Engineer and Technician » PLC Programming

Video: How to Program a TON Timer in RSLogix 500

Neal Babcock — Fri, 29 May 2009 02:10:59 +0000

Let’s say we have a rotating beacon that is mounted near a conveyor. For safety purposes, the beacon turns on when the conveyor is running to let the plant personnel know that the conveyor is operating.

Everyone agrees it would be better if the beacon came on for 5 seconds before the conveyor starts.

The conveyor is automatically started from another section of the plant.

We want the beacon to turn on as soon as the run command is given.

However, we need to delay the start of the conveyor for 5 seconds after the run command has been given. That way, the beacon will be on for 5 seconds before the conveyor starts.

We will do that with a TON (Timer On Delay) instruction. Watch the video to see how.

Addressing in RSLogix 5000

Neal Babcock — Mon, 04 May 2009 18:39:08 +0000

Before you can program any PLC system, you have to understand how the addressing is done in that particulaur PLC. I have included a actual screenshot from RSLogix 5000 below that shows examples of addressing in RSLogix 5000.

Read on to get the details.

INSTRUCTION – RSLogix 5000′s Relay Ladder Logic command language is comprised of “instructions”. An XIC (it looks like a normally open contact –] [-- ) is an instruction. A timer is an instruction. A few of the most common instructions are described below.

BIT - an address within the PLC. It can be an input, output or internal coil, among others.

RUNG - A section of the PLC ladder program that terminates in an output function of some type. Just like in an electrical ladder diagram, a rung has some type of output that is turned on or turned off by the preceding entities in the rung. The first rung in a ladder program is always 0.

HARDWIRED INPUT - a physical connection to the PLC from an input device (switch or sensor, etc.).

RSLogix 5000 defines the address of the input, based on the input cards that you configure.

We'll see how this works later on, but here is an example of a hardwired input:

Local:4:I.Data.3

Here is what each part of the address means:

Local:4:I.Data.3
"Local" means that the module is connected to a controller across a backplane or with a parallel link, keeping the module within a few inches of the controller.

Local:4:I.Data.3
"4" means that the module is module 4 (located in the 5th slot in the rack).

Local:4:I.Data.3
"I" means the bit is an input

Local:4:I.Data.3
"Data" indicates the type of data (this is the default for I/O)

Local:4:I.Data.3
"3" indicates that the bit is 4th input on the card (the bits start with 0).

By the way, don't get the capital "I's" confused with ones.

So, in evaluating our example, we would describe the bit as "Module 4, bit 3".

Here is where some confusion comes in. Because the Rockwell numbering system starts with 0, and the processor resides in Slot 0, our example bit is actually in slot 5. Our bit 3 is actually the 4th bit. We could also describe the bit as "Slot 5, position 4".

You will have to learn to transpose these ways of describing a bit back and forth in your head. If you are troubleshooting a problem, and you want someone to look for a signal on our example bit, you might have to tell him to look at the 4th position on the 5th slot. That will lead him to the physical point on the PLC.

However, you need to keep in mind that the corresponding bit in your program will be labeled Local:4:I.Data.3.

It can be confusing, but you will get used to it.

HARDWIRED OUTPUT - a physical connection from the PLC to an output device (relay or pilot light, etc.)

Outputs are addressed the same way.

Local:5:O.Data.4
"Local" means that the module is connected to a controller across a backplane or with a parallel link, keeping the module within a few inches of the controller.

Local:5:O.Data.4
"5" means that the module is module 5 (located in the 6th slot in the rack).

Local:5:O.Data.4
"O" means the bit is an output

Local:5:O.Data.4
"Data" indicates the type of data (this is the default for I/O)

Local:5:O.Data.4
"4" indicates that the bit is 5th output on the card (the bits start with 0).

INTERNAL COIL
This is a programmable bit used to simulate a relay within the PLC. The internal coil has no connection to the outside world. It does not connect to an output card. Internal coils are used to store information. The "contacts" of this "relay" can then be used multiple times in other parts of the program.

