Omron SYSMAC C series User manual

Cat. No. W83-E1-5
-|omron| C20
Programmable
Controller
USER'S
MANUAL

Introduction
Features
This user's manual! provides all the information you need to install,
operate, and maintain the OMRON SYSMAC-C20 Programmable
Controller, which can be used for a wide range of industrial applications.
The C20 is OMRON’s response to the demand for a low-cost, versatile
industrial control system that can be quickly installed and easily operated
by technicians with little or no experience with programmable controllers.
Improved cost effectiveness
All functions for a small-scale control system are integrated in a single
housing. The C20 can be used in the place of conventional relay control
panels and is ideally suited for economical control of manufacturing and
other applications which require fewer input/output control points.
Slim profile for convenient mounting
The C20's compact design allows it to be installed in locations where
space is at a premium. Hardly larger than a standard text book, the C20
can easily be mounted either horizontally or vertically near the machinery
it operates.
Choice of memory chips
Either RAM or EPROM memory chips can be installed in the C20. RAM
offers the convenience of a read/write memory for control programs that
are under development or subject to modification; EPROM chips can be
used for “permanent” storage for rarely changed programs or when
multiple copies are needed for programs.
Optional optical-fiber communication
As with the other models of the SYSMAC C-Series programmable
controllers, I/O linkage can be accomplished using a remote I/O master
unit and optical-fiber cables. This state-of-the-art technology enables
noise-free communications over long distances at ultra-high speeds.
Detachable terminal block
An easily removable terminal block greatly facilitates wiring the I/O lines
and makes maintenance more convenient.
Upwardly compatible peripheral devices
The programming console, printer interface unit, and PROM writer can be
shared with all programmable controllers in the SYSMAC C-Series.
Moreover, with the addition of a peripheral interface, it is possible to
program and operate the PC through a portable graphic programming
console (either LCD or CRT) as well as a multisupport base unit.

Introduction
About this manual
This
manual
was
designed,
written,
and
illustrated
to
be
highly
useful
to
people
at
ali
levels
of
understanding
and
experience
with
program-
mable
controllers,
including
the
first-time
user.
it
has
also
been
organized
and
indexed
to
allow
easy
access
to
specific
information.
The
only
special
knowledge
you
need
is
an
understanding
of
ladder
diagrams
using
the
familiar
symbols
adopted
for
conventional
relay-
based
industrial
systems.
Here's
what
you
will
find
in
the
following
chapters
and
appendixes:
Chapter 1
Programmable
controllers
in
general
and
the
C20
in
particular
are
introduced.
Each
part
of
the
C20
is
illustrated
and
explained,
and
different
system
configurations
are
presented.
Chapter 2
What
you
need
to
know
to
install
the
C20
is
explained,
including
important
environmental,
wiring,
mounting,
and
power
supply
considerations.
Chapter 3
You're
introduced
to
programming
and
the
eight
steps
used
to
create
a
PC
control
program.
This
chapter
focuses
on
the
first
three
—
assessing
the
controlied
operation,
assigning
input/output
points,
and
writing
relay
ladder
diagrams.
Chapters 4 and5
These
two
chapters
cover
using
the
programming
console
to
write
your
application
program
and
put
it
to
work
controlling
your
equipment
or
process
through
the
C20.
Chapter 6
Examples
are
given
which
show
you
in
simplified
form
some
of
the
numerous control applications that can be realized by the C20.
Appendixes
The
many
peripheral
devices
available
to
expand
the
C20
system
and
provide
valuable
support
services
are
introduced.
Also
covered
are
the
basic
and
special
instructions
used
in
programming,
as
well
as
product
specifications
and
other
detailed
technical
information.
A
comprehensive
index
is
included
for
easy
reference
to
this
manual.

Contents
Chapter 1 Introduction to the C20
1-1 Overview
4-1 PC Basics
1-1 input/output section
1-2 Central processing unit
1-2 Programming device
1-2 About the SYSMAC-C20
1-2 €20 Components
1-2 CPU
1-4 Programming console
1-5 Basic system configuration
Chapter 2 Installing the C20
2-1 Overview
2-1 But first...
l 2-1 Static electricity
2-1 Memory chips
2-2 Installing memory chips
2-3 Testing the environment
2-3 Dealing with problems of noise
2-3 Control panel wiring
2-3 External wiring
2-4 Noise-suppression measures
2-4 Power supply
2-4 Grounding
2-5 Control panel mounting
2-5 Mounting dimensions
2-6 Power supply
2-6 Grounding
2-7 Battery
2-7 Protections against power failure
, Chapter 3 Introduction to programming
J 3-1 Overview
3-1 Relay ladder diagram method
3-1 Eight basic programming steps
3-2 Step 1—Assessing the controlled operation
3-2 input/output requirements
3-2 Sequence, timing, and relationship assessment
3-3 Step 2—Assigning I/Os
3-3 Points and channels
3-3 I/O channe! assignment
3-3 Assigning internal auxiliary relays
3-4 Assigning numbers to timers and counters
3-4 Step 3—Writing a relay ladder diagram
3-5 Step 4—Turning the ladder diagram into PC code
3-6 Sample program for coding practice

