LADDER DIAGRAM
A ladder diagram is a means of graphically
representing the logic required in a relay logic
system.
Rail
start
PB1
emergency stop
PB2
R1
Rung
R1
R1
A
PLC WIRING DIAGRAM
A
Input
01
B
02
Output
PLC
01
02
20
11
12
20
03
20
11
External
switches
Stored program
C
SCAN
A PLC resolves the logic of a ladder diagram
(program) rung by rung, from the top to the
bottom. Usually, all the outputs are updated
based on the status of the internal registers.
Then the input states are checked and the
corresponding input registers are updated. Only
after the I/Os have been resolved, is the
program then executed. This process is run in a
endless cycle. The time it takes to finish one
cycle is called the scan time. In some controllers
the idle state is eliminated. In this case, the scan
time varies depends on the program length.
PLC Scan
Output
Input
begin
Idle
Scan cycle
Resolve
logic
PLC Ladder Diagram
INSTRUCTIONS







1) Relay,
2) Timer and counter,
3) Program control,
4) Arithmetic,
5) Data manipulation,
6) Data transfer, and
7) Others, such as sequencers.
LOGIC STATES


ON : TRUE, contact closure, energize, etc.
OFF: FALSE, contact open , de-energize,
etc.
Do not confuse the internal relay and program with the external
switch and relay. Internal symbols are used for programming.
External devices provide actual interface.
(In the notes we use the symbol "~" to represent
negation. AND and OR are logic operators. )
PROGRAMMING
Normally Open
(NO)
Normally Closed
(NC)
Power flows through these contacts when they are closed. The
normally open (NO) is true when the input or output status bit
controlling the contact is 1. The normally closed (NC) is true
when the input or output status bit controlling the contact is 0.
7
Coils
Coils represent relays that are energized when power
flows to
them. When a coil is energized it causes a
corresponding
output to turn on by changing the state of the status bit
controlling
the output to 1. That same output status bit maybe used
to control
normally open or normally closed contact anywhere in
the program.
8
Boxes
Boxes represent various instructions or functions that are
Executed when power flows to the box. Some of these
Functions are timers, counters and math operations.
9
AND OPERATION
A
B
C
Rung
Each rung or network on a ladder program represents
a logic operation. In the rung above, both inputs A and B
must be true (1) in order for the output C to be true (1).
10
OR OPERATION
A
C
Rung
B
In the rung above, it can be seen that either input A or B
is be true (1), or both are true, then the output C is true (1).
11
NOT OPERATION
A
C
Rung
In the rung above, it can be seen that if input A is be true (1),
then the output C is true (0) or when A is (0), output C is 1.
AND and OR LOGIC
PB1
PB2
R1
R1 = PB1.AND.PB2
AND
PB3
PB4
R2
R2 = PB2.AND.~PB4
PB1
R1
R1 = PB1 .OR. PB2
OR
PB2
COMBINED AND & OR
R1 = PB1 .OR. (PB2 .AND. PB3)
R1
PB1
PB2
PB3
PROGRAMMING EXAMPLE 1
Bar code reader
microswitch
Stopper
Conveyor
Part
Robot
Machine
Operation
id
MSI
R1
C1
R2
R3
C2
R4
C3
C4
description
microswitch
output to bar code reader
input from bar code reader
output robot
output robot
input from robot
output to stopper
input from machine
input from machine
state
1
1
1
1
1
1
1
1
1
explanation
part arrive
scan the part
right part
loading cycle
unloading cycle
robot busy
stopper up
machine busy
task complete
PLC WIRING DIAGRAM
Input
MS1
C1
C2
C3
C4
01
02
03
04
05
Programmable
Controller
PLC
Output
11
12
13
14
15
R1
R2
R3
R4
Operation




Rung 1. If part arrives and no part is stopped,
trigger the bar code reader.
Rung 2. If it is a right part, activate the
stopper.
Rung 3. If the stopper is up, the machine is
not busy and the robot is not busy, load the
part onto the machine.
Rung 4. If the task is completed and the robot
is not busy, unload the machine.
PLC Ladder
01
11
14
14
02
14
04
05
03
03
12
13
Sequential Function Chart
Action Qualifiers:
N
non-stored, executes while the
step is active
R resets a store action
S sets an action active
L time limited action, terminates
after a given period
D time delayed action.
P a pulse action, executes once in a
step
SD stored and time delayed
DS time delayed and stored
SL stored and time limited
Sequential Function Chart
A Detailed Design Process

