Category Archives: PLC Basics

Building a PLC Program That You Can Be Proud Of – Part 3

In part 1 we looked at writing PLC programs to control a traffic light using discrete bits and then using timed sequencing using indirect addressing. Part 2 used indirect addressing for inputs as well as output to control the sequence of pneumatic (air) cylinders in the program. We will now return to the traffic light application and expand our program significantly.


Let’s build on the traffic light sequencing used in part one with inputs for pedestrian and car detection. We will also throw in the time of day so that during weekday peak hours an advanced arrow will be used. Advanced green (flashing) will be used when the traffic is detected in the turning lane during the off-peak hours.
Pedestrian Walk Signals Traffic Light with Turn

Programming using this method of sequencing requires a lot of time up front before we start the actual PLC program. However, this method makes the program easier to understand, troubleshoot and modify in the future.

Remember that the PLC programmer must know everything about the machine and operation before programming.

If we just start writing code, then we will constantly be correcting and modifying based on trial and error. I use a spreadsheet program to plot out the inputs, outputs and mask tables. We will go into the details of this below.

Let’s look at the inputs:
Sequencer Inputs

We set up the input table in words V0 to V499. Each bit in the table will be compared to the signals coming to the actual signals wired or programmed in the PLC.
Bit 0 is the time input which will control the entire program interval. Next, we have the Car Detection signals on Bits 1 to 4. The pedestrian signals are pushbuttons coming from Bits 5 and 6. The left turn signals are located in Bits 7 to 10.  The real-time clock functions will come from Bits 12 to 15.

Let’s look at the outputs:
Sequencer Outputs

The output table will be in words V1000 to V1499. The input word will be compared to the actual inputs and the corresponding output channel will be moved to the actual outputs. All of the output bits control lights. You will notice that there is a green light for each direction. This will give us the greatest flexibility when writing our PLC program. Pedestrian signals have a flashing output bit and just an output bit. There will be only one output but this bit will determine if it is flashing or not.

Masking Inputs:
Sequencer Mask

The masking table will be in words V500 to V999. The masking bits will correspond directly to the input table bits. Using the mask will allow us to ignore the status of certain bits when using the compare instruction and setting the outputs.

The Mask table will be used by using an ‘AND’ word instruction. If the mask bit is on for the input then it will be used in the compare instruction. If the mask bit is off for the input then the value is always off using the compare.
Example:
1 ‘AND’ 1 = 1
1 ‘AND’ 0 = 0
0’AND’ 1 = 0
0 ‘AND’ 0 = 0

Once we have this all laid out in the spreadsheet, we can start filling out our sequence of events. Fill in the events based upon the time frame from input bit 0. I usually start by thinking about what happens when power is applied to the unit the first time. This exercise can be a struggle because you must know exactly what you want the sequence to be in order to fill out the table.

We will continue this next time by writing the code to do what our tables want. A review of the numbering systems can be found on this post. A copy of the spreadsheet can be obtained at the following link: PLC Traffic Sequence

Part 4 will continue with the programming of the logic in the PLC.

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.




How PLC Outputs Work

This post is a further follow up from my original ‘Here’s a Quick Way to Understand PLC Inputs and Outputs’. There are basically two different kinds of PLC outputs, Discrete and analog. Discrete outputs are either ‘ON’ or ‘OFF’; 1 or 0. You can think of them as a single light bulb. Analog outputs have a range to them. They are outputs that usually will control proportional valves, drive speeds, etc. They usually have one of the following signals that are outputted from the PLC: 4-20mA, 0-10VDC, 1-5VDC.


Discrete Outputs
PLC Outputs

The above diagram has three outputs. A coil, light, and motor. The Ladder outputs Y0, Y1, and Y2 control the outputs respectfully. You will notice that when the Ladder output turns on, the corresponding output card bit LED turns on. This then will energize the output hardwired to the device.

The outputs are turned on or off at the end of every PLC Scan. The PLC logic is solved left to right, top to bottom in most PLCs. Physical outputs are not set/reset until an I/O refresh is performed at the end of every scan. This means that if I have a scan of 1msec, then the maximum time it will take to turn on/off the output is 1msec.

