Engineer and Technician » Ladder Logic

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.

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 500

RSLogix 5000 “Serial” Logic

Neal Babcock — Mon, 22 Dec 2008 21:21:15 +0000

RSLogix 5000 allows the use of “serial” logic that does not conform to traditional, electrical ladder logic.

For example, both of the rungs shown below are valid in RSLogix 5000.

Clearly, the second version would not work if wired that way in an equivalent electrical circuit. It would not be allowed in RSLogix 500, either.

The main advantage, in my opinion, to writing the code as it is shown in the second version is that you can get more instructions on the screen, and that involves less scrolling. And, the logic is slightly different; if something turns off the “SystemReady” bit somewhere else in the program, PL1 would not come on.

The main disadvantage, in my experience, is that the second version will drive electricians and maintenance people crazy, if they are not familiar with RSLogix 5000. Their managers will most likely request that you re-write the rung in “traditional” ladder logic.

Setting up the Analog Input Card in RSLogix 5000

Neal Babcock — Fri, 14 Nov 2008 20:24:01 +0000

Let’s assume that the output of our scales has been calibrated for 0-10 VDC. Zero volts equals 0 pounds, and 10 volts equals 2000 pounds.

Now we know the signal type is DC voltage, the range is 0-10 and the engineering units are pounds.

Right-click on the 1756-IF8 card in the Controller Organizer and choose “Properties”. Click on the “Configuration” tab and you will see this.

You’ll see that Channel 0, which is our Scales channel, is selected.

Click on the dropdown menu for “Input Range” and select “0V to 10V”.

Change the “Low Signal” field to 0.

Change the “High Engineering” field to 2000.

Change the “Low Engineering” field to 0.

That is all we really need to do. However, we are going to take advantage of the fact that there is a filter available. This filter smoothes input transitions.

Set the “Digital Filter” field to 1000 ms.

Click “Apply” and we are done with the scales.

Ladder Logic and the Dialect of PLCs

Neal Babcock — Tue, 11 Nov 2008 19:50:04 +0000

Ladder logic is a programming language, like Visual Basic or C. When you learn any language, it is always important to understand the basic terms.

If you are just getting started, here are a few terms you should know. These apply to any PLC.

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

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.

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

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

--]/[– 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.

-( )- Output Coil
When used with a hardwired output, this function is off until the logic in the program allows it to turn on. It then becomes “true”, and will energize the device that is wired to the respective output. 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.

+———–+
TIMER |
+– SEC —+ Timer
This function is used to supply a programmable delay. It requires the use of its “timer finished” bit, like a time delay relay uses its contact.

+————+
COUNTER |
+— 000 —+ Counter
The counter function is used to count events. It could be used to keep track of machine cycles, count parts, etc. It can be programmed with a preset value that triggers another event when the count is reached.