box (shell) and convert that low
voltage signal to a 4 to 20 ma (current) signal as soon as possible. For
example, if you have a thermocouple in an oven it is probably generating a
signal in the millivolts range (hundredths of a volt). If your controller
is a hundred feet away then you can buy a hundred feet of special thermocouple
wire and try to eliminate noise throughout the hundred foot run back to the
controller. Or you could mount a thermocouple to milliamp (ma) current loop
transmitter on the outside of (or close to) the oven and then the ma current
runs back to the controller and is less susceptible to noise. You still want to use a high quality, twisted pair, shielded cable on
a 4 to 20 ma analog differential input for lowest noise. My point being -- there is
less noise in transmitting a 4 to 20 ma current over distances then transmitting
a thermocouple (millivolt) signal over the same distances. So convert
analog signals, at their source (as soon as possible) to 4 to 20 ma current
analog signals.
5 volts (TTL)
does not really belong in the low voltage group but TTL is such low current it
does require the same precautions as these other signals.
You can not do without high voltage devices either. The best you
can do is keep the AC voltage cabling and devices separated and isolated from
the rest of the system.
Current
The most commonly used analog signal in automation today is 4 to 20 milliamps.
You will see analog I/O modules and converters for all kinds of other voltages
and currents but 4 to 20 ma is now "the standard" since it is the
least susceptible to noise. Some
analog converters may have other current ranges, such as 0 to 20 ma, but most
devices today use 4 to 20 ma.
For measuring high
currents, such as reading current draw on a large motor, there are current
transformers that convert 0 to 200 amps at 480 volts AC to 4 to 20 ma at 24
VDC. There are also voltage to current converters. For example you can
buy transformers that convert 0 to 480 volts AC to 4 to 20 ma
current.
Noise
Noise is everywhere. The lower your voltage and current the more
susceptible to noise you are. There are many things you can do to cut down
on noise:
Shield cables, enclosures, conduit
Proper grounding
Use isolation on power and signals
Use different methods to filter signals
Use twisted pair, shielded cables
Use differential inputs instead of single ended inputs
The best way to "see" noise is to get an oscilloscope and a good
isolation transformer. Look (on the oscilloscope) at the voltage going into the
isolation transformer
versus the voltage coming out of the transformer. You can make similar
tests using other signals and filters.
One important concept is signal to noise ratio. The higher the
signal power and the lower the noise -- the better your signal to noise ratio
and the more accurate results you will receive.
One important question is -- how much noise is bad? It depends on your system and what you need
to achieve. For example, if you have all AC
powered discrete I/O and no analog data then there should be little problem with
noise. You still need protection such as
fuses and surge protectors. But we would not recommend an expensive
isolation transformer -- we would probably just use something like a Corcom AC
filter.
However, If you need to accurately measure analog signals in a system with
variable speed drives then you need to
take a lot of extra effort to isolate and eliminate noise. This means starting
with isolating the power going to your control system and using isolation on the
power going to the variable speed drives.
One of the worst sources of noise are motors and variable frequency
drives. For sensitive applications (i.e. those using analog I/O) it is
important to keep the power and signals isolated between your controller and
drives. To reduce noise on the power line from drives you can use
isolation transformers, reactors, and filters. Note that isolation and filters
blocks most of the noise both
ways -- it eliminates much of the noise from getting into your system -- and in
the case of drives -- it prevents the noise from the drives from getting
back out onto the main power lines and then getting into other
equipment.
Ferrite coils are often used to cut down on noise, although we will have to
find a few websites that explain how and when to use them.
Filtering
Filtering a signal cuts down on the noise but reduces the response time.
For example, an encoder must have little filtering so that it can produce pulses
at a fast speed. Whereas most proximity switch applications can have heavy
filtering and are less prone to false signals due to noise. There are
several ways to achieve filtering.
Most input modules have a resistor and capacitor that filter the incoming
signal. If you check the specifications of most discrete input modules
they will tell you that the input module has a delay or response time of
typically 10 milliseconds (ms). Therefore you can not measure signals with
response times faster than the specified response time. Note that there
are high speed input modules but they have less filtering and are more
susceptible to noise.
Really good input modules will allow you to set the input filtering
time. For example, if you are trying to measure the force to press a switch
this is high accuracy and fast sampling. So you would use less filtering
(higher response time) and noise is more of a concern. Whereas if you were
measuring the outside temperature then you could dampen the signal a lot
(because outside temperature changes very slowly) and
noise would not be much of a concern.
Another way to filter signals in code is to use a moving average. If
you average results you reduce the peaks and valleys (fluctuations) of the
signal. We would recommend that you first try to use electronics filtering
and use moving averages only as a last resort.
It is important to understand how much noise will effect your system and
to take effective measures to reducing noise.
Every Input & Output
Signal conditioning and protection is something that has to be considered
for every input and output. For example you can not connect an encoder
that produces a 5 VDC TTL level signal to a high speed counter designed for 24 VDC. Like wise
you can not connect an output that sources 1/4 amp to a coil that draws 1/2
amp. Any conversion of signals is a pain but luckily many manufacturer's make
signal conditioning modules. These modules typically are DIN rail
mounted.
For discrete inputs and outputs we always try to use 24 VDC. If we have signals that are
something other than 24 VDC we try to convert them, at their source, to 24 VDC and
then run the wires back to our controller. If there are only a few signals
to convert then you can get DIN rail mounted converters. If you have more
than 6 or 8 signals to convert then you may want to use a rack mount or board
mounted converters to achieve higher densities and lower total cost.