PLCs will sometimes have the ability to update the I/O in the middle of a scan. Please refer to your PLC manufacturers manual for this instruction. This can be used for updating the I/O quickly or controlling stepper drives for motors by giving them a pulse train output from the discrete PLC output. A pulse train is just a quick series of on/off states of the output.

Analog Outputs
PLC Analog Output to Motor Speed

An analog output converts a digital value to a voltage or current level that can be used to control (vary) physical outputs. In the example above we are controlling the speed of the motor. Words in the PLC will control the analog value.
Example:
4 – 20 mA current Output – 8-bit resolution
4 mA = 00000000 base 2 = 00 base 16
20 mA = 11111111 base 2 = FF base 16
For a review of numbering systems, follow the link below:
What Everybody Ought to Know About PLC (Programmable Logic Controller) Numbering Systems

In the industrial environment noise from variable frequency drives, improper grounding, etc. can interfere with your analog input. The following post will show a quick method to reduce this noise.
The Secret Of Getting Rid Of Noise On Your Analog Signal

Previous Post:
How PLC Inputs Work

Watch on YouTube: How PLC Outputs Work

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.




How PLC Inputs Work

This post is a further follow up from my original ‘Here’s a Quick Way to Understand PLC Inputs and Outputs’. There are basically two different kinds of PLC inputs, Discrete and analog. Discrete inputs are either ‘ON’ or ‘OFF’; 1 or 0. You can think of them as a single switch. Analog inputs have a range to them. They are inputs that usually will sense pressure, temperature, height, weight, etc.  They usually have one of the following signals that are inputted into the PLC: 4-20mA, 0-10VDC, 1-5VDC.


Discrete Inputs
PLC Input

The above diagram has two inputs. A normally open (NO)  and a normally closed (NC). When we talk about normally open and close, think of the condition of the input if no one touches anything. A normally open contact will not turn on the input to the PLC card in its ‘normal’ state. The normally closed contact will turn on the input to the PLC card in its ‘normal’ state.

Normally Open Input
The NO contact when activated will complete a circuit and turn on the PLC input. Ladder logic will then turn on if you use a normally open (Examine On) input in your program. See the above diagram.

Normally Closed Input
The NC contact when activated will break a circuit and turn off the PLC input. Ladder logic will then turn off if you use a normally open (Examine On) input in your program. See the above diagram.

As you can see with the diagram above this can get tricky to determine the on/off condition of the input. PLC logic can convert any signal by using normally closed (Examine Off) inputs in the program. In the field, I usually look at the PLC input lights, and wiring diagrams to determine the current state of the input. This is before diving into the program to troubleshoot.

Here is a link to wiring up discrete 3-wire sensors in the field. Here’s a Quick Way to Wire NPN and PNP devices

Analog Inputs
BinMaster Analog Input

An analog input converts a voltage or current level into a digital value that can be stored and processed in the PLC. They use words to determine the signal coming from the device.
Example:
4 – 20 mA current Input – 8-bit resolution
4 mA = 00000000 base 2 = 00 base 16
20 mA = 11111111 base 2 = FF base 16
For a review of numbering systems, follow the link below:
What Everybody Ought to Know About PLC (Programmable Logic Controller) Numbering Systems

In the industrial environment noise from variable frequency drives, improper grounding, etc. can interfere with your analog input. The following post will show a quick method to reduce this noise.
The Secret Of Getting Rid Of Noise On Your Analog Signal

Here are some additional posts that you might find helpful.
How to make a Start / Stop / Jog circuit in a PLC
The Secret of Using Timers
The Secret of Using Counters

Watch on YouTube: How PLC Inputs Work

Watch on YouTube: Wiring (Testing) Analog PLC Input Click

Watch on YouTube: Wiring (Testing) Analog PLC Input Omron CP1H

Watch on YouTube: Wiring Contact (Discrete) PLC Inputs

Watch on YouTube: Wiring PNP Sensor to PLC

Watch on YouTube: Wiring NPN Sensor to PLC

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.




How you can learn PLC Programming without spending a dime!

