<s>
A	O
programmable	B-Architecture
logic	I-Architecture
controller	I-Architecture
(	O
PLC	O
)	O
or	O
programmable	O
controller	O
is	O
an	O
industrial	O
computer	O
that	O
has	O
been	O
ruggedized	B-Device
and	O
adapted	O
for	O
the	O
control	O
of	O
manufacturing	O
processes	O
,	O
such	O
as	O
assembly	O
lines	O
,	O
machines	O
,	O
robotic	O
devices	O
,	O
or	O
any	O
activity	O
that	O
requires	O
high	O
reliability	O
,	O
ease	O
of	O
programming	O
,	O
and	O
process	O
fault	O
diagnosis	O
.	O
</s>
<s>
PLCs	O
can	O
range	O
from	O
small	O
modular	O
devices	O
with	O
tens	O
of	O
inputs	O
and	O
outputs	O
(	O
I/O	B-General_Concept
)	O
,	O
in	O
a	O
housing	O
integral	O
with	O
the	O
processor	O
,	O
to	O
large	O
rack-mounted	O
modular	O
devices	O
with	O
thousands	O
of	O
I/O	B-General_Concept
,	O
and	O
which	O
are	O
often	O
networked	O
to	O
other	O
PLC	O
and	O
SCADA	O
systems	O
.	O
</s>
<s>
They	O
can	O
be	O
designed	O
for	O
many	O
arrangements	O
of	O
digital	O
and	O
analog	O
I/O	B-General_Concept
,	O
extended	O
temperature	O
ranges	O
,	O
immunity	O
to	O
electrical	O
noise	O
,	O
and	O
resistance	O
to	O
vibration	O
and	O
impact	O
.	O
</s>
<s>
Programs	O
to	O
control	O
machine	O
operation	O
are	O
typically	O
stored	O
in	O
battery-backed-up	O
or	O
non-volatile	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
PLCs	O
were	O
first	O
developed	O
in	O
the	O
automobile	O
manufacturing	O
industry	O
to	O
provide	O
flexible	O
,	O
rugged	O
and	O
easily	O
programmable	B-Architecture
controllers	I-Architecture
to	O
replace	O
hard-wired	O
relay	O
logic	O
systems	O
.	O
</s>
<s>
Since	O
then	O
,	O
they	O
have	O
been	O
widely	O
adopted	O
as	O
high-reliability	O
automation	B-Application
controllers	O
suitable	O
for	O
harsh	O
environments	O
.	O
</s>
<s>
The	O
hard-wired	O
nature	O
made	O
it	O
difficult	O
for	O
design	O
engineers	O
to	O
alter	O
the	O
automation	B-Application
process	O
.	O
</s>
<s>
When	O
general-purpose	B-Language
computers	O
became	O
available	O
,	O
they	O
were	O
soon	O
applied	O
to	O
control	O
logic	O
in	O
industrial	O
processes	O
.	O
</s>
<s>
The	O
PLC	O
provided	O
several	O
advantages	O
over	O
earlier	O
automation	B-Application
systems	O
.	O
</s>
<s>
It	O
was	O
easily	O
extensible	O
with	O
additional	O
I/O	B-Architecture
modules	I-Architecture
,	O
while	O
relay	O
systems	O
required	O
complicated	O
hardware	O
changes	O
in	O
case	O
of	O
reconfiguration	O
.	O
</s>
<s>
With	O
a	O
simple	O
programming	O
language	O
focused	O
on	O
logic	O
and	O
switching	O
operations	O
,	O
it	O
was	O
more	O
user-friendly	O
than	O
computers	O
using	O
general-purpose	B-Language
programming	I-Language
languages	I-Language
.	O
</s>
<s>
About	O
this	O
same	O
time	O
,	O
Modicon	O
created	O
Modbus	B-Protocol
,	O
a	O
data	O