RSLogix 5000 has greatly simplified the process of describing an internal coil. We can simply give it a name, known as a tag.

For example, if you have an internal coil that is the result of, say, three hardwired safety gate limit switches, we could label the coil "SafetyGatesClosed".
Note the lack of spaces in the tag name. RSLogix 5000 does not allow spaces, or other special characters, in the tag name.

Some people use underscores, so the tag might be "Safety_Gates_Closed". Either way is fine; it just depends on what your company or your client prefers.

TIMER
A timer is a programmable instruction that lets you turn on or turn off bits after a preset time.

The two primary types of timers are TON for "timer on delay" and TOF for "timer off delay".

Timers in RSLogix 5000 use tag names for identification.

COUNTER
A counter is a programmable instruction that lets you turn on or turn off bits after a preset count has been reached.

There are different types of counters available in the RSLogix, but the CTU (counter up) instruction covers everything we will talk about here.

Counters in RSLogix 5000 use tag names for identification.

--] [-- Normally Open Contact
When used with a hardwired input, this instruction is off until there is a voltage applied to the input. The bit address then goes high, or on, and the instruction becomes "true." It works the same way when it has the same address as an internal coil, except that the coil must be turned on by logic in the program.

Allen-Bradley calls these normally open contacts "XIC", or "eXamine If Closed" instruction.

An XIC instruction can reference a hardwired input, a hardwired output, an internal coil or a timer done bit, among others.

--]/[– Normally Closed Contact
This is an inverted normally open contact.

When used with a hardwired input, this instruction is “true” until there is a voltage applied to the input. It then goes low, or off, and becomes “false.”

It also can be used with an internal coil, becoming true when the coil is off and becoming false when the coil is on.

Allen-Bradley calls these normally closed contacts “XIO”, or “eXamine If Open” instructions.

If it is used as an internal coil, it will toggle the instructions associated with it. That is, it will close a normally open instruction and open a normally closed instruction.

Allen-Bradley calls these outputs “OTE”, or “OutpuT Energize”.

An OTE may be used with a hardwired output or an internal coil.

TRUE – A state that indicates an instruction is allowing logic to “flow” through it.

Also, if the logic in a rung turns on the output of the rung, then the rung is said to be true.

FALSE – Without stating the obvious, this is the opposite of true.

Excerpted from PLC Programming with RSLogix 5000

Connecting to an Allen-Bradley SLC and Going Online

Neal Babcock — Thu, 12 Mar 2009 04:29:16 +0000

I get a lot of questions about going online with a PLC. Here is a section from PLC Programming with RSLogix 500 that covers going online with an SLC 5/03.

You have written your program and now you are ready to download, or send, the Batching program to the computer.

WARNING
There are dangerous voltages present on terminals of the PLC. Follow all warnings and instructions from the Allen-Bradley manual for connecting power to the PLC. If you are not familiar with hazards and the potential dangers of these voltages STOP RIGHT NOW. Consult a trained professional who is able to assist you.

First, backup up your original file and put it in a safe place.

Check the serial port settings of your computer in Control Panel. Make sure the baud rate (or bits per second) of your COM port is set as high as possible 57600 should work there.

Allen-Bradley provides a cable to connect the serial port of your computer to the 9-pin serial port on the SLC 500 processor. Connect the cable and turn power on to the PLC.

Turn the key operated switch on the processor to the center position PROG (program).

If you do not have RSLinx installed, you will have to do that now.

After you install RSLinx, you will have to configure it for your computer and your PLC.

To do that,

Choose
Start > Programs > Rockwell Software > RSLinx > RSLinx

In the Notification are of your Taskbar (the lower right of your Windows screen) that looks like this.

The one on the left is the RSLinx Communications Service. The other one is for the RSI Directory Service.

Click on the left one, the RSLinx Communications Service. The RSLinx screen will appear.

Choose

Communications > Configure Drivers

The driver screen appears.