I have been writing PLC programs for over 20 years. I often get asked what is the best way to learn PLC programming. Programming in the way I was taught in college was with the Motorola 6809. (Yes, I know that I am dating myself) This was microprocessor programming, but it was the best way to sometimes explain the methods behind PLC programming. Manufacturers of PLCs had proprietary software that was not even related in their appearance and methods of programming. Today we have a few standards that have changed the look and feel of the programming software packages so each manufacturer is similar. The following is the best recommendation that I have for beginners to start to learn PLC programming today.


start stop 003

The first place to start in order to learn PLC programming is free publication by Kevin Collins. This PDF will teach you PLC programming without just telling you what a PLC is and how it functions. He also includes some test questions along the way in order for you to retain and understand the important points that he is making.

PLC Programming for Industrial Automation
by Kevin Collins
(Note: This book is now for sale on Amazon.)

Topics covered include:

  • PLC Basics
  • Ladder Programming
  • Conditional Logic
  • Ladder Diagrams
  • Normally closed contacts
  • Outputs and latches
  • Internal relays
  • Timers
  • The Pulse Generator
  • Counters
  • Sequential Programming Introduction
  • Evolution of the Sequential Function Chart
  • Programming using the Sequential Function Chart
  • Entering the SFC program into the PLC
  • Modifying an SFC Program
  • Selective Branching
  • Parallel Branching

GreyToBinaryCode

Simulator

After learning the basics from the above manual, practice. Create programs yourself and test what you have learned. You can accomplish this by using simulators. Allot of the programming software will have simulators. The simulator will mimic the PLC hardware so you can test your programs before installing in the field. Traditionally I have not been a fan of simulators, but recently Automation Direct has introduced a simulator with their Do-More PLC. It is the Do-More Designer Software. This software simulator includes the entire instruction set (Not Just a Bit Logic) as well as communication protocols. It can be downloaded and installed for free from the above link.

Indirect Addressing 2 Pointer

The next step I recommend is then to advance into some of the advanced instructions. An understanding of the numbering systems in the PLC will be a benefit. Math, PID, register manipulation and conversion instructions are just a few of the advanced programming you can learn. All of these and more instruction information can be obtained from reviewing the documentation from the PLC manual that you are programming. Once again all of these instructions are included in the Do-More Designer Software.

Indirect Addressing Animation

The program structure is the next topic. Allot of programmers would stop here and can do well with developing software, however, there is much more than you can lean.  Sequencers give programmers the methods to change the logic on the fly and allow troubleshooting the system easier. This method of programming can benefit you greatly and reduce the development time of your logic.

Omron HostLink Frame_Responseadu_pdu

The last step that I recommend learning is the sharing of information. I am meaning the information that you program through an HMI and/or SCADA package. This refers to the understanding of the ways in which information can be gathered from the PLC and displayed in different ways. Here are a couple of previous articles that have been written on this subject:

How to Implement the Omron PLC Host Link Protocol 

Robust PLC Data Logger

iis107 display

As you can see, there is a lot of information available to you to begin and lean PLC programming without spending a dime!  Remember that PLCs are similar to computers, (Moore’s Law) they increase in size and ability. Systems are expanding and changing every day. Happy programming.

Do you know of additional tips or methods to share?
PLC Beginner’ s Guide – There are many different PLC manufacturers with different hardware and software. All of the programmable logic controllers have similar basic features. Here is how I would approach learning about basic PLCs.

Watch on YouTube: How you can learn PLC Programming without spending a dime!

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.




How to Troubleshoot a PLC

Your control system does not work. Where do you start? Let’s walk through a series of questions in order to determine where the problem lies.

footprints




Is this a new installation or previous installation that was running fine? Determine if the system has been running well in the past and has currently stopped working correctly. This is the indication that the problem relies upon the system.

Is there anything that has happened outside of the system? Has there been a lightning strike, blown drives on other systems, etc?  This can point to the original cause of the malfunction.

build-2Bgear-2Btower

What is the system doing now and what should it be doing? Gather all of the information you can from every resource you can.

  • Supervisors  – machine, location, time of the error, other happenings in the plant, etc
  • Operators – What is it currently doing? What should it be doing? What do you think is wrong?
Operators of the equipment are your key resource in finding, correcting and ensuring the error does not happen again. They know the equipment from an operational point of view which can assist you greatly in troubleshooting.