communications	O
protocol	O
used	O
with	O
its	O
PLCs	O
.	O
</s>
<s>
Modbus	B-Protocol
has	O
since	O
become	O
a	O
standard	O
open	O
protocol	O
commonly	O
used	O
to	O
connect	O
many	O
industrial	O
electrical	O
devices	O
.	O
</s>
<s>
In	O
a	O
parallel	O
development	O
Odo	O
Josef	O
Struger	O
is	O
sometimes	O
known	O
as	O
the	O
"	O
father	O
of	O
the	O
programmable	B-Architecture
logic	I-Architecture
controller	I-Architecture
"	O
as	O
well	O
.	O
</s>
<s>
He	O
was	O
involved	O
in	O
the	O
invention	O
of	O
the	O
AllenBradley	O
programmable	B-Architecture
logic	I-Architecture
controller	I-Architecture
and	O
is	O
credited	O
with	O
inventing	O
the	O
PLC	O
initialism	O
.	O
</s>
<s>
Allen-Bradley	O
(	O
now	O
a	O
brand	O
owned	O
by	O
Rockwell	O
Automation	B-Application
)	O
became	O
a	O
major	O
PLC	O
manufacturer	O
in	O
the	O
United	O
States	O
during	O
his	O
tenure	O
.	O
</s>
<s>
As	O
programming	O
terminals	B-General_Concept
evolved	O
,	O
it	O
became	O
more	O
common	O
for	O
ladder	O
logic	O
to	O
be	O
used	O
,	O
because	O
it	O
was	O
a	O
familiar	O
format	O
used	O
for	O
electro-mechanical	O
control	O
panels	O
.	O
</s>
<s>
Up	O
to	O
the	O
mid-1990s	O
,	O
PLCs	O
were	O
programmed	O
using	O
proprietary	O
programming	O
panels	O
or	O
special-purpose	O
programming	O
terminals	B-General_Concept
,	O
which	O
often	O
had	O
dedicated	O
function	O
keys	O
representing	O
the	O
various	O
logical	O
elements	O
of	O
PLC	O
programs	O
.	O
</s>
<s>
Some	O
proprietary	O
programming	O
terminals	B-General_Concept
displayed	O
the	O
elements	O
of	O
PLC	O
programs	O
as	O
graphic	O
symbols	O
,	O
but	O
plain	O
ASCII	O
character	O
representations	O
of	O
contacts	O
,	O
coils	O
,	O
and	O
wires	O
were	O
common	O
.	O
</s>
<s>
The	O
oldest	O
PLCs	O
used	O
non-volatile	B-General_Concept
magnetic	O
core	O
memory	O
.	O
</s>
<s>
an	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
interface	O
,	O
where	O
the	O
controller	O
receives	O
and	O
sends	O
data	O
from/to	O
external	O
devices	O
,	O
</s>
<s>
Modern	O
PLCs	O
generally	O
contain	O
a	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
,	O
such	O
as	O
OS-9	B-Operating_System
or	O
VxWorks	B-Operating_System
.	O
</s>
<s>
Second	O
design	O
type	O
–	O
a	O
modular	O
PLC	O
–	O
has	O
a	O
chassis	O
(	O
also	O
called	O
a	O
rack	O
)	O
that	O
provides	O
space	O
for	O
modules	O
with	O
different	O
functions	O
,	O
such	O
as	O
power	O
supply	O
,	O
processor	O
,	O
selection	O
of	O
I/O	B-Architecture
modules	I-Architecture
and	O
communication	O
interfaces	O
–	O
which	O
all	O
can	O
be	O
customized	O
for	O
the	O
particular	O
application	O
.	O
</s>
<s>
Either	O
a	O
special	O
high-speed	O
serial	O
I/O	B-General_Concept
link	O
or	O
comparable	O
communication	O
method	O
is	O
used	O
so	O
that	O
racks	O
can	O
be	O