Contents
Chapter 4 Using the programming console
4-1 Overview
4-1 The keyboard
4-1 Numeric keys
4-1 CLR key
4-2 Operation keys
4-2 Instruction keys
4-3 Made switch
4-3 Console display
4-4 PROGRAM mode
4-4 To erase existing memory
4-5 Getting started, summarized
4-5 Exampie program
4-6 Step 5—Entering the program in the CPU
4-9 Deleting instructions
4-9 Inserting instructions
Chapter
5
Checking
and
running
your
program
5-1
Overview
5-1
Step
6—Editing
the
program
5-1
Quick-search
editing
functions
5-1
Going
directly
to
a
known
address
5-2
Searching
for
a
specific
instruction
5-3
NO
END
INSTRUCTION
message
5-3
Searching
relay
contacts
5-4
Search
function
summary
5-4
Address
search
5-4
Specific
instruction
search
5-4
Specific
contact
search
5-5
Step
7—Testing
for
errors
5-5
Status
check
5-5
Forced
set/reset
5-6
Forced
relay
set
5-6
Forced
relay
reset
5-7
Forced
timer
set
5-7
Changing
set
value
of
timer
or
counter
5-8
Input/Output
monitor
5-9
Rapid
check
of
counter/timer
values
5-9
Rapid
check
of
relay
status
5-10
Step
8—Saving
your
program
to
cassette
tape
5-12
Loading
and
verifying
the
program
5-12
Verifying
the
program
Chapter
6
Application
examples
6-1~
Overview
6-1
Automatic
control
of
warehouse
door
6-3
Automatic
lubricating
oil
supplier
6-4
Conveyor
belt
motor
control
6-6
Automatic
car
washing
machine
6-7
Bottle
label
detection

Contents
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
Appendix G
Appendix H
System expansion and peripherals
A-1 Overview
A-2 Expansion 1/0 unit
A-3 1/0 link unit
A-5 1/0 link unit wiring
A-6 Channel and end station setting
A-14 PROM writer
A-14 Graphic programming console
A-17 Multisupport base
A-18 Printer interface unit
A-19 Link adapter
Instruction words
B-1 Overview
B-10 Special Instructions
Scan time
C-1 Overview
C-3 Listof instruction execution times
C-4 Sean time calculation examples
Assignment of 1/0 channel and relay numbers
D-1 1/0 channel assignment
D-1 Relay assignment
Maintenance and troubleshooting
E-1 Overview
E-1 Inspection
E-4 Troubleshooting
Specifications
F-{ Overview
F-1 Available types
F-3 Ratings
F-3 Characteristics
F-5 Input/Output specifications
F-19 10 Sink unit
F-20 Dimensions
Mounting kit
G-1 Overview
List of instructions
H-1 Basic instruction
H-2 Applied instructions

Chapter 1
Introduction to the C20
Overview
PC Basics
This introductory chapter explains why programmable controllers have
become such a valuable part of modernfactories. The SYSMAC-C20 is
described, including its basic components and operating principles.
Programmable controllers, or PCs, evolved as industries sought econo-
mical ways to automate their production lines, particularly those involved
in the manufacturing of equipment and other heavy industry products.
The PCs took the place of relay-based control systems which were
comparatively slower, less reliable, and which presented formidable
wiring and maintenance requirements.
PCs
operate
by
monitoring
input
signals
from
such
sources
as
pushbut-
tons,
sensors,
and
limit
switches.
When
changes
are
detected
in
the
signais,
the
controller
system
reacts,
through
user-programmed
internal
logic,
to
produce
output
signals.
These
signals
operate
the
external
loads
of
the
controlled
system,
such
as
relays,
motor
controls,
indicator
lights,
and
ajarms.
This
type
of
control
system
eliminates
much
of
the
wiring
and
rewiring
that
was
necessary
with
the
conventional
relay-based
system.
Instead,
the
programmed
logic
provides
the
“wiring
network”
which
can
be
changed
as
required
by
simply
reprogramming
the
PC.
Thus
the
automated
processes
of
a
production
line
can
be
controlled
and
modified
at
will,
for
highly
economical
adaptability
to
a
changing
manufacturing
environ-
ment.
Atypical programmable controller has three basic components—an
input/output
section,
a
central
processing
unit
(CPU),
and
a
programming
device.
Input/output section
This
section
consists
of
wiring
and
interfacing
relays
that
connect
the
PC
to the equipment being controlled.
Central processing unit
The
CPU
contains
the
control
circuitry
as
well
as
the
memory
that
stores
the
contro!
plan
that
guides
equipment
operation.
It
is
the
heart
of
the
PC
and
organizes
ail
controller
activity
by
scanning
the
control
plan
along
with
the
status
of
the
inputs
and
executes
specified
commands
to
specified
outputs.