PLC fatal and non-fatal errors:
If the machine is still running partially then this is an indication of a non-fatal error. Cannot run at all is usually a fatal error.

Do_More CPU Units

Take a look at the PLC indicator lights on the CPU. Refer to the operation manual for the PLC for troubleshooting specific lights on the CPU. The following are general tips:

If no lights are on then the possible cause is a power supply. This is usually the most common of errors on a PLC system. Mean time before failure (MTBF) is rated on the lowest rating of components which is usually the power supply.

If the run light is on and an error light flashing this usually indicates internal errors such as batteries, scan time, etc. It is usually not the reason for the lack of operation.

If the run light is on and no other errors are seen on the CPU we can put the PLC program on the bottom of the list of items that could be the cause.

Check the input cards of the PLC. You should see the individual sensors lighting up the inputs. If not then check the power supply to the input card/cards.

Ask the operator what is happening and what is suppose to happen. Try to follow the sequence of events in the PLC to determine either an input or output device not working.
Some items to watch:

If this is a new PLC program that you are doing start with a logic flow diagram. This will determine the procedure to start programming.
Every program can be done in several ways. The best method is the most documented one.

Documentation is the mark of a good program.

PLC-2BScan

Some trouble with new programs can be racing conditions. This is usually a case of not understanding how the PLC scans logic. In general, the PLC will scan from left to right, top to bottom. The output bits/words are available to the inputs of the next rung of logic. (Modicon PLC’s will scan differently.) Actual outputs and inputs are not read until the end of the scan of the PLC. Racing conditions happen when the output is set on multiple rungs, but will not get actually set until the end of the scan. Think of it as the last action will always win. So if this happens to move the logic to the end of the program and see if it works. Then go back and see where the output was also set.  Cross-reference guides are ideal for this purpose. (Refer to your programming software on how to get cross-references.)

start stop 011

We have discussed just a few troubleshooting techniques. Hopefully, now you know how to start looking for the errors on your system. Let me know how you make out.

Watch on YouTube: How to Troubleshoot a PLC

Do you know of additional tips or methods to share?

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.




Here is a Method That is Helping PLC Programmers to Program Faster

PLC programming involves both direct and indirect addressing. Direct address programming involves writing each ladder logic rung to do the operation required. We often forget about using powerful indirect addressing to solve our logic.


The below-animated picture will show a simple example of using indirect addressing. This will use the MOVE instruction and transfer a word indirectly to output word V100. V[V0] means that the value in V0 will point to the V memory to get the value to move. You can think of this as a pointer for the memory location to move.

Indirect Addressing Animation

Of course, we need to monitor V0. Our values are in sequence from V1 to V6. We need to ensure that V0 is always in the range from 1 to 6.

Let’s take a look at a program sample using the Do-more Designer Software. We will set up the sequence similar to the animation above but expand the program.
Just like above we will set up the pointer at V0 and the output at V100 memory locations. V1 to V37 will hold our output data sequence. This is outputs that we want to set on each event and/or time frame. You can see some of the registers and the corresponding values. These are set as a hexadecimal value. The following link will provide a review of the numbering systems in the PLC. (WHAT EVERYBODY OUGHT TO KNOW ABOUT PLC (PROGRAMMABLE LOGIC CONTROLLER) NUMBERING SYSTEMS)

Indirect Addressing 4 Data

This is the logic to set up the move instruction. The source is V[V0] which means the pointer is V0 in this memory area. The destination will be V100.

Indirect Addressing 1 MOV

An internal timing bit ST5($100ms) is used to increment the pointer V0. This could also be done by an event or series of events. The pointer is then compared to ensure that it is between 1 and 37.

Indirect Addressing 2 Pointer

Finally, the output word is then transferred to the physical outputs. This is done by using MAPIO instruction. Each bit can be set independently.

This example uses indirect addressing to program a sequence based upon time. We could just as easily used indirect addressing to compare inputs to a table and set the outputs accordingly. You can see how this method can greatly reduce the amount of time to develop your program. This holds especially true if the sequence needs to be changed. It would be just a matter of changing data values in the table.