distributed	O
away	O
from	O
the	O
processor	O
,	O
reducing	O
the	O
wiring	O
costs	O
for	O
large	O
plants	O
.	O
</s>
<s>
Options	O
are	O
also	O
available	O
to	O
mount	O
I/O	B-General_Concept
points	O
directly	O
to	O
the	O
machine	O
and	O
utilize	O
quick	O
disconnecting	O
cables	O
to	O
sensors	O
and	O
valves	O
,	O
saving	O
time	O
for	O
wiring	O
and	O
replacing	O
components	O
.	O
</s>
<s>
Examples	O
of	O
devices	O
providing	O
a	O
discrete	O
signal	O
include	O
limit	O
switches	O
,	O
photoelectric	B-Algorithm
sensors	I-Algorithm
and	O
encoders	O
.	O
</s>
<s>
In	O
such	O
cases	O
to	O
increase	O
the	O
system	O
availability	O
in	O
the	O
event	O
of	O
hardware	O
component	O
failure	O
,	O
redundant	O
CPU	O
or	O
I/O	B-Architecture
modules	I-Architecture
with	O
the	O
same	O
functionality	O
can	O
be	O
added	O
to	O
hardware	O
configuration	O
for	O
preventing	O
total	O
or	O
partial	O
process	O
shutdown	O
due	O
to	O
hardware	O
failure	O
.	O
</s>
<s>
Other	O
redundancy	B-Application
scenarios	O
could	O
be	O
related	O
to	O
safety-critical	O
processes	O
,	O
for	O
example	O
,	O
large	O
hydraulic	O
presses	O
could	O
require	O
that	O
both	O
PLCs	O
turn	O
on	O
output	O
before	O
the	O
press	O
can	O
come	O
down	O
in	O
case	O
one	O
output	O
does	O
not	O
turn	O
off	O
properly	O
.	O
</s>
<s>
Programmable	B-Architecture
logic	I-Architecture
controllers	I-Architecture
are	O
intended	O
to	O
be	O
used	O
by	O
engineers	O
without	O
a	O
programming	O
background	O
.	O
</s>
<s>
,	O
the	O
majority	O
of	O
PLC	O
systems	O
adhere	O
to	O
the	O
IEC	O
61131-3	O
standard	O
that	O
defines	O
2	O
textual	O
programming	O
languages	O
:	O
Structured	O
Text	O
(	O
ST	O
;	O
similar	O
to	O
Pascal	B-Application
)	O
and	O
Instruction	O
List	O
(	O
IL	O
)	O
;	O
as	O
well	O
as	O
3	O
graphical	O
languages	O
:	O
Ladder	O
Diagram	O
,	O
Function	B-Application
Block	I-Application
Diagram	I-Application
(	O
FBD	O
)	O
and	O
Sequential	B-Language
Function	I-Language
Chart	I-Language
(	O
SFC	O
)	O
.	O
</s>
<s>
Modern	O
PLCs	O
can	O
be	O
programmed	O
in	O
a	O
variety	O
of	O
ways	O
,	O
from	O
the	O
relay-derived	O
ladder	O
logic	O
to	O
programming	O
languages	O
such	O
as	O
specially	O
adapted	O
dialects	O
of	O
BASIC	O
and	O
C	B-Language
.	O
</s>
<s>
While	O
the	O
fundamental	O
concepts	O
of	O
PLC	O
programming	O
are	O
common	O
to	O
all	O
manufacturers	O
,	O
differences	O
in	O
I/O	B-General_Concept
addressing	O
,	O
memory	O
organization	O
,	O
and	O
instruction	O
sets	O
mean	O
that	O
PLC	O
programs	O
are	O
never	O
perfectly	O
interchangeable	O
between	O
different	O
makers	O
.	O
</s>
<s>
PLC	O
programs	O
are	O
typically	O
written	O
in	O
a	O
programming	O
device	O
,	O
which	O
can	O
take	O
the	O
form	O
of	O
a	O
desktop	O
console	O
,	O
special	O
software	O
on	O
