2. Introduction to Ladder Logic Generic Terms & Techniques

Below are the summary of the terms and techniques you need to know if you want to work with ladder logic. There is simply no way around it. Also included is a program for a simple machine that lets you really comprehend how Ladder Logic works.

To effectively write a program, or even edit one, you the programmer must know how to visualize the effects of the changes you will make. In other words, you have to be able to look at the logic "on paper" and imagine how it will work when it is entered into the PLC. This book will teach you how to do that.

2.1 The Dialects of PLCs (Rung, XIC, XIO, OTE, OTL, & OTU)

We will mow look at the definitions of some generic terms and symbols that we can use when we are talking about PLCs:

Bit: It's an address within a PLC. It can be an input, output or internal coil, among others.

Rung: This is a section of the PLC ladder program that terminates in an output function of some type.

Hardwired input: It's a physical connection to the PLC from an input device (switch, sensor, etc). A hardwired input is labeled INPUT in our examples.

Hardwired output: This is a physical connection from the PLC to an output device (relay, a pilot light, etc). A hardwired output is labeled OUTPUT in our examples.

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. An internal coil is labeled COIL in our examples.

--] [--

Normally Open Contact or NO (called XIC - eXamine If Closed - by Allen-Bradley): When you use a NO contact with a hardwired input, the instruction is OFF or LOW until a voltage is applied to the input. The bit address then goes ON or HIGH, that is, 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.

--] / [--

Normally Closed Contact or NC (called XIO - eXamine If Open - by Allen-Bradley): This is an inverted normally open contact. When you use a NC contact with a hardwired input, this instruction is ON or HIGH (that is "true") until there is a voltage applied to the input. It then goes OFF or LOW, and becomes "false." It can also be used with an internal coil, becoming true when the coil is off and becoming false when the coil is on.

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Output Coil (called OTE - Output Energize by Allen-Bradley): When you use an output coil 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.

-< L >-

Output Latch (OTL): An OTL instruction will force a single bit of logic into a HIGH state if all the conditions leading to it evaluate to true. It's a powerful basic instruction but can cause you a lot of grief if you use it improperly or too frequently.

The OTL instruction is often found on the right side of a ladder logic rung. It switches a bit to a HIGH state once it executes. Unlike the OTE Instruction, the OTL will never turn the bit LOW. In order to make that happen, you can leverage other instructions. Another instruction most commonly paired with OTL is an OTU (Output Unlatch).

-< U >-

Output Unlatch (OTU): The OTU instruction will set a bit to a LOW state if all the conditions leading to it evaluate to true. This instruction is one of the fundamental instructions for PLCs. However, it should be used with caution. This instruction will be found on the right side of a ladder logic structure and turn a bit to a LOW state if the preceding instructions evaluate to true.

Study closely the icons and their names as shown in Figure 2.1.1. This is a summary of basic ladder login symbols (non-generic) as used by Allen-Bradley. Note that there are two possible symbols you can use for each of Normally Closed Contact, Output and Not Output logic.

Figure 2.1.1 Summary of basic ladder logic symbols used by Allen-Bradley.

These symbols are ladder logic instructions. The PLC scans (executes) these symbols. Every PLC manufacturer uses instruction symbols. There are some variations in symbols by manufacturers.

Timer: The timer 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: 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.

TRUE: An indication that a bit is "on". If you press a pushbutton switch that is wired to an input, then the bit is said to be true. Also, if the logic in a rung turns on the output of the rung, then the rung is said to be true.

FALSE: Obviously this is the opposite of true. That is, a bit is "off".

2.1.2. Summary of the Ladder Logic Generic Terms & Techniques

Below is a summary of what you just learned. I have also added a few simple applications you should take a look at.

This summary explains Normally open, Address notation, Tag names, and Sequential function chart. In addition, it will touch upon XOR and XNOR operations. These two operations are similar but have different functions. XNOR, on the other hand, limits the control of an input variable to the negated output. In addition, it simplifies the XOR operation by connecting the internal relay to the output in a break mode.

Normally open

The Normally Open Contact, or NOR, is one of the basic symbols of ladder logic programming. Often referred to as the "Examine If Closed" XIC Instruction, this type of logical check is a direct reapplication of the relay-based contact used in early electrical drawings. Using this symbol in PLC programming allows for simple logic checks and the ability to control outputs. The following are examples of NOR operation.

Now, let's loo at the boolean equation. This can be implemented in many ways, including implementing the equations in boolean algebra. A burglar alarm is a typical application. The inputs are A, B, C, and D. In addition, the outputs are the state of the alarm. As you can see, boolean algebra and ladder logic are closely related, which is why the two types of logic are so closely interrelated.

A retentive timer requires reset instructions. A counter counts up or down until the limit is reached. Once the counter has reached the preset limit, it resets the input bit. In addition, MCRs can be used to force off a section of ladder logic. A delay timer can be an OFF or an ON delay timer. The latter type has status bits, which are used to enable and disable the timer.

Another form of NOR logic involves the use of a PLC and relays. The same rules apply to both types, and the logic ladder diagram is drawn similarly. In this diagram, the rails represent the active connections, and the rungs are zero-volt connections. When the inputs are connected to the PLC terminals, they are represented by a normally open or closed symbol. There are other variations of NOR logic, which can be found in the following chapter.

Address Notation

In ladder logic, addresses are referred to as "bits" or "addresses." These bits represent the flow of electrical current through the logic. For example, if any sensor closes the output, it switches off the output. The program's outputs may be switched on or off in a specified sequence depending on the inputs. There are also multiple types of ladders. For example, problem one could turn off the output of sensor B and switch it to B. The other type of ladder logic would switch off the output of the sensor unless the output was turned on.

A timer is a basic unit of logic. It is often used to time a program. One type of timer is a TON timer. A TON timer works by switching on one input, and turning it off in another. This sequence repeats until a specified number of seconds is reached. The delay time of a TON timer is typically ten seconds. In this example, the output will turn on after a 5.5-second delay.

Ladder logic can be very simple or as complex as the developer decides. For example, one rung of the ladder can be turned on or off based on alternate input conditions. The "OR" condition is a common way to implement this. A ladder logic program can also include multiple "AND" statements to create a loop. Unlike traditional logic, ladder logic can be written in any programming language, and it is easier to use than ever.

Using address notation in ladder logic can be helpful. Address notation identifies the input and output and the logical expression memory addressing structure. Addresses are also described in ladder diagram comments. The comments at the beginning of each rung...