The following are separate posts that use indirect addressing:

Building a PLC Program You Can Be Proud Of – Part 1
This uses the control of an intersection traffic light to demonstrate direct versus indirect addressing.

Building a PLC Program That You Can Be Proud Of – Part 2
A sample program to control valves. This uses indirect addressing for the inputs as well as the outputs.

Now You Can Have Robust Data Logging for Free – Part 1
Using indirect addressing, this sample program will log information in the PLC to be retrieved at a later time.

Indirect addressing is a powerful method of programming to simplify and program faster than you ever thought possible. You can even use indirect addressing in the PLC to scale a non-linear analog input signal. Let me know your thoughts on using indirect addressing. What can you come up with?

Watch on YouTube: Here is a Method That is Helping PLC Programmers to Program Faster

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.




The Secret of Using Counters

funny_counter

Counters are used in the majority of PLC programs. This is especially true if part of your SCADA system. Counters like the animated picture above count things. In this situation, we are counting the number of turns the little guy makes. The counter is displaying the total number. This is considered a totalizing counter. If an output turned on to do something then it would be a preset (target number entered for the count) counter. There are also a wide variety of off the shelf industrial counters that you can use. The implementation of counters can be vast, however, it all starts with a TIMING CHART. This is the same as the timing charts we discussed in ‘The Secret of Timers’ post.


A timing chart is a secret behind understanding of the counter that you need in your application. Making a timing chart before writing the program will ensure that all of the information will be accounted for.

The timing chart is mapped out on an x and y plain. The ‘y’ plain has the state of the input on/off (1 or 0). The ‘x’ plain will show time.

The following shows a timing chart for a counter:
timing chart counterAs you can see in this timing chart, you have an input, output and display.

Inputs:
Inputs are used usually sensors that are wired to the counter (PLC) to indicate the items that we need to count. They can be switches, photoelectric sensors, proximity sensors, encoders, etc. (Wiring of NPN / PNP devices) A counter will generally have only one input. In the case of an encoder input, it is still only one input, however, this is wired usually as A, B and Z phase. Z is always the reset. A and B indicate the pulses and are leading or trailing each other by 90 degrees depending on direction. Allot of counters will also allow you to add a direction input signal. However, this is all still only one input.
Output ModesInput Modes

Outputs:
Outputs from counters are generally discrete. This means that they are on or off, similar to the inputs. Outputs will trigger when the count value matches the set value. The duration that the output is on depends on the reset signal, to start the count again. (DC Solenoids protection) Allot of the counters today will allow you to have multiple outputs. These multifunction counters can have several preset outputs that trigger when the counter set value has been reached. Batch outputs are also available on some of the industrial counters. A batch output counts the number of times that the preset has been reached. This output will be turned on when the number entered for the batch has been reached.

Set Value – SV:
This is usually on the display and shows the preset value. It is the target number of counts.

Present Value – PV:
This is usually on the display and shows the current or accumulated value.

Roller Measurment

The PLC programming is usually not that much different then the industrial counter. Allot of the manufactures will have an up counter, down counter and/or an up/down counter. Just as the name implies the display is either counting up or down. You have to refer to the instruction manual of the manufacturer you are programming for the way in which the counter will be programmed.

Do-More Up and Down Counter

In the above example Do-More PLC program we have an up and a down counter. X0 is the input and X1 is the reset on both of these counters. (CT0, CT1)
The preset value is stored in memory location D0. This value is set to the number 3.
When the present value (accumulated) reaches the set value (preset) then the CT0. A done bit goes on and the output Y0 is active. Y0 will remain on until the reset input goes on.
The only difference for the down counter is the display. You will see that the present value will count down to zero (0) before the CT1. A done bit is turned on.
These counters are memory retentive. So in order to make the counter non-memory retentive, use the first scan bit of the PLC to trigger the reset of the counter. (ST0 – $FirstScan)

Every PLC has counters. They all have different types depending on what you are trying to achieve. It will all start with your Timing Chart.

Watch on YouTube: Learn PLC Programming – Free 9 – The Secret of Counters

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.