Introduction
to
the
C20
About the SYSMAC-C20
C20 Components
CPU
Programming device
This
device
is
used
to
enter
the
control
plan
into
the
CPU's
memory.
An
operator
keys
in
the
instructions
used
to
sequentially
contro!
the
application
process.
There
are
several
programming
methods;
in
the
case
of
the
SYSMAC
C-Series,
relay
ladder
diagram
prog
ramming
is
used
because
plant
personnel
are
familiar
with
that
format
and
it
provides
for
convenient
record-keeping.
The
C20
comes
in
two
versions,
a
basic
unit
and
an
expandable
unit.
The
capabilities
of
the
basic
unit
include
up
to
1194
program
statements,
28
I/O
points,
and
136
internai
auxiliary
relays.
The
expandable
unit
is
functionally
identicai
to
the
basic
unit
except
that
it
can
be
expanded
using
expansion
/O
units
to
include
up
to
140
I/O
points.
The
C20
offers
the
flexibility
of
either
RAM-
or
EPROM-based
operation.
When
a
RAM
chip
is
installed
as
the
memory,
programs
written
by
the
user
can
be
modified
and
rewritten.
EPROM
provides
a
semi-permanent
storage
for
completed
programs.
The
programming
console
can
be
detached
and
is
upwardly
compatible
with
the
full
line
of
SYSMAC
C-Series
programmabie
controllers.
Due
to
its
compact
design,
the
C20
incorporates
the
detachable
I/O
terminals
and
microprocessor
functions
in
a
single
housing
called
the
CPU.
The
detachable
programming
console
functions
as
the
program-
ming
device.
Additionally,
various
optional
peripheral
devices
are
available
to
support
system
expansion.
The
two
main
components
are
explained
and
illustrated
in
the
next
several
pages.
Central processing unit
The
CPU
is
the
main
enclosure
of
the
C20
incorporating
the
micro-
processor
and
I/O
terminals.
The
controlling
program
written
using
a
programming
device
is
stored
and
executed
in
the
CPU.

1 Console screws
These stud screws are removed with a flat-bladed screwdriver to open
the cover for initial setting and maintenance purposes.
2 Input terminals
These terminals are used to connect the wiring carrying input signals
from the devices of the controlled system.
3 Input/output indicators
These LEDs show the state of the signals (by turning red when ON and
dark when OFF) for each of the input and output terminals.
4 LED indicators (POWER, RUN, ERROR/ALARM)
These LEDs light up to show the state of the main power supply
(ON/OFF), when the C20 is in the RUN mode, and if any error has
occurred.
5 Power terminals
These terminals connect the AC or DC power source and are grounded at
a resistance of less than 100Q.
6 Output terminals
These terminals are used to connect the wiring carrying output signals
from the CPU to the output devices.
7 Connector for expansion 1/0 units
This is available only on the expandable type CPU and is used to connect
to expansion I/O units to increase the number of I/O points.
8 Connector for peripheral devices
This connects the programming console or another peripheral device.
PC