a	O
personal	B-Device
computer	I-Device
,	O
or	O
a	O
handheld	O
programming	O
device	O
.	O
</s>
<s>
It	O
is	O
stored	O
either	O
in	O
non-volatile	B-General_Concept
flash	B-Device
memory	I-Device
or	O
battery-backed-up	O
RAM	B-Architecture
.	O
</s>
<s>
In	O
some	O
programmable	B-Architecture
controllers	I-Architecture
,	O
the	O
program	O
is	O
transferred	O
from	O
a	O
personal	B-Device
computer	I-Device
to	O
the	O
PLC	O
through	O
a	O
programming	O
board	O
that	O
writes	O
the	O
program	O
into	O
a	O
removable	O
chip	O
,	O
such	O
as	O
EPROM	B-General_Concept
.	O
</s>
<s>
A	O
program	O
written	O
on	O
a	O
personal	B-Device
computer	I-Device
or	O
uploaded	O
from	O
PLC	O
using	O
programming	O
software	O
can	O
be	O
easily	O
copied	O
and	O
backed	O
up	O
on	O
external	O
storage	O
.	O
</s>
<s>
The	O
main	O
difference	O
from	O
most	O
other	O
computing	O
devices	O
is	O
that	O
PLCs	O
are	O
intended-for	O
and	O
therefore	O
tolerant-of	O
more	O
severe	O
conditions	O
(	O
such	O
as	O
dust	O
,	O
moisture	O
,	O
heat	O
,	O
cold	O
)	O
,	O
while	O
offering	O
extensive	O
input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
to	O
connect	O
the	O
PLC	O
to	O
sensors	O
and	O
actuators	B-Algorithm
.	O
</s>
<s>
PLC	O
input	O
can	O
include	O
simple	O
digital	O
elements	O
such	O
as	O
limit	O
switches	O
,	O
analog	O
variables	O
from	O
process	O
sensors	O
(	O
such	O
as	O
temperature	O
and	O
pressure	O
)	O
,	O
and	O
more	O
complex	O
data	O
such	O
as	O
that	O
from	O
positioning	O
or	O
machine	B-General_Concept
vision	I-General_Concept
systems	O
.	O
</s>
<s>
PLC	O
output	O
can	O
include	O
elements	O
such	O
as	O
indicator	O
lamps	O
,	O
sirens	O
,	O
electric	O
motors	O
,	O
pneumatic	O
or	O
hydraulic	O
cylinders	O
,	O
magnetic	O
relays	O
,	O
solenoids	B-Algorithm
,	O
or	O
analog	O
outputs	O
.	O
</s>
<s>
The	O
input/output	B-General_Concept
arrangements	O
may	O
be	O
built	O
into	O
a	O
simple	O
PLC	O
,	O
or	O
the	O
PLC	O
may	O
have	O
external	O
I/O	B-Architecture
modules	I-Architecture
attached	O
to	O
a	O
fieldbus	O
or	O
computer	B-Architecture
network	I-Architecture
that	O
plugs	O
into	O
the	O
PLC	O
.	O
</s>
<s>
The	O
functionality	O
of	O
the	O
PLC	O
has	O
evolved	O
over	O
the	O
years	O
to	O
include	O
sequential	O
relay	O
control	O
,	O
motion	O
control	O
,	O
process	O
control	O
,	O
distributed	O
control	O
systems	O
,	O
and	O
networking	B-Architecture
.	O
</s>
<s>
The	O
data	O
handling	O
,	O
storage	O
,	O
processing	O
power	O
,	O
and	O
communication	O
capabilities	O
of	O
some	O
modern	O
PLCs	O
are	O
approximately	O
equivalent	O
to	O
desktop	B-Device
computers	I-Device
.	O
</s>
<s>
PLC-like	O
programming	O
combined	O
with	O
remote	O
I/O	B-General_Concept
hardware	I-General_Concept
,	O
allows	O
a	O
general-purpose	B-Language