The Secret of Using Timers

Timers are used in the majority of PLC programs. There are also a wide variety of off the shelf industrial timers that you can use. The implementation of timers can be vast, however, it all starts with a TIMING CHART.


A timing chart is a secret behind understanding of the timer that you need in your application. Making a timing chart before writing the program will ensure that all of the information will be accounted for.

The timing chart is mapped out on an x and y plain. The ‘y’ plain has the state of the input on/off (1 or 0). The ‘x’ plain will show time.

Let’s take a look at a timing chart for an On-Delay Timer. This is the basic operation for an Omron H3BR industrial timer.

Timing Chart On-Delay Basic

Power –  When dealing with PLC’s we must consider when power to the unit is removed what happens to the current time and output conditions.
Start – In this case, the start signal is momentary to start the time cycle. (t) We could modify this signal to be maintained until the output switches.
Output – The output will show when it turns on. This can also indicate the opposite, and show when it turns off.
Time – Time is shown by the relationship between the start signal and the output. Our example shows timing starts on the leading edge of the Start. This could have also been on the trailing edge.

Here is the same on-delay timing chart with some more detail. Several conditions are added to the chart.

Timing Chart On-Delay Details

These conditions prompt us to ask the following questions.
What happens when:

  • Power is removed/restored
  • Multiple start signals are received
  • Do we need a Reset signal? If so what happens during its operation
  • Do we need a display of the time? Present Value (PV) / Set Value (SV)

As you can see the timing chart is vital in determining how the sequence will be performed. This is the exact same method that I use when determining timing sequences in a PLC program.

Let’s look at an example.

Motor_Sequence

When we hit the start button, the warning light then comes on. After a fixed time the warning light goes off and the motor starts. The motor will run until the stop button is hit.

We will start by using the Start / Stop Circuit we did earlier.

Timer Program

You will notice that we have added an internal memory bit (C0) as our Start Sequence. This is a memory retentive bit, so we can use the (ST0) $FirstScan to make this circuit non-memory retentive. If the power goes off, or the PLC is put into program mode the circuit does not remember the last state. It will default to be off.
The sequence is as follows:

  • Start pressed
  • TMR starts to time (10seconds)
  • Warning output comes on
  • After TMR (10seconds)
    • Warning output goes off
    • Motor output comes on
  • Stop pressed
    • TMR is reset to 0
    • Warning light off
    • Motor is off

Every PLC has timers. They all have different types depending on what you are trying to achieve. It will all start with your Timing Chart.

Watch on YouTube : Learn PLC Programming – Free 8 – The Secret of Timers

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.




Here’s a Quick Way to Wire NPN and PNP devices

Here’s a Quick Way to Wire NPN and PNP devices

I get asked often on how to wire NPN and PNP devices to the programmable logic controller. This can be confusing at first when looking at the wiring diagrams. I have managed to destroy a few sensors in the process….. so let’s get started and I will share my experiences.


E2A_7069B_7l E2A_7002A_7l
NPN and PNP refer to the transistor in the output device.
NPN – Negative Positive Negative Switching. Sometimes referred to as ‘Sinking’ the load.  People have told me that when the NPN sensor blows it has a tendency to blow in an open state. (No Signal)
PNP – Positive Negative Positive Switching. Sometimes referred to as ‘Sourcing’ the load. People have told me that when the PNP sensor blows it has a tendency to blow in a closed state. (Signal On)

When the sensor blows, (malfunctions) it usually will also take out the power supply. (Fuse) It generally does not matter if you use NPN or PNP sensors provided they are all connected to the PLC using isolated commons.

You cannot mix PNP and NPN sensors on the same common point for inputs to the PLC. If you do mix the sensors, then the different common points on the PLC must be isolated from each other. This means that the commons are not connected internally to each other. Not ensuring this takes place will provide a short across the power supply and blow your sensors and supply. In general, machines tend to use all NPN or all PNP only.

Colour coding of the wires vary. Do not always rely on the colour code of the wires for connection. Refer to the wire diagrams in the documentation.