The great thing about signal conditioning is that there are so many
options. Since many I/O modules are designed for different signals, remote
I/O can be used. For example, we love 24 VDC but suppose we have to
interface to another machine whose inputs and outputs are all 120 VAC. Our
options would be:
We can buy 120 VAC input and output modules for our controller and pull
the 120 VAC back into our controller and let the AC wiring generate a lot of
noise.
We can buy a bunch of 120 VAC to 24 VDC converters (solid state relays
or similar), and mount them on the other machine and run only 24 VDC in and
out of our controller.
We can take a small rack of 120 VAC remote I/O, stick it on their
machine, and only the communications is brought back to our
controller. Note that the communications is usually
isolated.
For discrete outputs you typically add a mechanical or solid state relay
for isolation and to boost the switching current of the output.
Since the circuit on each side of the relay is supplying its own voltage and
current, relays allow you to change current and voltage for incompatible
circuits.
Types of I/O
The main types of I/O are:
PLC I/O
Embedded controller I/O
Computer I/O on ISA, PCI, VME bus
Remote I/O
When you buy a PLC you can of course buy I/O that works with the PLC.
You can also buy PLC I/O with or without a CPU and just use the I/O. PLC
I/O is designed so well that if you have a computer controller and you like a
certain PLC manufacturer's I/O (or perhaps it is already in use at the customer
site) that you simply have a communications link between the computer and the
PLC I/O.
Embedded controllers (computers) will have their own I/O as well.
Computer I/O can be extremely fast. These boards plug into a PCI
computer slot and a program can quickly read the I/O and write the data to disk.
Thousands of samples per second is easy to accomplish. Companies such as
National Instruments offer sampling rates as
fast as millions of samples per second. At this rate, they can't write the
values to disk in real-time but memory is so cheap that they can write thousands
of measurements to memory and then write these values to disk once the reading
phase is completed.
Remote I/O or distributed I/O is much like PLC I/O without the CPU.
It is industrial quality I/O with a communications link that allows a controller
to read and write I/O that are mounted away from the controller. For
example, if you are controlling equipment that is physically located over a 50
by 50 foot area you could pull all the wires for every discrete and analog
signal back to one cabinet or you could use remote (distributed) I/O and place
the I/O in several different points around the area. This reduces how far
the wires are pulled and having to bring thousands of wires into one cabinet.
Each remote I/O "drop" is connected to the others and the main controller using
a single communications cable.
Links
Many of these manufacturer's have products and tutorials.
Access I/O Products
-- digital and analog I/O PCI cards, remote I/O, bus expansion cards, watchdog
timers
Beckhoff -- see Fieldbus components then bus terminals
Dataforth
-- hundreds of different industrial signal conditioning products. The
application notes and signal conditioning tutorial are worth signing up for.
Gespac
-- 3U CompactPCI Industrial I/O and Signal Conditioners
Grayhill -- I/O modules can be used for isolation and
signal conditioning
Moore Industries -- signal
transmitters, isolators, converters; temperature interfaces; limit alarms;
process controllers & displays
M-System --
4 wire signal conditioners, limit alarms, rack mount signal conditioners,
power transducers, gateways, remote I/O, multiplexers, PC recorders,
lightning arrestors, panel meters
National Instruments -- wide range of signal conditioning products and data acquisition
Wilkerson
Instrument -- signal conditioners,
controllers, indicators, transmitters,
timers, counters
Tips & Ticks
Each system is different in regards to noise problems and what you need
to do. You do not have
to do much for systems with only discrete I/O as you do with systems that
contain high accuracy analog signals and variable frequency
drives.
Use isolation where possible: on power and on the signals going between
systems
Use networks to read and write analog values instead of using analog
inputs and outputs for two reasons:
Devices handling analog values are typically
limited to just one or two analog I/O. With networks you can typically
read and write a lot more values.
Errors in an analog signal are created by converting a number into an analog
signal (D to A conversion), during the propagation of the
analog signal (i.e. the wiring), and then when the analog signal
is converted back to some value (A to D conversion). All of these errors are eliminated
with networking.
Use circuit breakers or fuses liberally to protect the users, the
equipment, and to facilitate troubleshooting. This includes (but not
limited to) a fuse on every output and a fuse on every power
input.
Convert low voltages and currents to higher voltage / current
For analog I/O, use differential signals instead of single ended for
higher accuracy.
For communications, use twisted pair shielded cable.
If there are multiple outputs on one module, there are typically two
ratings -- one for each output and another rating for all outputs
combined. For example, suppose you have a 16 point discrete output
module. It may specify that each output can source 1/4 amp of power
but the module (all outputs combined) can only source a total of 2 amps. So if all 16 outputs
were on at the same time, and all were trying to source 1/4 amp that would
be 4 amps total and would exceed the 2 amp maximum limit.
Put a diode across the coils of all DC relays and inductors to reduce
surges.
Put a metal oxide varistor (MOV) across the coils of all AC relays and
inductors to reduce surges.
Carefully read and follow the manufacturer's manual in detail. Most
manufacturers include "best practices" on how to install their systems but
most users want to "cut corners" and ignore this advice. People that
"cut corners" get a lot more support calls over the years.
We try to offer a fair and balanced opinion on every
page of our website. We would appreciate more
information from other users to express their opinions which we will
then incorporate. If you have questions or comments please post them
on our message board (see button in left hand column) so that others can
read and benefit.