Introduction to the C20
Programming console
This is the standard programming device used with the C20. The control
programs written by the programming console are stored and run in the
CPU.
PROIS
PROGRAMMING CONSOLE
9 Mounting screws
These two screws secure the detachable programming console to the
CPU.
10 LCD
This displays the program as it is being written and is used for checking
and monitoring program operation. It also displays error messages.
11 Contrast control
The contrast control adjusts contrast level of display.
12 Keypad
The color-coded keypad is functionally divided into the following areas:
White 10 keys used to input program addresses, timing values, and
other types of numeric entry
Red One key used to clear the display
Yellow 12 keys used to provide editing functions while writing and
correcting the control program
Gray 16 keys used to input instruction words used in the program
The function of each key is detailed in Chapter 4.
13 Mode selector switch
This three-position switch selects one of three operation modes of the PC:
program, monitor, and run. These modes are explained in detail in
Chapters 4, and 5.
14 Jacks for connecting cassette tape recorder
Programs
may
be
saved
to
a
standard
cassette
tape
recorder
connected
to
the
output
(MIC)
jack.
Previously
written
programs
can
also
be
supplied
to
the
CPU
via
the
input
(EAR)
jack.
These
operations
are
explained
in
Chapter
5.

Introduction to the C20
Basic system configuration
LAMA 1
The figure below shows some of control systems employing the C20. The
maximum number of I/O points per system is 28 if the DC input type
nonexpandable CPU is used. With the AC input type, it is 26.
With the DC input type CPU, the number of I/Os can be increased to 56,
84, 112, or 140 points. This is accomplished by using up to two expansion
\/O units. With the AC type. the number of |/O points can be 52, 78, 104, or
130.
i í f
Expandable CPU
Expandable CPU
Data
from/to
C€500/C250/C 120
via SYSBUS
I se
VO link unit
Expansion I/O unit Expansion I/O unit
Programming console
(horizontal type)
Programming console
(vertical type)
Peripheral devices connectable to C20
Peripheral interface unit
unit
Graphic programming Graphic programming Multisupport base
console (GPC) console (CRT) (MSB)
Note:
Although there are several additional peripheral devices available for
other PCs in the SYSMAC C-Series, only those shown here can be used
with the C20.
1-5

Chapter 2
Installing the C20
Overview
But first...
This chapter tells you how to install the C20. You may wish to skip this
section and go first to Chapter 3, Introduction to programming, which
provides important information on how to determine the configuration of
your controlling system.
The following are some important points that should always be observed
to ensure correct installation and operation of the C20.
Static electricity
in low-humidity conditions, static electricity may cause problems with the
C20's components. Therefore, before touching the C20, be sure to first
touch a grounded metallic object to discharge any static electricity
buildup.
Memory chips
The C20 offers a choice of using either a RAM or EPROM chip for the
program memory, according to your application.
With the nonexpandabie CPU, there are stiil 170 program addresses
available as standard even when no RAM chip is mounted and by
mounting a RAM chip, the number of addresses can be increased to 512.
With the expandable CPU, the standard memory capacity is 1194
addresses.
The foltowing table provides the necessary information as to the type of
chip that can be used in the expandable and nonexpandable type CPUs
as well as the DIP switch settings required for each type of chip.
[_Wonevpandabie CPU | cpana U O
RAM
Use
Type
RAM-G
|
RAM
RAM
not
required
Memory capacity | 512 addresses Memory capacity | 1,194 addresses
DIP switch setting DIP switch setting
quana E
12 3 1234
4
EPROM | Use Type ROM-H
Memory capacity | 1,194 addresses
DIP switch setting
TARAN
123 4
EPROM Use Type ROM-H
Memory capacity ‘| 1,194 addresses
DIP switch setting
TAUB
123 4

Installing the C20
Installing memory chips
The procedure for installing memory chips is described below:
1. Use the information given in the table above to confirm that the type of
chip you have is correct.
2. Remove the front panel of the CPU by loosening the mouting screws
with a flat-bladed screwdriver. You'll be able to see the CPU’s circuit
board.
Caution: Before opening the CPU, always be sure that the power supply to the C20
has been turned OFF.
Location of memory chip and
DIP switch
RS MMMMany
3. Insert the RAM or EPROM chip into the 28-pin IC socket on the lower
left corner of the circuit board.
4. Set the DIP switch to the correct setting for the type memory chip you
have installed. For this, refer to the table on the previous page.