A Detailed Design Process
1. Understand the process
2. Hardware/software selection
3. Develop ladder logic
4. Determine scan times and memory
requirements
Specifications

OUTPUT-PORT POWER RATINGS

Each output port should be capable of
supplying sufficient voltage and current to
drive the output peripheral connected to it.

SCAN TIME
This is the speed at which the controller
executes the relay-ladder logic program. This
variable is usually specified as the scan time
per 1000 logic nodes and typically ranges
from 1 to 200 milliseconds.
MEMORY CAPACITY
The amount of memory required for a
particular application is related to the length
of the program and the complexity of the
control system. Simple applications having
just a few relays do not require significant
amount of memory. Program length tend to
expand after the system have been used for
a while. It is advantageous to a acquire a
controller that has more memory than is
presently needed.

PLC Status Indicators
•
Power On
•
Run Mode
•
Programming Mode
•
Fault
Troubleshooting
1. Look at the process
2. PLC status lights
HALT - something has stopped the CPU
RUN - the PLC thinks it is OK (and probably is)
ERROR - a physical problem has occurred with
the PLC
3. Indicator lights on I/O cards and sensors
4. Consult the manuals, or use software if available.
5. Use programming terminal / laptop.
List of items required when
working with PLCs:
1. Programming Terminal - laptop or desktop PC.
2. PLC Software. PLC manufacturers have
their own specific software and license key.
3. Communication cable for connection from Laptop
to PLC.
4. Backup copy of the ladder program (on diskette,
CDROM, hard disk, flash memory). If none, upload it
from the PLC.
5. Documentation- (PLC manual, Software manual,
drawings, ladder program printout, and Seq. of
Operations manual.)
Examples of PLC
Programming Software:
1. Allen-Bradley – Rockwell Software RSLogix500
2. Modicon - Modsoft
3. Omron - Syswin
4. GE-Fanuc Series 6 – LogicMaster6
5. Square D- PowerLogic
6. Texas Instruments – Simatic
7. Telemecanique – Modicon TSX Micro
8. Mitshibishi – MelSoft (GX Developer)
Outputs & Power Supply
Communication
Ports (RS-485)
PLC
Inputs
PLC Internal Architecture
PLC Input/Output
PLC Input Devices

Push buttons

Switches (limit switches, level switches, etc.)

Sensors

...
PLC Output Devices

Relay contacts

Solenoid valves

Signal devices (such as lamps, alarms, etc.)

Motors

...
Programming terminal
Programming terminal

Programming is done through programming
terminal

Programming terminal translates engineering
language (logic control) to machine language
(binary code)
Programming through standard
computer

Most PLC manufacturers offer software
packages that allow a standard computer to be
used as a programming terminal
Programming through standard
computer
Relating the program to inputs
and outputs
SWITCHES
Non-locking
Locking
Normally Ope n
Normally Clos e d
DPST
P1
SPDT
P2
Multiple Throw
Multiple Pole
Bre ak-before -make
Make -be fore -bre ak
TERMS
Throw - number of states
Pole - number of connecting moving parts (number of
individual circuits).
SPDT
A serial switch box (A-B box) has
two 25 pin serial ports to switch from.
A
B
Output
DPST
Input
Knob
How is this switch classified?
TYPES OF SWITCHES