desktop	B-Device
computer	I-Device
to	O
overlap	O
some	O
PLCs	O
in	O
certain	O
applications	O
.	O
</s>
<s>
Desktop	B-Device
computer	I-Device
controllers	O
have	O
not	O
been	O
generally	O
accepted	O
in	O
heavy	O
industry	O
because	O
desktop	B-Device
computers	I-Device
run	O
on	O
less	O
stable	O
operating	O
systems	O
than	O
PLCs	O
,	O
and	O
because	O
the	O
desktop	B-Device
computer	I-Device
hardware	O
is	O
typically	O
not	O
designed	O
to	O
the	O
same	O
levels	O
of	O
tolerance	O
to	O
temperature	O
,	O
humidity	O
,	O
vibration	O
,	O
and	O
longevity	O
as	O
the	O
processors	O
used	O
in	O
PLCs	O
.	O
</s>
<s>
Desktop	O
logic	O
applications	O
find	O
use	O
in	O
less	O
critical	O
situations	O
,	O
such	O
as	O
laboratory	O
automation	B-Application
and	O
use	O
in	O
small	O
facilities	O
where	O
the	O
application	O
is	O
less	O
demanding	O
and	O
critical	O
.	O
</s>
<s>
PLCs	O
use	O
built-in	O
ports	O
,	O
such	O
as	O
USB	B-Protocol
,	O
Ethernet	O
,	O
RS-232	O
,	O
RS-485	O
,	O
or	O
RS-422	O
to	O
communicate	O
with	O
external	O
devices	O
(	O
sensors	O
,	O
actuators	B-Algorithm
)	O
and	O
systems	O
(	O
programming	O
software	O
,	O
SCADA	O
,	O
HMI	B-Application
)	O
.	O
</s>
<s>
Communication	O
is	O
carried	O
over	O
various	O
industrial	O
network	O
protocols	O
,	O
like	O
Modbus	B-Protocol
,	O
or	O
EtherNet/IP	B-Protocol
.	O
</s>
<s>
PLCs	O
used	O
in	O
larger	O
I/O	B-General_Concept
systems	O
may	O
have	O
peer-to-peer	O
(	O
P2P	O
)	O
communication	O
between	O
processors	O
.	O
</s>
<s>
These	O
communication	O
links	O
are	O
also	O
often	O
used	O
for	O
HMI	B-Application
devices	O
such	O
as	O
keypads	O
or	O
PC-type	O
workstations	O
.	O
</s>
<s>
A	O
human-machine	B-Application
interface	I-Application
(	O
HMI	B-Application
)	O
is	O
employed	O
for	O
this	O
purpose	O
.	O
</s>
<s>
HMIs	O
are	O
also	O
referred	O
to	O
as	O
man-machine	B-Application
interfaces	I-Application
(	O
MMIs	O
)	O
and	O
graphical	O
user	B-Application
interfaces	I-Application
(	O
GUIs	O
)	O
.	O
</s>
<s>
If	O
the	O
system	O
contains	O
remote	O
I/O	B-General_Concept
—	O
for	O
example	O
,	O
an	O
external	O
rack	O
with	O
I/O	B-Architecture
modules	I-Architecture
—	O
then	O
that	O
introduces	O
additional	O
uncertainty	O
in	O
the	O
response	O
time	O
of	O
the	O
PLC	O
system	O
.	O
</s>
<s>
Special-purpose	O
I/O	B-Architecture
modules	I-Architecture
may	O
be	O
used	O
where	O
the	O
scan	O
time	O
of	O
the	O
PLC	O
is	O
too	O
long	O
to	O
allow	O
predictable	O
performance	O
.	O
</s>
<s>
Precision	O
timing	O
modules	O
,	O
or	O
counter	O
modules	O
for	O
use	O
with	O
shaft	B-Algorithm
encoders	I-Algorithm
,	O
are	O
used	O
where	O
the	O
scan	O
time	O
would	O
be	O
too	O
long	O
to	O
reliably	O
count	O
pulses	O
or	O
detect	O
the	O
sense	O
of	O
rotation	O
of	O
an	O