Choose RS-232 DF1 devices from the “Available Driver Types” drop-down menu.

Click “Add New”.

Click “OK”.

You probably have your cable connected to COM1 of your computer. Click “Auto-Configure”.

In the box to the right of the “Auto-Configure” button, you should see a message that says “Auto Configuration Successful”. In addition, the “Device:” box should show “SLC-CH0/Micro/PanelView”.

Click “OK”. You should see a screen like this.

Click “Close”. You may close the RSLinx screen – RSLinx will still run, as indicated by the icon in the Notification area of your taskbar.

Open RSLogix.

Choose Comms > System Comm… from the menu. You will see a screen like this. You may have to maximize the screen and resize the panels to get the right view.

In the explorer panel on the left, click on the “+” next to “AB_DF1-1, DH-485”. You should see animation in the icon, as the tiny blue square moves around the little network symbol. The computer icon for address 00 indicates your computer. The icon for address 01 indicated the PLC. In our case, it is an SLC-5/03.

Select the SLC in the left window and click “OK”. The communications window will close.

In the RSLogix 500 main screen, choose File. Select your program from the Recent File List (it is probably 1).

Choose “Controller Properties” from the Project tree. Click on the “Controller Communications” tab.

Click on the “Driver” dropdown box and select “AB_DF-1-1”.

Click “Apply” then “OK”.

Choose Comms > Download

You should get a screen similar to this.

Choose “Yes”.

The program will begin downloading.

Since we are downloading a new configuration, this screen will appear.

Choose “Apply”. The program is now in the PLC.

Sometimes it is necessary to cycle power on your computer. Making the first connection to a PLC can be tricky. Prepare yourself by having the phone number for your Rockwell rep handy.

If things go well, you may be able to go online right away.

Choose Comms > Go Online

Choose “Browse” and find the folder where you file is located. Click “OK”.

The file name will show in the bottom box. Click on it to select it, then click “Upload Use File”.

You may need to change the Baud Rate of the PLC. Click “Channel Configuration” in the Project tree and click on the “Chan. 0 – System” tab. Choose “19200” from the Baud dropdown menu. Click “Apply” and “OK”.

Tip: In Allen-Bradley PLC vernacular, upload means get the program from the PLC and load it in RSLogix on your computer.

Download means send the program from RSLogix on your computer to the PLC.

You will now see a screen like this. Notice that the ladder icon is animated to indicate you are online.

Setting up an SCP in RSLogix 500

Neal Babcock — Mon, 23 Feb 2009 02:18:12 +0000

An SCP (Scale with Parameters) is really handy instruction in RSLogix 500. It scales any range of values to a range defined by the programmer.

One of the common uses of the SCP instruction is to take the value from an analog input and scale it to an engineering value.

In the example below, you can see how the output from a set of scales can be scaled to show the actual weight of the tank.

In RSLogix 500, double-click on 0000. This opens up the ASCII editor.

Type SCP in the box and press enter. SCP stands for “Scale with Parameters”. It allows you to take an analog input from a sensor and scale it to the output units you want. [Note: You could certainly insert the instruction using the toolbar. I just wanted to show how to use the ASCII editor to insert an instruction.]

Before we start scaling, let’s take a moment to see how the Allen-Bradley NI4 converts a 0-10VDC signal to a number. The NI4 is an analog-to-digital processor that takes the 0-10VDC signal and converts it to a number between 0 and 16384.

The NI4 will yield a number from 0 to 16384 that is directly proportional to the 0 to 10VDC signal that is applied at the input.

In other words, zero volts on the input of the NI4 means that the NI4 will provide 0 as a value to the PLC. Ten volts on the input will yield 16384. Five volts on the input will yield half of 16384, or 8192, and so on.

The SCP instruction starts out looking like this.

Each of the six parameters (Input, Input Min., Input Max., Scaled Min., Scaled Max. and Output) has two fields associated with them. Each field is currently filled with a question mark. The first field is a value that you assign. The second field is the actual value returned by the processor.