The following is a wiring diagram of an open collector PNP sensor. You will notice that the load appears between the 0V (Blue)  and Switching wire (Black). When connecting to the PLC, the PLC input acts as the load. The 0V (Blue) will be attached to the common input and the Switching wire (Black) will be attached to the input number.PNP1

The following is a wiring diagram of an open collector NPN sensor. You will notice that the load appears between the +V (Brown)  and Switching wire (Black). When connecting to the PLC, the PLC input acts as the load. The +V (Brown) will be attached to the common input and the Switching wire (Black) will be attached to the input number.NPN1

As you can see a direct short will be created if NPN and PNP sensors are wired into the PLC on the same common. The following shows an example of wiring of the 3 wire sensors into a PLC with isolated commons.

NPN_PNP_PLC

Watch on YouTube : Wiring NPN Sensor to PLC

Watch on YouTube : Wiring PNP Sensor to PLC

Watch on YouTube : Wiring Contact (Discrete) PLC Inputs

Wiring Interposing Relays
Watch on YouTube
: Wiring NPN and PNP Sensors into the PLC with an Interposing Relay
If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

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How to make a Start / Stop / Jog circuit in a PLC

Looking at a stop-start jog circuit in the PLC will help us in understanding the differences in hard wiring the circuit and programming.

Let’s start with the basic stop stop circuit.
Here is what it looks like hard wired. (Physical switches wired to outputs devices, such as motor contactor and relays.)
Latching Relay Circuit
When the start pushbutton (NO) is pressed the power is passed through the stop pushbutton (NC) to the control relay (CR). The CR contact closes and ‘seals in’  the start pushbutton. The start pushbutton can now be released because the CR contacts allow the power to pass through to the CR.


NO – Normally Open – This refers to the state of the input device if nothing acts upon it. 
NC – Normally Closed – This refers to the state of the input device if nothing acts upon it.

Let’s take a look at the PLC program for the above wiring diagram.
start stop 001
The first thing that you will notice is that the input for Stop is NO contact and not NC. This is because the actual signal wired in the input is NC and we do not want to inverse this signal. You can see that the stop input is currently on in the program.
If we hit the start pushbutton then the circuit is complete and the output CR turns on.

start stop 002Letting go of the start pushbutton, the output remains on because of the CR input seals in the start pushbutton.
start stop 003
Pressing the stop pushbutton will break the circuit and turn off CR.
start stop 004
Letting go of the stop pushbutton will return us back to the original state shown above.

Adding a jog input to the hard wiring diagram will look something like this:
start stop Jog
You can see that the diagram will work the exact same as the circuit above with the start and stop pushbuttons. The jog when pushed will break the sealing contact, and then make a bypass of the start pushbutton. This will keep the M coil on as long as the jog button is pressed. Letting go of the jog will stop the bypass of the start pushbutton which will stop M coil. When the jog returns to the original state M input will already be off so it will not keep M coil on.
The action on the jog is referred to as a Break before Make device. The jog pushbutton will break the circuit before making another connection.

Sometimes in programming a PLC it can be beneficial to think of the inputs as Make before Break. Inputs are made before the previous ones are broken. The programmable controller will scan the program from left to right, top to bottom. The outputs from the rung above are available to the rungs below. Here is a previous article on PLC scanning.
Lets take a look at PLC program with a jog that will not work.
start stop 010
Even though this looks like it would work… Remember that the contacts in the PLC are making before breaking. You can jog the unit and it will turn on but as soon as you release your finger off of the pushbutton the not jog input will seal the CR in. The output will not be able to turn off.
start stop 011

We must consider the delay from on to off when looking at the PLC program for this circuit.
Here is a circuit that will work:
start stop 020

Notice that we create a delay from on to off by turning on an intermediate bit in the program.

Another way to do start-stop circuits in the PLC is to use the instructions Set (SET)  and Reset (RST).
The set will have all of the conditions to turn on a bit in memory and the reset will have all of the conditions to turn off a bit in memory. These instructions are used to make the program easier to view and troubleshoot.
Here is the same logic above using the set and reset instructions.
start stop 021

Notice that X10 Jog2 is in parallel with the Start. We use a trailing edge one shot in parallel with the Stop. This sets our delay so the output will turn off.

Watch on YouTube: Learn PLC Programming – Free 4

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.