RATING:
Selector switches
Pushbutton switches •24 Volts AC/DC
•48 Volts AC/DC
Photoelectric
•120 Volts AC/DC
•230 Volts AC/DC
switches
•TTL level
(Transistor-to-transistor
Limit Switches
±5V)
Proximity switches •Isolated Input
Level switches
Thumbwheel
switches
Slide switches
RELAYS
A switch whose operation is activated by an electromagnet is called a "relay"
contact
coil
input
Relay coil
R1
Output contact
R1
RELAY Ladder Logic
Example 1:
For a process control, it is desired to have the
process start (by turning on a motor) five seconds
after a part touched a limit switch. The process is
terminated automatically when the finished part
touches a second limit switch. An emergency
switch will stop the process any time when it is
pushed.
L1
PB1
LS1
LS2
R1
R1
TIMER
R1
R2
PR=5
PB1
LS1
LS2
TIMER
5
R1
Motor
R2
Example 2:
One motor with two pushbuttons: start and stop
State variables: PB1(for start), PB2(for stop), M (for
motor)
Logic

PB1 is on -> CR1 energized, normally open
contact 1 is closed -> M=1

PB2 is on -> CR2 energized, normally close
contact 2 is open -> M=0

Rung 1: CR1=(PB1+CR1) CR2

Rung 2: CR2=(PB2)

Rung 3: M=CR1 CR2
Cylinder Pneumatic

Also called Actuator
ElectroPneumatic Valve

Directional Control Valve which acts as a ‘switch’
to direct compressed air to each side of
pneumatic actuator.
5-Port 2 Way Valve

Also called Double Acting Pneumatic Actuator and 5/2
way solenoid operated directional control valve.

Two ports to allow air in, one for outstroke (extend) and
one for in-stroke (retract).
Cylinder & Valve Assembly
TIMER
TIMER
A timer consists of an internal clock, a count value register, and an
accumulator. It is used for or some timing purpose.
Clock
Accumulator
reset
Register
contact
Contact
output
Clock
Reset
Output
Count
0
1
2
3
4
Time 5 seconds.
5
ON-DELAY TIMER (TON)
For this example the timer has been set for 5 seconds.
When S1 is closed, TR1 begins timing. When 5
seconds have elapsed, TR1 will close its associated
normally open TR1 contacts, illuminating pilot light
PL1. When S1 is open, de-energizing TR1,
the TR1 contacts open, immediately extinguishing
PL1. This type of timer is referred to as ON delay..
TON Example
When the switch is closed input 4 becomes a logic
1, which is loaded into timer T37. T37 has a time
base of 100 ms (.100 seconds). The preset time
(PT) value has been set to 150. This is equivalent
to 15 seconds (.100 x 150 ). The light will turn on
15 seconds after the input switch is closed.
Retentive On-Delay (TONR)
The Retentive On-Delay timer (TONR) functions
in a similar manner to the On-Delay timer
(TON). There is one difference. The Retentive
On-Delay timer times as long as the enabling
input is on, but does not reset when the input goes
off. The timer must be reset with a RESET (R)
instruction.
TONR Example
TONR Example Cont.
The same example used with the On-Delay timer
will be used with the Retentive On-Delay timer.
When the switch is closed at input I0.3, timer T5
(Retentive timer) begins timing. If, for example,
after 10 seconds input I0.3 is opened the timer
stops. When input I0.3 is closed the timer will
begin timing at 10 seconds. A RESET (R)
instruction can be added. Here a pushbutton is
connected to input I0.2. If after 10 seconds
input I0.3 were opened, T5 can be reset by
momentarily closing input I0.2. T5 will be reset
to 0 and begin timing from 0 when input I0.3 is
closed again.
OFF-DELAY (TOFF)
The Off-Delay timer is used to delay an output off
for a fixed period of time after the input turns
off. When the enabling bit turns on the timer bit
turns on immediately and the value is set to 0.
When the input turns off, the timer counts until
the preset time has elapsed before the timer bit
turns off.
TIMER MAX VALUES
TIMER EXAMPLE

Start PB Pressed >> Pump1 ON (5 sec) >>
Pump2 ON (3 sec) >> Mixer ON (60 sec) >>
Drain Valve ON >> Pump 3 (8 sec)
Counter
COUNTER
Digital counters output in the form of a relay contact when a preassigned count value is reached.
input
Register
5
Accumulator
reset
contact
output
Input
Reset
Output
Count
0
12
3
4
5
0
1
CTU, CTD, and CTUD
UP COUNTER (CTU)
Down Counter (CTD)
UP/DOWN COUNTER (CTUD)
COUNTER EXAMPLE
Example
Example (cont.)