PLC learning series 4 : How a ladder logic diagram works? How to read ladder logic ?

How a Ladder Logic diagram works?

Ladder diagrams describe programs in graphical form, used in PLC programming. This diagram is developed from structured relay contacts that describe the flow of electric current.

In the ladder diagram there are two vertical lines where the left vertical line is connected to the positive voltage source of the power supply and the right line is connected to the negative voltage source of the power supply.

The ladder program is written using a pictorial shape or symbol that is generally similar to a relay control circuit. The program is displayed on the screen with elements such as normally open contact, normally closed contact, timer, counter, sequencer etc.
displayed as in a pictorial form.

Under the right conditions, electricity can flow from the left-hand rail to the right-hand rail, a railroad track like this is called a ladder line.

The general rules in describing a ladder diagram are:

  • Power flows from the left rail to the right rail
  • The coil output cannot be connected directly to the left rail.
  • No contacts are placed to the right of the coil output
  • Only one coil output is allowed on the ladder line

Autocade simples for ladder logic:

The symbols displayed are given in an AutoCAD drawing for learners:

How to read a ladder logic program?

  • Ladder diagrams are drawn vertically with inputs on the left and outputs on the right.
  • Each rung of the ladder has output
  • An individual output device can appear on the ladder diagram, read from left to right
  • An individual physical input device (limit switch, push-button, pressure switch, etc.) may be used on the ladder diagram in both N.O. and N.C. configurations.
  • Completing one rung, read the below rung as a continuation, till the end marked as stop.
  • Internal contacts of the PLC are represented as conventional control relays and contacts.
  • Control relay coils (outputs) appear on the ladder diagram.

PLC learning series:

PLC learning Series I: What is PLC? Functions of PLC
PLC learning series 2: PLC programming languages
PLC learning series 3: PLC Architecture and wiring of PLC
PLC learning series 5: 8 Rules for ladder diagram programming
PLC learning series 6: PLC process Scan basics
PLC learning series 8: Instruction List programming
PLC learning series 9: PLC selection criteria.
PLC learning Series 10: PLC timers
PLC learning series 11 : How to interface PLC with SCADA?
PLC learning series 12: Troubleshooting PLC
PLC learning Series 13: Counters in PLC


Instrumentation Engineer

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