Setting up an SCP to calculate Tank Weight
The Input parameter is the value that will be scaled. Let’s use this SCP instruction for the Scales. The input we will use is the address we assigned to the analog input card; that is, I:1.0.

The Input Min parameter is the value that is read by the analog card when there is no liquid in the tank. With our scales, this value is 0.

The Input Max is the value that is read by the card when the tank is full. Our Mixing tank weighs 2000 lbs. when it is full of liquid. We measured the voltage that the Scale put out when the tank was full and found it to be 10 volts.

So, in this case, 2000 lbs. in the tank equals 10 volts, which means the NI4 will read out 16384 when the tank has 2000 lbs. of liquid in it.

The Scaled Min parameter is the lowest value you want the SCP to calculate in the units you want. In this case, it is 0, and we are using pounds as our units.

The Scaled Max parameter is the highest value you want the SCP to calculate. For the Scales in our project, it is 2000.

The Output parameter is typically an address where you want to store the result of the SCP. We are going to put it in the N7 file (integer). We will store it specifically in N7:0.

Here is how the SCP instruction for our Scales looks.

Admittedly, the numbers rarely work out like this, but for simplicity’s sake, I made them easy to work with.

The real beauty of the SCP is apparent after you go online. Let’s say that you couldn’t really calibrate the scales previously by using voltmeter.

After you go online, and you are getting live data from the SCP instruction, you can visually verify that the tank is empty.

Hoverer, you are reading 133 from the NI4. You simply enter 133 as the Input Min parameter.

When the tank is full, you see that the reading is 14733. Enter 14733 as the Input Max parameter and the SCP will calculate the rest for you.

The important thing to remember is that the value in N7:0 is the actual weight of the tank in pounds. We will use that when we program the setpoint logic.

This is how N7:0 is used. In the example below, we want to add 1275 pounds of water to a tank. As long as the weight of the tank (N7:0) is below 1275 pounds, the city water valve will remain open.

Excerpted from PLC Programming with RSLogix 500

Troubleshooting A PLC With RSLogix 5000

Neal Babcock — Mon, 16 Feb 2009 18:17:17 +0000

Excerpted from How To Troubleshoot With A PLC

Let’s consider a batching system just like the one we worked on earlier, except that the PLC is an Allen-Bradley ControlLogix. The software we use to program a ControlLogix PLC is Rockwell’s RSLogix 5000.

TROUBLESHOOTING OPERATION #3

BATCHING SYSTEM WON’T PUMP FINISHED PRODUCT

The problem is that the system is not working. A completed batch was being pumped to the filing lines when the pumped stopped and the “System Fault” pilot light came on. Pressing the “Start” or “Stop” buttons has no effect.

Shown below is a graphical layout of the system. In fact, apart from the program, this is all the information you have.

The operator station looks like this.

The “System Fault” pilot light is on.

As you go online, this screen appears.

We want to find the rung that generates a system fault.

In the Controller Organizer (the window on the left), click on the “+” folder icon to open the “Tasks” folder.

Open the “Main Task” folder.

Open the “Main Routine”.

The screen looks like this.

Finding The Problem

We could use CTRL-F to find the system fault bit, but it conveniently appears in the first rung.

The instruction is false, which keeps the output “SystemEnable” false.

Right-click on the tag name (SystemFault) and choose “Find All SystemFault”. The search results box appears at the bottom of the screen.

We are looking for the output, or OTE. Click on the blue line that says “Found: Rung 22, OTE Operand 0: OTE(SystemFault).

We are taken to Rung 22.

The output is true, but there is no other instruction in the rung that is highlighted.

Take a closer look at the GRT instruction. A GRT instruction is true when Source A is greater than Source B.

In our GRT instruction, Source A is 100 and Source B is 95. That makes the instruction true, which is turning on the “SystemFault” output, which is keeping the system from running.

Not all instructions in RSLogix change color when they are true.

