PLC learning series 5: 8 Rules for ladder diagram programming

There are a set of universal laws that governs how a ladder diagram are laid out. By implementing these rules, we introduce a work practice that is understood by all technicians

1. Signals, Decisions and Actions:

All control circuits have three main parts known as the Sections of Signal, Decision and Action.

Signal section: Starts or stops current flow to the circuit. Allows circuit to operate.

Decision section: Uses logic from multiple devices to determine what work is to be done. Decision is used to control a load device.

Action section: Work to be performed based upon decision. (I.e. turn a load device on or off.)

2. Input devices (Control devices) can be connected Parallel or Series:

  • Control systems are connected to the load unit (working coil) between L1.
  • In order to regulate a load (working coil), two or more input systems can be linked in series or parallel.
  • In ladder logic the input can be arranged in either series or parallel.

3. Output or Load device connection:

  • Load devices (lights, solenoid, relay coil, etc) must be powered by full (rated) voltage to operate correctly.
  • Multiple loads must be placed in parallel.
  • Multiple loads must be placed in parallel to achieve full-voltage.
  • One side of a load is always connected to L2 (0V).

4. Line number & Numerical Cross-Reference

A row number is allocated for each horizontal rung. The number of the line is situated on each horizontal ring’s left side. Displayed three row numbers

  • On the correct side of a coil are numerical cross references
  • They identify the line number that their contacts are located on.

5. Same input can use in multiple times in a program:

  • A single input can be used to repeatedly in the different rungs.
  • Using this feature we can trigger different output with single switch simultaneously.

6. Output cannot be used multiple times in one program, except in set/reset and latch/ unlatch functions.

  • Same output cannot be used for different inputs in one program.
  • In the function set/reset, the same output address is used.

Look at the below figure, Using the set and reset feature, different inputs (I1 and I2) are connected to the single output (Q1).

  • The input L1 is to set the outline and L2 for reset.

Latching: The output latch (OTL) instruction is a retentive output instruction that is used to maintain, or latch, an output. If this output is turned on, it will stay on even if the status of the output logic that caused the output to energize becomes false.

7. Outputs address can be used as inputs address:

The output coil works as the input coil with the use of latch/unlatch function. As shown in above example.

8. Input address cannot be used as an output address:

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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