encoder	O
.	O
</s>
<s>
In	O
his	O
book	O
from	O
1998	O
,	O
E	O
.	O
A	O
.	O
Parr	O
pointed	O
out	O
that	O
even	O
though	O
most	O
programmable	B-Architecture
controllers	I-Architecture
require	O
physical	O
keys	O
and	O
passwords	O
,	O
the	O
lack	O
of	O
strict	O
access	O
control	O
and	O
version	O
control	O
systems	O
,	O
as	O
well	O
as	O
an	O
easy-to-understand	O
programming	O
language	O
make	O
it	O
likely	O
that	O
unauthorized	O
changes	O
to	O
programs	O
will	O
happen	O
and	O
remain	O
unnoticed	O
.	O
</s>
<s>
Modern	O
programmable	B-Architecture
controllers	I-Architecture
generally	O
contain	O
a	O
real-time	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
which	O
can	O
be	O
vulnerable	O
to	O
exploits	O
in	O
a	O
similar	O
way	O
as	O
desktop	O
operating	O
systems	O
,	O
like	O
Microsoft	B-Application
Windows	I-Application
.	O
</s>
<s>
,	O
these	O
concerns	O
have	O
grown	O
as	O
networking	B-Architecture
is	O
becoming	O
more	O
commonplace	O
in	O
the	O
PLC	O
environment	O
connecting	O
the	O
previously	O
separate	O
plant	O
floor	O
networks	O
and	O
office	O
networks	O
.	O
</s>
<s>
In	O
February	O
2021	O
,	O
Rockwell	O
Automation	B-Application
publicly	O
disclosed	O
a	O
critical	O
vulnerability	O
affecting	O
its	O
Logix	O
controllers	O
family	O
.	O
</s>
<s>
Secret	O
cryptographic	O
key	O
used	O
to	O
verify	B-Algorithm
communication	I-Algorithm
between	O
the	O
PLC	O
and	O
workstation	O
can	O
be	O
extracted	O
from	O
Studio	O
5000	O
Logix	O
Designer	O
programming	O
software	O
and	O
used	O
to	O
remotely	O
change	O
program	O
code	O
and	O
configuration	O
of	O
connected	O
controller	O
.	O
</s>
<s>
These	O
differ	O
from	O
conventional	O
PLC	O
types	O
by	O
being	O
suitable	O
for	O
safety-critical	O
applications	O
for	O
which	O
PLCs	O
have	O
traditionally	O
been	O
supplemented	O
with	O
hard-wired	O
safety	B-Algorithm
relays	I-Algorithm
and	O
areas	O
of	O
the	O
memory	O
dedicated	O
to	O
the	O
safety	O
instructions	O
.	O
</s>
<s>
PLCs	O
are	O
well	O
adapted	O
to	O
a	O
range	O
of	O
automation	B-Application
tasks	O
.	O
</s>
<s>
These	O
are	O
typically	O
industrial	O
processes	O
in	O
manufacturing	O
where	O
the	O
cost	O
of	O
developing	O
and	O
maintaining	O
the	O
automation	B-Application
system	O
is	O
high	O
relative	O
to	O
the	O
total	O
cost	O
of	O
the	O
automation	B-Application
,	O
and	O
where	O
changes	O
to	O
the	O
system	O
would	O
be	O
expected	O
during	O
its	O
operational	O
life	O
.	O
</s>
<s>
PLCs	O
contain	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
devices	O
compatible	O
with	O
industrial	O
pilot	O
devices	O
and	O
controls	O
;	O
little	O
electrical	O
design	O
is	O
required	O
,	O
and	O
the	O
design	O
problem	O
centers	O
on	O
expressing	O
the	O
desired	O
sequence	O
of	O
operations	O
.	O
</s>
