1. Discrete Versus Analog Devices
Discrete devices are inputs
and outputs that have only
two states: on and off.
ON/OFF
toggle switch
Analog devices are inputs
and outputs that can have
an infinite number or states.
Analog
control valve

2. Analog Input Interface Module
A varying low DC
voltage proportional
to the temperature
being monitored is
produced by the
thermocouple.
This voltage is amplified and digitized
by the analog input module and then
sent to the processor on command from
a program instruction.
There are two basic types of analog
input modules available: current
sensing and voltage sensing.
Typical analog inputs may vary from
0 to 20 milliamperes, 4 to 20 milliamperes,
or 0 to 10 volts

3. Analog Output Interface Module
Both analog inputs and outputs are word-oriented.
The analog output
interface module
receives from the
processor digital
data, which are
converted into a
proportional voltage
or current to control
an analog field device.

4. Analog I/O Control System
The valve is initially
open 100%.
As the fluid level in the tank
approaches the preset point,
the processor modifies the output,
which adjusts the valve to maintain
a set point.

5. High-Speed Counter Module
Special I/O Modules
High-Speed Counter Module
Used to provide an interface for
applications requiring counter
speeds that surpass the capacity
of the PLC ladder program.
They have the electronics needed
to operate independently of the
processor.
A typical count rate is 0 to 75 kHz,
which means the module would be
able to count 75,000 pulses per
second.

6. I/O Specifications Nominal Input Voltage This AC or DC value specifies
the magnitude and type of voltage
signal that will be accepted.
On State Input Voltage Range
This value specifies the voltage
at which the input signal is
recognized as being absolutely on.
Nominal Current Per Input
This value specifies the minimum input current that the input devices must be capable of driving to operate
the input circuit.

7. I/O Specifications Number of Inputs and Outputs per Card
This value indicates the number
of inputs or outputs that can be
connected to the module.
Off-State Leakage Current per Output
This value specifies the maximum value
of leakage current that flows through
the output in the OFF state.
Maximum Output Current Rating Per Output
and Module
These values specify the maximum current of a single
output and the module as a whole.

8. CPU - Processor The term CPU is often used interchangeably with the
term processor. However, by strict definition, the CPU
term encompasses all the necessary elements that form
the intelligence of the system.

9. Processor Unit The processor executes the operating system, manages
memory, monitors inputs, evaluates the user logic, and
turns on the appropriate outputs.
Status indicators provide system diagnostic information.
Keyswitch allows you select different modes of operation.

10. Typical Processor Modes Of Operation
RUN Position
Places the processor in the Run mode
Executes the ladder program and
energizes output devices
Prevents you from performing online
program editing in this position
Prevents you from using a
program/operator interface device
to change the processor mode

11. Typical Processor Modes Of Operation
PROG Position
Places the processor in the program
mode
Prevents the processor from scanning
or executing the ladder program, and
controller outputs are de-energized
Allows you to perform program
entry and editing
Prevents you from using a
program/operator interface device
to change the processor mode

12. Typical Processor Modes Of Operation
REM Position
Places the processor in the Remote
mode: either REMote Run, REMote
Program, or REMote Test mode
Allows you to change the processor
mode from a programmer/operator
interface device
Allows you to perform program
online program editing

13. Electrostatic Discharge Protection
Many electronic components
found in modules are
sensitive to electrostatic
voltages.
Ground yourself before handling static-sensitive parts.
Wear a wrist strap that provides a path to bleed
off charges.
Be careful not to touch the backplane connector or
connector pins of the PLC system.
Create a static free work area by covering your
work bench and floor area with a conductive surface
that is grounded.

14. Memory Design Memory is a physical space inside the CPU where the
program files and data files
are stored and manipulated.
Program File
Data File

15. Memory Requirements
The complexity of the program determines the amount of
memory required.
The program is stored in memory as 1’s and 0’s,
which are typically assembled in the form
of 16-bit words.

