logic functions

OR OperationA coil is not energized until either normally open switch A or B is closed.

Boolean Logic Diagram

Logic Functions

NOR Operation

There has to be an output when neither A nor B has input.

When there is input to A or B the output ceases.

Logic Functions

NAND Operation

There is no output when both A and B have an input.

Logic Functions

EXCLUSIVE-OR (XOR) Operation

No output when the status of the inputs is same

No output when there is no input to both Input 1 and 2 and when there is an input to both Input 1 and Input 2

Logic Functions

Testing a Program Through Simulation

Status Functions


Forcing

Used to temporarily override the input or output status of the application in order to test and debug the program


Forcing


Case Study -1

Discrete Inputs/Outputs

Wiring

Discrete Inputs/Outputs

Case Study – 1 contd…

Motor starter to control a three-phase AC motor: Hard-wired Approach

Case Study - 2


Momentarily depressing the Start pushbutton completes the path of current flow and energizes the motor starter (M).

Motor Starter – Hard-wired Approach


Motor Starter – PLC Approach



When Start pushbutton is pressed, the CPU receives a logic 1 from input I0.0. This causes the I0.0 contact to close. All three inputs are now a logic 1. The CPU sends a logic 1 to output Q0.0. The motor starter is energized and the motor starts.



The output status bit for Q0.0 is now a 1. On the next scan, when normally open contact Q0.0 is encountered, the contact will close and output Q0.0 will stay on even if the Start pushbutton is released.



When the Stop pushbutton is pressed, input I0.1 turns off, the I0.1 contact opens, output coil Q0.0 de-energizes, and the motor turns off.


Expanding the Application - Run & Stop Indicators Lamps


When Q0.0 is off, the normally open Q0.0 contact on Network 2 is open and the RUN indicator off. At the same time, the normally closed Q0,0 contact is closed and the STOP indicator is on.



Start button is pressed, output Q0.0 is on. The normally open Q0.0 contact on Network 2 is closed and the RUN indicator on. The normally closed Q0.0 contact on Network 3 is open and the STOP indicator light connected to output Q0.2 is off.



Adding a Limit Switch

A limit switch could be used to stop the motor or prevent the motor from being started.



If the access door is open, the normally open contacts of LS1 connected to input I0.3 are open and the motor will not start.



When the access door closes, LS1 closes. Input I0.3 is now on, and the motor will start when the Start pushbutton is pressed.


Further Expansion

Analog Inputs and Outputs

Analog Inputs

Application Example

Analog Inputs

Expanding the Application – A Conveyor System

As packages move along the conveyor, they are weighed.

A package that weighs at or greater than a specified value is routed along one conveyor path.

A package that weighs less than a specified value is routed along another conveyor path, where it will later be inspected for missing contents.

Analog Outputs

The transducer takes the voltage signal

Depending on the requirement, amplifies, reduces, or changes it into another signal which controls the device.

The 0 - 10 VDC signal controls the 0 - 500 Lbs. scale analog meter

Timers

Timers are devices that count increments of time

The output of the timer is a logic 0 as long as the current time is less than the preset time.

When the current time is greater than the preset time, the timer output is a logic 1

Timers

Hard-Wired Timing Circuit (On-delay)

When S1 closes, TR1 begins timing.

When 5 seconds (preset) have elapsed, TR1 will close its associated normally open TR1 contacts, illuminating pilot light PL1.

When S1 opens, de-energizing TR1, the TR1 contacts open, immediately extinguishing PL1.

Timers

On-Delay Timer (TON)

When the On-Delay Timer (TON) receives an enable (logic 1) at its input (IN), a predetermined amount of time (preset time - PT) passes before the timer bit (T-bit) turns on.

The timer resets to the accumulated time to zero when the enabling input goes to a logic 0.

Timers

ExampleA switch is connected to input I0.3, and a light is connected to output Q0.1.

If the switch were opened before 15 seconds has passed, then re-closed, the timer would again begin timing at 0. Because this type of timer does not retain its accumulated time when its input (IN) goes to logic 0, it is said to be non-retentive.

Timers

T37 is now a normally closed contact.

What is the function of this circuit now?

Function:

Causes the indicator light to turn off only when the timer times out

Timers

Retentive On-Delay Timer (TONR)

Just like the On-Delay timer (TON), the Retentive On-Delay Timer (TONR) times as long as the enabling input is on, but does not reset when the input goes off.

Timers

Example

If, for example, after 10 seconds input I0.3 turns off, the timer stops.

When input I0.3 turns on again, the timer begins timing at 10 seconds.

The light connected to Q0.1 turns on 5 seconds after input I0.3 has been closed for the second time.

Timers

Off-Delay Timer

Used to delay turning 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 time value is set to 0.

When the input turns off, the timer times until the preset time has elapsed. At that time, the timer bit turns off.

Timers

Timer Example

A tank is filled with two chemicals, the solution is mixed, and the tank is drained.

Timers

Steps

1. When the Start button is pressed, input I0.0 turns on, and the program starts pump 1, controlled by output Q0.0. Pump 1 runs for 5 seconds, adding the first chemical to the tank, then shuts off.

2. The program then starts pump 2, controlled by output Q0.1. Pump 2 runs for 3 seconds adding the second chemical to the tank. After 3 seconds pump 2 shuts off.

3. The program then starts the mixer motor, connected to output Q0.2 and mixes the two chemicals for 60 seconds.

4. Next, the program opens the drain valve, controlled by output Q0.3, and starts pump 3 controlled by output Q0.4. Pump 3 shuts off after 8 seconds and the process stops.

Counters Counters compare an accumulated value to a preset value to control

circuit functions.

Used to initiate an operation when a count is reached or to prevent an operation from occurring until a count has been reached.

Counters

Counters

Count Up Counter (CTU)

Counts up from the current value each time the count up (CU) input goes from off to on.

When the current value is greater than or equal to the preset value (PV), the counter bit is a logic 1.

The course resets when the reset (R) input turns on. The counter stops counting when it reaches its maximum value of 32,767.

Counters

Count Down Counter (CTD)

Counts down from the current value each time the count down (CD) input goes from off to on.

When the current value is equal to zero, the counter bit is a logic 1. The counter stops counting at zero.

The counter resets and loads the current value with the preset value (PV) when the load input (LD) turns on.

logic functions

Documents