<s>
Programmable	B-Architecture
controllers	I-Architecture
are	O
widely	O
used	O
in	O
motion	O
,	O
positioning	O
,	O
or	O
torque	O
control	O
.	O
</s>
<s>
Similar	O
to	O
traditional	O
PLCs	O
,	O
but	O
their	O
small	O
size	O
allows	O
developers	O
to	O
design	O
them	O
into	O
custom	O
printed	O
circuit	O
boards	O
like	O
a	O
microcontroller	B-Architecture
,	O
without	O
computer	O
programming	O
knowledge	O
,	O
but	O
with	O
a	O
language	O
that	O
is	O
easy	O
to	O
use	O
,	O
modify	O
and	O
maintain	O
.	O
</s>
<s>
It	O
's	O
between	O
the	O
classic	O
PLC	O
/	O
Micro-PLC	O
and	O
the	O
Microcontrollers	B-Architecture
.	O
</s>
<s>
For	O
high-volume	O
or	O
very	O
simple	O
fixed	O
automation	B-Application
tasks	O
,	O
different	O
techniques	O
are	O
used	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
cheap	O
consumer	O
dishwasher	B-Application
would	O
be	O
controlled	O
by	O
an	O
electromechanical	O
cam	O
timer	O
costing	O
only	O
a	O
few	O
dollars	O
in	O
production	O
quantities	O
.	O
</s>
<s>
A	O
microcontroller-based	O
design	O
would	O
be	O
appropriate	O
where	O
hundreds	O
or	O
thousands	O
of	O
units	O
will	O
be	O
produced	O
and	O
so	O
the	O
development	O
cost	O
(	O
design	O
of	O
power	O
supplies	O
,	O
input/output	B-General_Concept
hardware	O
,	O
and	O
necessary	O
testing	O
and	O
certification	O
)	O
can	O
be	O
spread	O
over	O
many	O
sales	O
,	O
and	O
where	O
the	O
end-user	O
would	O
not	O
need	O
to	O
alter	O
the	O
control	O
.	O
</s>
<s>
Single-board	B-Device
computers	I-Device
using	O
semi-customized	O
or	O
fully	O
proprietary	O
hardware	O
may	O
be	O
chosen	O
for	O
very	O
demanding	O
control	O
applications	O
where	O
the	O
high	O
development	O
and	O
maintenance	O
cost	O
can	O
be	O
supported	O
.	O
</s>
<s>
"	O
Soft	O
PLCs	O
"	O
running	O
on	O
desktop-type	O
computers	O
can	O
interface	O
with	O
industrial	O
I/O	B-General_Concept
hardware	I-General_Concept
while	O
executing	O
programs	O
within	O
a	O
version	O
of	O
commercial	O
operating	O
systems	O
adapted	O
for	O
process	O
control	O
needs	O
.	O
</s>
<s>
The	O
rising	O
popularity	O
of	O
single	B-Device
board	I-Device
computers	I-Device
has	O
also	O
had	O
an	O
influence	O
on	O
the	O
development	O
of	O
PLCs	O
.	O
</s>
<s>
Traditional	O
PLCs	O
are	O
generally	O
closed	B-Application
platforms	I-Application
,	O
but	O
some	O
newer	O
PLCs	O
(	O
e.g.	O
</s>
<s>
groov	O
EPIC	O
from	O
Opto	O
22	O
,	O
ctrlX	O
from	O
Bosch	O
Rexroth	O
,	O
PFC200	O
from	O
Wago	O
,	O
PLCnext	O
from	O
Phoenix	O
Contact	O
,	O
and	O
Revolution	O
Pi	O
from	O
Kunbus	O
)	O
provide	O
the	O
features	O
of	O
traditional	O
PLCs	O
on	O
an	O
open	B-Application
platform	I-Application
.	O
</s>
<s>
These	O
are	O
similar	O
to	O
PLCs	O
and	O
are	O
used	O
in	O
light	O
industries	O
where	O
only	O
a	O
