Ladder logic
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- The smart thing about ladder logic is that it looks very similar to electrical relay circuits.
- Basics of PLC programming
- Basics of logic gates like OR, AND, NOT, NOR, NAND, XOR.
- NO PRIOR EXPERIENCE IN ANYTHING
- This course is mostly at a BEGINNER level in general
Ladder logic is a programming language that is used to program a PLC (Programmable Logic Controller). It is a graphical PLC programming language which expresses logic operations with symbolic notation using ladder diagrams, much like the rails and rungs of a traditional relay logic circuit. It is used by engineers and electricians to execute logical, sequential, counting, timing and arithmetic tasks in order to carry industrial automation applications.
In the good old days, machine and process automation was accomplished using a hard wired control system known as relay logic. With the advent of microprocessors and the invention of the PLC, relay logic quickly became superseded by programming languages such as ladder logic.
- Beginner in PLC programming
- Beginner in INDUSTRIAL INSTRUMENTATION
- Beginner in INDUSTRIAL AUTOMATION
- Beginner in LOGIC CIRCUITS
Necessity is the mother of invention. Programming logic controller (PLC) invention is yet another example that supports this truth. A Programmable Logic Controller or PLC for short, is simply a special computer device used for industrial control systems. The etymology of this innovation of electronics research is very simple and interesting. In late 1960's, General Motors were in a find of an alternative mechanism for analog relays in control circuitry. Engineers developed a standard machine controller that is capable of performing the task of analogy relays. In simple terms, it can be termed as a significant evolution in the industrial automation revolution.
Rules of ladder logic programming
The vertical lines of the ladder diagram represent the power rails. The circuits are connected between these vertical lines.
The power flow is taken to be from the left hand vertical across a rung.
Each rung on the ladder defines one operation in the control process.
A ladder diagram is read from left to right and from top to bottom.
The top rung is read from left to right. Then the second rung down is read from left to right and so on.
When the PLC is in its run mode, it goes through the entire ladder program to the end.
The end rung is indicated by a block with the word END or RET for return.
Each rung must start with an input or inputs and must end with atleast one output.
A particular device can appear in more than one rung of a ladder. The same letters and/or numbers are used to label the device.
The inputs and outputs are all identified by their addresses, the notation used differs from PLC manufacturer to manufacturer.
Electrical devices are shown in their normal condition. Thus a switch which is normally open until some object closes, it is shown as open on the ladder diagram. A switch that is normally closed is shown closed.
The term input is used for a control action, such as closing the contacts of a switch ie an input to the PLC. The term output is used for a device connected to the output of a PLC.
The end rung of the program is being mentioned with clarity, the program promptly resumes at the start. This procedure of going through all the rungs of the program is termed as a "cycle".
The AND gate is a basic digital logic gate that implements logical conjunction - it behaves according to the truth table to the right. A HIGH output (1) results only if all the inputs to the AND gate are HIGH (1). If none or not all inputs to the AND gate are HIGH, a LOW output results. The function can be extended to any number of inputs.
The OR gate is a digital logic gate that implements logical disjunction – it behaves according to the adjacent truth table. A HIGH output (1) results if one or both the inputs to the gate are HIGH (1). If neither input is high, a LOW output (0) results. In another sense, the function of OR effectively finds the maximum between two binary digits, just as the complementary AND function finds the minimum.
Inverting NOT gates are single input devices which have an output level that is normally at logic level “1” and goes “LOW” to a logic level “0” when its single input is at logic level “1”, in other words it “inverts” (complements) its input signal. The output from a NOT gate only returns “HIGH” again when its input is at logic level “0” giving us the Boolean expression of: A = Q.
In digital electronics, a NAND gate (NOT-AND) is a logic gate which produces an output which is false only if all its inputs are true; thus its output is complement to that of an AND gate.
The NOR gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output (1) results if both the inputs to the gate are LOW (0); if one or both input is HIGH (1), a LOW output (0) results. NOR is the result of the negation of the OR operator.
An XOR gate circuit can be made from four NAND gates. In fact, both NAND and NOR gates are so-called "universal gates" and any logical function can be constructed from either NAND logic or NOR logic alone.