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PLC Concepts

This chapter introduces basic and advanced concepts of ladder logic, which is the mostly

adopted programming language of PLC. Users familiar with the PLC concepts can move to

the next chapter for further programming concepts. However, for users not familiar with the

operating principles of PLC, please refer to this chapter to get a full understanding of PLC

concepts.

Table of Contents

1.1

PLC Scan Method ............................................................................................................... 1-2

1.2 Current Flow........................................................................................................................ 1-3

1.3 NO Contact, NC Contact .................................................................................................... 1-3

1.4 PLC Registers and Relays ................................................................................................. 1-3

1.5 Ladder Logic Symbols ....................................................................................................... 1-4

1.5.1 Creating a PLC Ladder Program ........................................................................... 1-5

1.5.2 LD / LDI (Load NO contact / Load NC contact) ..................................................... 1-6

1.5.3 LDP / LDF (Load Rising edge trigger/ Load Falling edge trigger) ......................... 1-6

1.5.4 AND / ANI (Connect NO contact in series / Connect NC contact in series) .......... 1-6

1.5.5 ANDP / ANDF (Connect Rising edge in series/ Connect Falling edge in series) .. 1-6

1.5.6 OR / ORI (Connect NO contact in parallel / Connect NC contact in parallel) ....... 1-6

1.5.7 ORP / ORF (Connect Rising edge in parallel/ Connect Falling edge in parallel) .. 1-6

1.5.8 ANB (Connect block in series) .............................................................................. 1-6

1.5.9 ORB (Connect block in parallel) ............................................................................ 1-7

1.5.10 MPS / MRD / MPP (Branch instructions) .............................................................. 1-7

1.5.11 STL (Step Ladder Programming) .......................................................................... 1-7

1.5.12 RET (Return) ......................................................................................................... 1-8

1.6 Conversion between Ladder Diagram and Instruction List Mode ................................. 1-9

1.7 Fuzzy Syntax ..................................................................................................................... 1-10

1.8 Correcting Ladder Diagram ............................................................................................. 1-11

1.9 Basic Program Design Examples ................................................................................... 1-13

Delta DVP-SA2 User Manual

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