Ladder Logic Fundamentals

Oct 25, 2026

Ladder Logic Fundamentals introduces PLC ladder logic as the dominant method for programming industrial controllers, emphasizing that inputs and ladder rungs evaluate to TRUE or FALSE. The material explains how rung structure places input instructions on the left and outputs on the right, and how processor scanning order affects control flow when jumps or subroutines execute. It also covers core logic functions (AND, OR, XOR), then contrasts logical continuity with electrical continuity to clarify when outputs energize.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages27

Reading Time1 h 8 min

AccessFree to download

PLC Programming Languages

Anatomy of a Ladder Program

Input and Output Placement

Processor Scanning Behavior

Logic Functions

AND

Exclusive OR (XOR)

Logical Continuity

Logical Continuity vs. Electrical Continuity

MET 382 1/14/2008 PLC Fundamentals – Ladder Logic Fundamentals MET 382 Controls & Instrumentation for Automation Spring ‘08 T. E. Kostek Topics 􀂄 PLC programming languages 􀂄 Anatomy of a ladder program 􀂄 Logic functions 􀂄 Logical continuity vs. electrical continuity 􀂄 I/O Mapping 􀂄 Mastering examine on and examine off instructions 􀂄 The PLC scanning process 2 Ladder Logic Fundamentals 1 MET 382 1/14/2008 PLC Programming Languages 􀂄 In the United States, ladder logic is the most popular method used to program a PLC 􀂄 This course will focus primarily on ladder logic programming 􀂄 Other programming methods include: 􀂅 Function block diagrams (FBDs) 􀂅 Structured text (ST) 􀂅 Instruction List (IL) 􀂅 Sequential function charts (SFCs) 3 | Anatomy of a Ladder Program
| 4 | | --- | --- | Ladder Logic Fundamentals 2 MET 382 1/14/2008 Anatomy of a Ladder Program (cont’d) 􀂄 Input instructions are entered on the left 􀂄 Output instructions are entered on the right 􀂄 The power rails simulate the power supply lines 􀂅 L1 and L2 for AC circuits and +24 v and ground for DC circuits 􀂄 Most PLCs allow more than one output per rung 5 Anatomy of a Ladder Program (cont’d) 􀂄 The processor (or “controller”) scans ladder rungs from top-to-bottom and from left-to-right. 􀂅 The basic sequence is altered whenever jump or subroutine instructions are executed. 6 Ladder Logic Fundamentals 3 MET 382 1/14/2008 Anatomy of a Ladder Program (cont’d) A 3-rung example ladder program Anatomy of a Ladder Program (cont’d) Rung Comment Rung Number Description assigned to alias tag Alias tag pointing to base address Base address Input Instruction 8 Ladder Logic Fundamentals 4 MET 382 1/14/2008 Logic Functions 􀂄 PLC programming is a logical procedure 􀂄 In a PLC program,“things”(inputs and rungs) are either TRUE or FALSE 􀂄 If the proper input conditions are TRUE: 􀂅 The rung becomes TRUE and an output action occurs (for example, a motor turns on) 􀂄 If the proper input conditions are not TRUE: 􀂅 The rung becomes FALSE and an output action does not occur 9 Logic Functions (cont’d) 􀂄 Ladder logic is based on the following logic functions: 􀂅 AND 􀂅 OR 􀂄 Sometimes called “inclusive OR” 􀂅 Exclusive OR 10 Ladder Logic Fundamentals 5 MET 382 1/14/2008 | Logic Functions-AND
| | | | | | | | | | --- | --- | --- | --- | --- | --- | --- | --- | --- | | Input 1
Input 2 | | | | | | Output | | | | Input 1 Input 2 Output 0 0 0 0 1 0 1 0 0 1 1 1 | | | | | | | 0 Æ False
1 Æ True | | | Contacts ANDed together | | | | | ( )~~ ~~ | | | 11 | | Logic Functions-OR
| | | | | --- | --- | --- | --- | | Input 1
Input 2 | | Output | | | Input 1 Input 2 Output 0 0 0 0 1 1 1 0 1 1 1 1 | | | 0 Æ False
1 Æ True | | Contacts ORed together

12 | | | | Ladder Logic Fundamentals 6 MET 382 1/14/2008 | Logic Functions-Exclusive OR
In addition to ANDing and ORing, the Exclusive OR (XOR) is also useful. When the inputs are DIFFERENT, the XOR output is true.
| | | | | --- | --- | --- | --- | | Input 1
Input 2 |
XOR
Logic | Output | | | Input 1 Input 2 Output 0 0 0 0 1 1 1 0 1 1 1 0 | | | 0 Æ False
1 Æ True | | 13 | | | | Logic Functions (cont’d) 􀂄 Example 1 – Inputs ANDed together in series Inputs Output Three input instructionsANDed together. All 3 input instructions must be true in order to energize the output 14 Ladder Logic Fundamentals 7 MET 382 1/14/2008 Logic Functions (cont’d) 􀂄 Example 2 – Inputs ORed together in parallel Inputs Output Three input instructions ORed together. If any of the 3 input instructions are true the output will be energized 15 Logic Functions (cont’d) 􀂄 Example 3 – A combination of ANDing and ORing 16 Ladder Logic Fundamentals 8 MET 382 1/14/2008 Logical Continuity 􀂄 Logical continuity in a ladder rung occ

FAQs

What makes ladder logic rungs evaluate to TRUE or FALSE?

PLC programs treat inputs and ladder rungs as logical states. When required input conditions are TRUE, the rung becomes TRUE and energizes the corresponding output action.

Where are input and output instructions located in a ladder program rung?

Input instructions are entered on the left side of the rung, while output instructions are entered on the right side. Power rails simulate the supply lines.

How does logical continuity differ from electrical continuity?

Logical continuity exists when there is a continuous path of TRUE conditions from the left power rail to the output instruction(s), causing the rung to become true. Electrical continuity refers to a complete current path in the input circuit, setting a bit in the PLC memory (input image table) to 1.

Oct 25, 2026

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