Installing the C20
Testing the environment
Dealing with problems of
noise
Noise source
C20
200mm min.
Grounding (at a ground resistance of
less than 100Q)
© SYSMAC I/O circuit
@ SYSMAC power circuit
General control circuit
@ Power circuit
When you're ready to install the C20, first give attention to the
environmental conditions under which it will normally operate. The PC is
ruggedized for reliable use under tough conditions, but you still need to
avoid using it in these areas:
Where the ambient temperature is below 0° or above 55°C.
Where abrupt temperature changes may cause condensation.
Where relative humidity is below 35% or above 85%.
Where corrosive or flammable gas may occur.
In extremely dusty conditions, or where subject to salt or iron particles.
Where subject to abnormal vibration or shock.
In direct sunlight.
Where it can be splattered with water, oil, or chemical substances.
One of the most important considerations when installing and wiring the
C20 is the problem of noise. The C20 should be mounted at least 200mm
from high-tension equipment or wiring.
Control panel wiring
Wherever possible, always use wiring ducts to contain and protect the
wiring for the I/O units. I/O wires should not be placed in the same duct
with a power line or other wiring.
External wiring
Use standard wiring conduits for routing I/O and power lines to the C20.
However, be sure to use separate conduits for the 1/0 wiring.
The following points are important when the controlled system requires
the laying of 400V 10A max. or 220V 20A max. power lines.
A minimum distance of 300mm must be provided between the |/O lines
and power cables when their conduits are run parallel to each other.
lf the cables must be placed in the same duct at the point of connection to
the equipment, be sure to screen them with a grounded metal plate.
Metal (iron) plate
Z a 200mm min.
Grounding (at a ground resistance of less than 1000)
2-3

Installing the C20
Noise-suppression measures
i Be sure to take appropriate noise-suppression measures when any
1 | i electrical device likely to produce noise is employed as a PC output load.
For example, electromagnetic relays and valves generating noise of more
than 1,200V require noise suppression.
Surge
suppressor For AC-supplied noise sources, connect a surge suppressor in parallel
with the coil of each device. For DC-operated noise sources, connecta
AC power source diode in parallel with the coil of each device.
When mounting a CPU and I/O expansion unit in a contro! panel, be sure
to completely ground the intermediate mounting plate. The mounting
plate must be finished with high-conductivity plating to insure noise
immunity.
Power supply
Where possible, use independent power sources for the input, the load,
and for the CPU and |/O units.
© DC power source
Input
power
supply
External
power
Load power supply
=
Cain |
aes Se
Input device ) Output device | Input device Output device
C20 CPU Expansion I/O unit
ty
p--i-L--
\ Power supply f
DC 24V/AC 100 to 120V/AC 200 to 240V
Grounding
Terminal LG is a noise filter neutral terminal which does not normally
require grounding. When electrica! noise is a problem, however, this
terminal should be short circuited with terminal FG.

Installing the C20
Control panel mounting
40
p= aes 200 — 825)
ppa peed t
H! HT
i CPU } | Expansion i oo
! H i| YOunit IRA
1
l
i
t
itt
l
[ae 210 30 210
4-M4 4-M4
eM ge wv th a
Pict yerery
Pipi eee ee 2 ere ef
Peripheral device
CERTiageuee ee
ra CERST CTEE
Some of the main considerations when mounting the PC in a control panel
are accessibility for operation and maintenance and protection against
heat.
These are some of the things you should consider:
Use M4x25 mounting screws.
Provide the PC with adequate space for ventilation.
Avoid mounting the controller directly above any heat-generating source,
such as a heater, transformer, or high-capacity resistor.
If the ambient temperature is above 55°C within the panel, you must install
a fan for forced-air ventilation.
Avoid mounting the PC in a panel in which high-tension wiring or
equipment is also installed.
Use wire with a cross-sectional area of at least 2mm?(AGW14) to prevent
possible voltage drop. Use twisted pair cables for the wiring.
Mounting dimensions
When the CPU and I/O unit are mounted together in the control panel, a
distance of about 30mm should be maintained between the two units.
This means that the holes for the CPU mounting screws and those for the
1/0 unit mounting screws should be separated by about 40mm.
Although the CPU is actually 59.5mm thick (measured at the I/O terminal
block), be sure to leave enough room in front of the unit to allow for
instailation of optional peripheral devices, such as the programming
console.
When the programming console is attached to the PC, the total depth
increases to 68mm.