16. Memory Requirements Memory sizes are commonly expressed in thousands
of words (K) that can be stored in the system.
2 M
ControlLogix
Controller
Up to 160 K through
2 M memory
Up to 128,000 Inputs
and Outputs
SLC 500
Controller
Up to 64 K memory
Up to 4096 Inputs
and Outputs
64 K
MicroLogic 1000
Controller
1 K memory
Up to 20 inputs
Up to 14 outputs
1 K

17. Memory Design Memory location refers to an address in the CPU’s
memory where a binary word can be stored. A word
usually consists of 16 bits.
Memory utilization refers to the number of memory
locations required to store each type of instruction.
A rule of thumb for memory
location is one location per
coil or contact. One K of
memory would then allow
a program containing 1,000
coils and contacts to be
stored in memory.
Contact
Coil

18. Input Status Table Or File
The memory of a PLC is divided into sections that have
specific functions.
If the input is closed the bit is
set to 1
Each input has
one corresponding
bit in memory
If the input is
open the bit is set to 0

19. Output Status Table Or File
Each output has one corresponding
bit in memory
If the bit is a 0, the output
will be OFF
If the bit is a 1, the output will be ON

20. Status Table
Status Table Files: Stores information such as scan time, fault
codes, and watch dog timer; and some have
precision timing bits for use in the control
program.

21. Timer Table
Timer Files: Are usually three words long. One word contains
timer status information; another contains the
preset value or set-point; the last contains the
accumulated value.

22. Counter Table Counter Files: Also three words long, having the same
configuration as the timer.

23. Volatile Memory Volatile memory will lose its stored information if
all operating power is lost or removed.
Volatile memory is easily
altered and quite suitable
for most applications when
supported by battery
backup.

24. Nonvolatile Memory Nonvolatile memory has the ability to retain stored
information when power is removed accidentally
or intentionally.
Although nonvolatile memory
generally is unalterable, there
are special types used in which
the stored information can be
changed.

25. Memory Types As the name implies, programmable logic controllers
have programmable memory that allows users to develop
and modify control programs. Data are stored in memory
locations by a process called writing. Data are retrieved
from memory by what is referred to as reading.
Read-Only Memory (ROM):
Is designed so that information
stored in memory can only be
read, and under ordinary
circumstances cannot be changed. ROM memory is used
by the PLC for permanent storage of the operating system.

26. Memory Types Random Access Memory (RAM):
Is designed so that information
can be written into or read from the
memory. Commonly used for the
user’s program, timer/counter values,
and input/output status.

27. Memory Types Electrically Erasable Programmable Read-Only Memory
(EEPROM):
Is a nonvolatile memory
that offers the same flexibility
as does RAM. The EEPROM
can be electrically overwritten
with new data. Since it is
nonvolatile, it does not require
battery backup.
Typically, an EEPROM
memory module is used
to store, back up, or transfer
PLC programs.

28. User Communications With PLC Circuits
Central
Processing
Unit
Input
Module
Output
Module
Programming
Device
The programming device
is used to input the desired
instructions. These
instructions determine what
the PLC will do for a specific
input.
Operator
Interface
The operator interface device
allows the process information
to be displayed and new control
parameters to be entered.

29. Personal Computer Programming Device
The computer can be used to document the program.
PC software provides cut-and-paste features for
program developing and editing.
Allows easy monitoring of data tables.
Manufacturers software required.
Large amounts of logic can be displayed.
Circuit elements can be highlighted in color
to indicate status.
More than one program can be stored on the
computers hard drive.

30. HMI Allows operator to change settings using a touch screen
Shows operation in graphic format for easier viewing
Allows operator to change settings using a touch screen
Shows alarms, complete with time of occurrence and
location
Displays variables as they change over time

31. PLC PRORAMMABLE LOGIC CONTROLLER BOOK READ PAGES 77-83