1. Programmable Logic Controller Internal Operations

2. covers the internal operations of a PLC
Learning Objectives
This topic
covers the internal operations of a PLC
illustrates the concept of scan time
explains I/O response time
discusses practical issues of input signal interface with PLC
discusses practical issues of output signal interface with PLC
Chapter PLC3

3. Applications of PLCs: Sequence control Motion Control Process Control
Field bus
Communication
Data Management
Chapter PLC3

4. Internal Operation of PLCs: Input / Output Addressing
Chapter PLC3

5. Internal Operation of PLCs: Input / Output Addressing
000.00
010.01
PROGRAM
Chapter PLC3

6. Internal Operation of PLCs: Input / Output Addressing
000.00
010.01
PROGRAM
Reads Input
Executes Program
Sets Output
Chapter PLC3

7. Input / Output Addressing
Reads Input
Executes Program
On Output
Input / Output Processing
Most common method is mass input/output copying
Stores Output Status at Output RAM
Transfer to Output Module
Reads All Input Status
Executes
Program
Turns on Output
Updates Status at Input RAM
read one at a time
Decides which Outputs to turn on
These outputs are latched and they retain their status until they are updated by the next I/O copying routine.
Chapter PLC3

8. I/O Refresh
The process of writing from RAM to the output relays and reading input relay status to RAM is known as I/O Refresh.
I/O Refresh.
START
I/O Copy
Program Execution
End I/O Copy
Program
Copy all inputs
into RAM
Fetch, decode and execute all instructions in sequence
Copy all outputs from O/P
RAM to output unit and input all inputs to I/P RAM
Time depends on length of total program
e.g. 1K program = 5 ms
Fixed length delay
e.g.5 ms
Chapter PLC3

9. Scan Time On power up, CPU goes through a series of internal processes
Common processes, such as watchdog timer resetting, checking the I/O bus, checking scan time, diagnostic operations and housekeeping on program memory – processes. Communication windows which include the link service of a host computer and communications with peripheral devices.
Executes Program
Instruction execution which includes an executive routine in which the base intelligence of the system is used to interpret the user program containing the instructions to be executed.
Reads
Input
Sets
Output
Data input and output when status of the input relays will be written into the input portion of the I/O RAM and status of the output portion will be reflected in the output relays.
On power up, CPU goes through a series of internal processes
Chapter PLC3

10. Scan Time One Scan One Scan
Reads
Input
Sets
Output
Executes Program
Reads
Input
Sets
Output
Executes Program
One Scan
One Scan
Scan time is the total time required for PLC to perform activities in each scan cycle.
Typical scan time for 500 instruction words varies between 3 ms and 10 ms with ladder logic programming.
Chapter PLC3

11. Scan Time Case 1 : One Scan One Scan And DETECTED !!
Reads
Input
Sets
Output
Executes Program
Reads
Input
Sets
Output
Executes Program
INPUT SIGNAL comes in …..
And DETECTED !!
Chapter PLC3

12. Scan Time Case 2 : One Scan One Scan No INPUT SIGNAL comes in …..
Reads
Input
Sets
Output
Executes Program
Reads
Input
Sets
Output
Executes Program
No INPUT SIGNAL comes in …..
Some INPUT SIGNAL comes in …..
And DETECT nothing !!
And DETECTED !!
Chapter PLC3

13. Scan Time Case 3 : One Scan One Scan And NOT DETECTED !!
Reads
Input
Sets
Output
Executes Program
Reads
Input
Sets
Output
Executes Program
INPUT SIGNAL comes in …..
And NOT DETECTED !!
Because of the cyclic nature of I/O copying routine, input signals of high frequency which change state after the READ routine will not be recognised until the next copy occurs.
Only certain PLCs incorporate a subroutine to interrupt the scan to pick up such signals.
Chapter PLC3

14. Interfacing On-Off Sensors with PLCs
Mechanical
Switches or
‘Dry Contacts’
DC proximity
sensors
AC/DC 2 -
wire
Proximity sensors
Load
24V DC
solid state output
relay output 1 2 3 4 5
COM NC NO
24V
NPN
PNP L
oad
sink
+24V DC ( - ) +
Load
source
+24V ( - ) +
Chapter PLC3

15. Interfacing On-Off Sensors with PLCs
Input Cards can be broadly classified into two categories:
a) source type (or common positive), &
b) sink type (or common negative).
solid state output
NPN
PNP L
oad
sink
+24V DC ( - ) +
Load
source
+24V
NPN (current sinking) sensor must be matched with a source type input card.
PNP (current sourcing) sensor must be matched with a Sinking type input card.
Chapter PLC3

16. Interfacing On-Off Sensors with PLCs: Input Units and Interface Circuits
NPN (current sinking) sensor must be matched with a source type input card.
PLC Sources
When Sensor is ON…
SENSOR Sinks
When Sensor is OFF…
Signal is detected
No Signal
Chapter PLC3

17. Interfacing On-Off Sensors with PLCs: Input Units and Interface Circuits
PNP (current sourcing) sensor must be matched with a Sinking type input card.
PLC Sinks
When Sensor is ON…
SENSOR Souces
Chapter PLC3

18. Interfacing On-Off Loads with PLCs
Output Module of PLCs are used to drive loads, such as
solenoids,
lights,
buzzers,
motor contactors, etc
Output Module
Output Module
Power
Supply
These loads are powered by an External Power Source not within the PLC.
Chapter PLC3

19. Types of Output Modules
Transisterised
npn
pnp
Triac
Relay
Chapter PLC3

20. Interfacing On-Off Loads with PLCs
Factors affecting selection of Output Module:
Load type
Load current
Response Time
RELAY
TRIAC
Transistorised (npn/ pnp)
Characteristics
Type of Output
AC/ DC AC DC
2A Typical
1A Typical
10ms Typical
3ms Typical
1ms Typical
Chapter PLC3

21. Interfacing On-Off Loads with PLCs
Transisterised NPN Type
LOAD + -
Chapter PLC3

22. Interfacing On-Off Loads with PLCs
Transisterised PNP Type
LOAD
Chapter PLC3

23. Interfacing On-Off Loads with PLCs
Triac Type
LOAD
Chapter PLC3

24. Interfacing On-Off Loads with PLCs
Relay Type
LOAD
Chapter PLC3

25. Interfacing On-Off Loads with PLCs
Relay Type
LOAD
Chapter PLC3

26. Interfacing On-Off Loads with PLCs
Relay Type
LOAD
Chapter PLC3

27. Summary covers the internal operations of a Programmable Controller
This chapter :
covers the internal operations of a Programmable Controller
illustrates the concept of scan time
explains I/O response time
discusses practical issues of input signal interface with Programmable Controller
discusses practical issues of output signal interface with Programmable Controller
Chapter PLC3