few	O
points	O
of	O
I/O	B-General_Concept
are	O
needed	O
,	O
and	O
low	O
cost	O
is	O
desired	O
.	O
</s>
<s>
Most	O
such	O
devices	O
include	O
a	O
tiny	O
postage-stamp-sized	O
LCD	O
screen	O
for	O
viewing	O
simplified	O
ladder	O
logic	O
(	O
only	O
a	O
very	O
small	O
portion	O
of	O
the	O
program	O
being	O
visible	O
at	O
a	O
given	O
time	O
)	O
and	O
status	O
of	O
I/O	B-General_Concept
points	O
,	O
and	O
typically	O
these	O
screens	O
are	O
accompanied	O
by	O
a	O
4-way	O
rocker	O
push-button	O
plus	O
four	O
more	O
separate	O
push-buttons	O
,	O
similar	O
to	O
the	O
key	O
buttons	O
on	O
a	O
VCR	O
remote	O
control	O
,	O
and	O
used	O
to	O
navigate	O
and	O
edit	O
the	O
logic	O
.	O
</s>
<s>
Most	O
have	O
a	O
small	O
plug	O
for	O
connecting	O
via	O
RS-232	O
or	O
RS-485	O
to	O
a	O
personal	B-Device
computer	I-Device
so	O
that	O
programmers	O
can	O
use	O
simple	O
applications	O
in	O
general-purpose	B-Language
OS	O
like	O
MS	B-Application
Windows	I-Application
,	O
macOS	B-Application
or	O
Linux	B-Application
,	O
that	O
have	O
user-friendly	O
(	O
G	O
)	O
UIs	O
,	O
for	O
programming	O
instead	O
of	O
being	O
forced	O
to	O
use	O
the	O
tiny	O
LCD	O
and	O
push-button	O
set	O
for	O
this	O
purpose	O
.	O
</s>
<s>
Unlike	O
regular	O
PLCs	O
that	O
are	O
usually	O
modular	O
and	O
greatly	O
expandable	O
,	O
the	O
PLRs	O
are	O
usually	O
not	O
modular	O
or	O
expandable	O
,	O
but	O
their	O
price	O
can	O
be	O
two	O
orders	B-Device
of	I-Device
magnitude	I-Device
less	O
than	O
a	O
PLC	O
,	O
and	O
they	O
still	O
offer	O
robust	O
design	O
and	O
deterministic	O
execution	O
of	O
the	O
logic	O
.	O
</s>
<s>
A	O
variant	O
of	O
PLCs	O
,	O
used	O
in	O
remote	O
locations	O
is	O
the	O
remote	B-Device
terminal	I-Device
unit	I-Device
or	O
RTU	O
.	O
</s>
<s>
An	O
RTU	O
is	O
typically	O
a	O
low	O
power	O
,	O
ruggedized	B-Device
PLC	O
whose	O
key	O
function	O
is	O
to	O
manage	O
the	O
communications	O
links	O
between	O
the	O
site	O
and	O
the	O
central	O
control	O
system	O
(	O
typically	O
SCADA	O
)	O
or	O
in	O
some	O
modern	O
systems	O
,	O
"	O
The	O
Cloud	O
"	O
.	O
</s>
<s>
Unlike	O
factory	O
automation	B-Application
using	O
high-speed	O
Ethernet	O
,	O
communications	O
links	O
to	O
remote	O
sites	O
are	O
often	O
radio-based	O
and	O
are	O
less	O
reliable	O
.	O
</s>
<s>
RTUs	O
,	O
being	O
PLCs	O
,	O
have	O
a	O
wide	O
range	O
of	O
I/O	B-General_Concept
and	O
are	O
fully	O
programmable	O
,	O
typically	O
with	O
languages	O
from	O
the	O
IEC	O
61131-3	O
standard	O
that	O
is	O
common	O
to	O
many	O
PLCs	O
,	O
RTUs	O
and	O
DCSs	O
.	O
</s>
<s>
On	O
sites	O
with	O
only	O
a	O
handful	O
of	O
I/O	B-General_Concept
,	O
the	O
RTU	O
may	O
also	O
be	O
the	O
site	O
PLC	O
and	O
will	O
perform	O
both	O
communications	O
and	O
control	O
functions	O
.	O
</s>