Installing the C20
Power supply
Use a commercially available DC 24V, AC 100 to 120V or AC 200 to 240V
power supply for the C20.
As the DC 24V power supply, Type 3G2A3-PS221 is available. Here are
the electrical specifications.
Power supply unit
Characteristics 3G2A3-PS221
AC 100/110/120V (AC 85 to 132V)/AC 200/220/240V
Input voltage {AC 170 to 264V), 50/60Hz+7Hz (AC 200V is the
factory-set condition)
Output voltage 24V+5%
Output capacity 1.5A max. (36W)
Efficiency 70% min.
Power failure Retains 10ms max.
Inrush current 5A max.
Fuse 2A, incorporated
Leakage current imA max. between FG terminal and ground
560g
Where possible, use independent power sources for the inputs, the load,
and for the CPU and expansion |/O units.
Grounding
Attach a dedicated grounding wire with a cross-sectional area of at least
2mm*(AWG14) to terminal FG to lessen the possibility of electrical shock.
Ground resistance must be less than 100Q.
Do not use a grounding wire longer than 20m. Care must be taken
because ground resistance is affected by the nature of the ground, water
content, season, and the amount of time that has elapsed since the wire
was laid underground.
In case a large noise occurs causing the C20 to malfunction, short-circuit
terminals LG and FG for grounding at a resistance of 100Q.
PC operation may be adversely affected if the grounding wire is shared
with other equipment or if grounding is attempted by attaching the
grounding terminal to the metal superstructure of a building.
Other equipment
Other equipment

Installing the C20
When the I/O expansion unit is used with the CPU, the FG terminals on
both devices are used for grounding.
Battery
The CPU has a built-in battery to back up the memory in case of power
failure. This backup battery is required regardless of whether a RAM or
EPROM chip is used as the program memory. The battery's service life is
about five years when the PC is used at a temperature of 25°C, and is
somewhat shortened in temperatures higher than this or where humidity
is unusually high. These factors should also be kept in mind when storing
the PC.
When the battery is almost completely discharged, the ALARM/ERROR
indicator on the CPU blinks. You then have one week to change the
battery before memory loss occurs. A fresh battery must be installed
within 3 minutes after the old one has been removed from its connector.
Protections against power failure
A power sequence circuit is incorporated in the PC to prevent
malfunctioning because of a momentary power failure or a drop in the
supply voltage.
If the voltage drops below 85%, the PC stops operating and the external
output relays are automatically turned off.
ACH
P
oS
:
Momentary
power
failure
PF
(power failure “| detection time | =
detection signal) : $5 jo
+5Ve
GRE {power reset]
co
4$
Approx. c
10msec Approx. 1sec k
$f
Relay output
2-7

Installing the C20
The PC ignores all momentary power failures when the interruption tasts
no longer than 10msec. If the interruption is more than 10msec, but less
than 20msec, it may or may not be detected. Failures longer than 20msec
will cause the PC to stop operating; external output relays are
automatically turned off.
Operation automatically resumes when the supply voltage is restored to
more than 85% of the rated voltage.
When the expansion I/O unit (3G2C7-MC ) is used, an external
power supply is required to drive the relays. This external power supply is
not necessary when only the CPU is used.
External power supply|
(for relay driving)
SYSMAC-C20 Expansion I/O unit
(8G2C7-MCOO0)
When a two-wire sensor is used for the input, the input signal may be
turned ON by a leakage current exceeding 3mA. To prevent this, connect
a bleeder resistor of 7.5kO or less as shown.
Power source
C20 input circuit
Two-wire
sensor

Chapter 3
Introduction to programming
y
Overview
This chapter introduces the eight steps you'll take in creating the program
the PC uses to carry out its control operations. You can skip this chapter if
you're already familiar with PC programming using relay ladder dia-
grams. Instead, you may wish to go directly to Chapters 4 and 5 which
focus on how the C20 programming console is used to install the program
in the memory of the CPU.
Relay ladder diagram method
If you're an old hand at relay-based control systems, you'll find a lot that's
familiar in the way a PC works. This is because the circuits and internal
logic of the PC take the place of the relays, timers, counters, and other
formerly discrete devices. The actual operation of the machinery takes
place as if those discrete devices were still in place, but with a great deal
more flexibility and reliability.
But even if the actual devices are gone, the symbols and other control
concepts used to describe their operation still are used. These are the
basis of the relay ladder diagram programming method. This chapter has
been written on the assumption that you are already experienced with this
relay symbology, or have access to information that can teach you about
it.
Eight basic programming steps
In creating the control program for the PC, you'll foliow these eight basic
steps:
1. Determine what the controlled system must do and in what order.
2. Assign input and output devices, that is, designate the external
devices that will send signals to and receive signals from the PC.
3. Draw a diagram, using relay ladder symbols. This represents, in the
correct sequence, all of the required functions and their relationship.
4. Code the ladder symbols into a form that can be input to the CPU by
the programming console.
5. Transfer these written instructions to the CPU via the programming
console.
6. Edit the program.
7. Test the program for errors.
8. Save the completed program.
Steps 1 through 4 are the focus of this chapter.