According to the description applied to the instruction, Source A is the liquid level in the mixing tank. It is being reported as 100 (whatever that means).

Source A gets its value from Local:2:I.Ch1Data. We can tell that this tag represents a hardwired input.

We can deduce that this is an analog input, since we are getting a value that is not just 1 or 0.

Investigating An Analog Input
In the Controller Organizer, double-click on “I/O Configuration”. On this PLC, slot 2 has a 1756-IF8 card. Right-click on slot 2 and choose “Properties”. The “Module Properties: Local:2 (1756-IF8 1.1)” window appears. Click on the “Configuration” tab. Click on the “Channel 1″ button.

The configuration is shown like this.

Now we see that the input range on this channel is 0 to 10 volts. The engineering units applied to the scaling suggests that the level in the tank is shown as a percentage. That is, 0 volts equals 0% and 10 volts equals 100%.

Let’s go back to Rung 22.

The GRT will become true if the level in the tank exceeds 95%, which sets a system fault. That makes sense.

The PLC thinks the tank is 100% full. As we look inside the tank, we can see that it is nearly empty.

We can conclude that the PLC program is functioning properly. It is doing what it is supposed to do.

This is very likely to be a hardware problem. There could be a short in the signal wire from the level sensor, the sensor could be defective or the input card on the PLC could be bad.

Your electrician measures the voltage on the input of the PLC card and sees 10 volts. He disconnects the signal wire from the PLC, and you see the value of Source A in the GRT instruction drop to zero.

He measures the voltage at the output of the level sensor and again finds 10 volts.

You can manually pump the remaining liquid out of the tank and monitor the voltage at the output of the sensor. If it does not change, then the next step would be to clean, repair or replace the sensor.

The sensor is replaced and you see a more accurate level indicated in the GRT.

Testing the sensor to make sure it shuts down the system if the level gets too high might be difficult without disrupting a batch. You may have to use your imagination here. I will leave that up to you.

Equivalent Logic in PLC Programming

Neal Babcock — Sat, 31 Jan 2009 01:00:48 +0000

If you understand basic electrical diagrams, it is much easier to understand ladder logic.

In fact, that was the whole idea behind ladder logic. It was supposed to look, and work, like real electrical circuits

Excerpted from “The Beginner’s Guide to PLC Programming”

In its elementary form, PLC logic is very similar to the hard-wired logic you would find in an electrical ladder diagram.

For example, if you wanted to turn on a light with a momentary pushbutton, you would wire it like the circuit below.

When you press PB1, the pilot light PL1 lights up.

Now let’s do the same thing in a PLC. To duplicate the hardwired circuit on a PLC, you would wire the switch PB1 to an input and wire the light PL1 to an output. Each PLC manufacturer gives you the details of wiring their particular modules. The I/O (hardwired inputs and outputs) is set up like this:

- There is a “PB1” pushbutton switch wired to INPUT1 of the PLC.
- There is a “PL1” pilot light wired to OUTPUT1 of the PLC.

Now let’s examine the sequence of events. When you first turn on the PLC, the PB1 pushbutton is off, or false. Therefore, the PL1 output is off. Pressing PB1 will make INPUT1 true, OUTPUT1 will come on and the light will be energized. It will stay on only as long as you hold the button in. Just like electrical current has to flow through the switch to turn on the light in the hardwired circuit, the logic has to “flow” through the normally open instruction (which is closed when you press the switch) of INPUT1 to energize the output that turns on PL1.

The programming terminal display will look something like this as you hold in PB1. The yellow highlight indicates the bit, or address, is “on” or “true”.

RSLogix 500 Addressing in an Allen-Bradley PLC

Neal Babcock — Sun, 04 Jan 2009 01:41:33 +0000

There are a number of addressing schemes used by PLC manufacturers. Let’s take a quick look at how memory locations (including hardware I/O) are accessed with RSLogix 500. Along the way, I will define some terms.

