1. PLC Hardware
Components

2. Discrete I/O Modules
Discrete type I/O interface modules connects field
devices of the ON/OFF nature.
Discrete output modules
are used with field load
devices such as lights,
small motors, solenoids,
and motor starters that
require simple ON/OFF
switching.
Discrete input modules
are used with field control
devices such as selector
switches, pushbuttons,
and limit switches.

3. Discrete I/O Modules
The classification of discrete I/O covers bit-orientated
inputs and outputs. In this type of input or output,
each bit represents a complete information element
in itself.
Each discrete I/O module is powered by some
field-supplied voltage source of a specified value.
Common voltage ratings are:
Input Interfaces Output Interfaces
12 V ac/dc V ac
24 V ac/dc V ac
48 V ac/dc V ac
120 V ac/dc V ac
230 V ac/dc V dc
5 V dc (TTL level) V dc (TTL level)
24 V dc

4. Discrete I/O Modules
The I/O modules receive their module voltage and
current from the power supply module through the
backplane of the rack enclosure into which they are
inserted. Power from this supply is used to power the
electronics, that reside on the I/O board. The relatively
higher currents required by the loads of an output
module are supplied by user-supplied power.

5. AC Discrete Input Module
The input circuit is composed of two basic sections:
the power section and the logic section.
The power and logic sections are normally coupled
together with a circuit, which electrically separates
the two.

6. AC Discrete Input Module Operation
When the pushbutton is closed,
120 VAC is applied to the bridge
rectifier through R1 and R2.
This produces low level DC voltage across the LED.
The resulting light switches the phototransistor into conduction and
the closed status of the pushbutton is communicated to the processor.

7. A Zener is not like a normal diode
A Zener is not like a normal diode. A normal diode lets current flow in only one direction and needs to be installed in the correct direction. A Zener diode is placed in the opposite direction, against the flow of current. A Zener diode will prevent current from flowing until it reaches a certain voltage, depending on the diode rating. Once this critical value is reached, current is allowed to flow. The diode will also try to keep the outgoing voltage at its rated level, say 5 volts. If you feed 10 volts through the diode, the voltage might climb to volts, but it will try to stay at the same voltage. If you feed too high a voltage through the diode, it will break down and fail.

8. Input Module Tasks Sense when a signal is received from a sensor
Convert the input signal to the
correct voltage level for the
particular PLC
Isolate the PLC from
fluctuations in the input signal’s
voltage or current
Send a signal to the processor
indicating which sensor originated
the signal

9. AC Discrete Output Module
The output circuit is composed of two basic sections:
the power section and the logic section, coupled by
an isolation circuit.
The output interface can be though of as a simple
electronic switch to which power is applied to
control the output device.

10. AC Discrete Output Module Operation
The processor sets the output status
according to the logic program.
This in turn switches the
Triac into conduction which,
in turn, turns on the lamp.
When the processor calls for
an output, a voltage is applied
across the LED of the isolator,
which switches the
phototransistor into conduction.

11. AC Discrete Output Module
Provided with LEDs that indicate
the status of each output.
Fuses are generally required for
each circuit. Some modules also
provide visual indicators for
fuse status.
Individual AC outputs are
usually limited to 1 or 2 amps.
The maximum current load
for any one module is also
specified.

12. Interposing Relay Connection
Used for controlling larger loads such as motors.
The low current output
from the module operates
the relay coil, the contacts
of which operate the
motor starter.

13. Types Of Discrete Output Modules
Are used to turn two-state devices either ON or OFF
Triac outputs can only be used only for control
of AC devices.
Transistor outputs can be used only for control
of DC devices.
Relay outputs can be used with AC or DC devices.
However they have a much slower switching
time compared to solid-state outputs.

14. Sinking And Sourcing References
Sinking and sourcing references are terms used to
describe a current signal flow relationship between
DC field input and output devices in a control system
and their power supply.
Sourcing device
with sinking
output module
circuit

15. Sinking and Sourcing DC References
The design of DC field devices typically require that
they be used in a specific sinking or sourcing circuit,
depending on the internal circuitry of the device.
Sourcing device
with sinking
input module
circuit

16. Sinking And Sourcing References
Sinking device
with sourcing
input module
circuit
Field devices connected to the negative side (DC common)
of the field power supply are sinking field devices
Sinking device
with sourcing
output module
circuit

17. PLC PRORAMMABLE LOGIC CONTROLLER BOOK READ PAGE 60