INSTRUCTION – RSLogix’s command language is comprised of “instructions”. An XIC (it looks like a normally open contact –] [-- ) is an instruction. A timer is an instruction. A few of the most common instructions are described below.

BIT - an address within the PLC. It can be an input, output or internal coil, among others.

In RSLogix, there are a couple of ways to show the address of a bit. The default is:

[type]:[word]/[bit]

For example, an address that references an output of an SLC 500 is O:5/0. That is:

O:5/0 means that it is a physical output.
O:5/0 means that it uses Slot 5 (the 6th physical slot) in the rack.
O:5/0 means that it is the first output on the card.

Remember that the first slot in an SLC 500 rack is Slot 0. That means a card that is installed in the 6th physical slot is addressed as Slot 5.

Allen-Bradley PLC slots, like many computers, always start with 0.

By the way, don’t get the capital “O’s” confused with zeroes.

RUNG – A section of the PLC ladder program that terminates in an output function of some type. Just like in an electrical ladder diagram, a rung has some type of output that is turned on or turned off by the preceding entities in the rung. The first rung in a ladder program is always 0000.

HARDWIRED INPUT – a physical connection to the PLC from an input device (switch or sensor, etc.).

Allen-Bradley uses the capital letter “I” to designate a hardwired input. An address that describes an input on an SLC 500 is I:4/0.

Similar to the output structure,

I:4/0 means that it is a physical input.
I:4/0 means that it uses Slot 4 (the 5th slot in the rack).
I:4/0 means that it is the first input on the card.

Don’t get the capital “I’s” confused with ones.

HARDWIRED OUTPUT – a physical connection from the PLC to an output device (relay or pilot light, etc.) As was said above, an address that references an output of an SLC 500 is O:5/0.

INTERNAL COIL
This is a programmable bit used to simulate a relay within the PLC. The internal coil has no connection to the outside world. It does not connect to an output card. Internal coils are used to store information. The “contacts” of this “relay” can then be used multiple times in other parts of the program.

In RSLogix, the “B3” (binary) file is commonly used for all the internal coils. There are many other words in other files that have bits you can use as internal coils, but we are going to stick with the B3 file for our application.

B3:0/0 means that it references an internal Binary file
B3:0/0 means that it uses the first word in the table
B3:0/0 means that it is the first bit in the word.

Note that, unlike the Output and Input files, you have to use the file number in the address. In this case, the default file number is 3.

TIMER
A timer is a programmable instruction that lets you turn on or turn off bits after a preset time.

The two primary types of timers are TON for “timer on delay” and TOF for “timer off delay”.

Timers in A-B SLC and MicroLogix processors use file 4 for their timers.

T4:0 means that it references an internal Timer file
T4:0 means that it uses the first timer in the table

The address T4:0 simply refers to the timer. Each timer has bits that turn on after the timing function is complete. You can address this bit by simply putting a “/DN” after the timer address. DN stands for “done”.

For example, if timer T4:0 is a TON (timer on delay), then the bit T4:0/DN will turn on after the timer has reached its preset value.

COUNTER
A counter is a programmable instruction that lets you turn on or turn off bits after a preset count has been reached.

There are different types of counters available in the RSLogix, but the CTU (counter up) instruction covers everything we will talk about here.

Counters in A-B SLC and MicroLogix processors use file 5.

C5:0 means that it references an internal Counter file
C5:0 means that it uses the first counter in the table

The address C5:0 simply refers to the counter. Each counter has bits that turn on after the counting function is complete. You can address this bit by simply putting a “/DN” after the counter address. DN stands for “done”.

For example, if counter C5:0 is a CTU (counter up), then the bit C5:0/DN will turn on after the counter has reached its preset value.

–] [-- Normally Open Contact
When used with a hardwired input, this instruction is off until there is a voltage applied to the input. The bit address then goes high, or on, and the instruction becomes “true.” It works the same way when it has the same address as an internal coil, except that the coil must be turned on by logic in the program.

Allen-Bradley calls these normally open contacts “XIC”, or “eXamine If Closed” instruction.