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PLC Programming Fundamentals - Coggle Diagram
PLC Programming Fundamentals
PLC Memory System
Memory Classification
Program Memory
Stores ladder logic instructions
Data Memory
Stores I/O and internal data
Program Storage
System Files
Processor configuration data
Main Program
Primary control logic
Data Storage
Image Tables
Input and output status
Control Files
Timers, counters, bits
Input Processing
Input Devices
Manual Inputs
Pushbuttons and switches
Automatic Inputs
Sensors and limit switches
Input Image Table
Binary Representation
1 = ON, 0 = OFF
Update Timing
Updated during input scan
Input Purpose
Signal Stability
Prevents false triggering
Consistency
Same data used during scan
Output Processing
Output Devices
Indicators
Lamps and alarms
Actuators
Solenoids and motors
Output Image Table
Output Commands
Written by logic results
Temporary Storage
Held until output scan
Output Action
Physical Control
Energizes field devices
Status Feedback
Reflects program outcome
PLC Scan Cycle
Input Scan
Signal Reading
Reads field input voltage
Memory Update
Updates input image table
Program Scan
Logic Evaluation
Solves ladder rungs
Output Decision
Writes output bits
Output Scan
Output Update
Transfers bits to modules
Transfers bits to modules
Diagnostics and communication
Scan Time
Definition
Scan Duration
One full PLC cycle
Measurement
Milliseconds
Influencing Factors
Hardware Speed
CPU clock rate
Program Size
Number of rungs
System Impact
Response Speed
Reaction to input change
Missed Signals
Fast pulses undetected
PLC Programming Languages
Standardization
IEC-61131
Defines PLC languages
Compatibility
Vendor consistency
Graphical Languages
Ladder Diagram
Relay-style logic
Function Block Diagram
Data-flow blocks
Text Languages
Instruction List
Mnemonic commands
Structured Text
High-level coding
Logic Instructions
Input Instructions
Examine If Closed
True when bit = 1
Examine If Open
True when bit = 0
Output Instruction
Output Energize
Acts like relay coil
Bit Control
Sets or resets output
Logic Interpretation
Memory Based
Uses data table bits
Not Physical Wiring
Logic driven, not hardware
Ladder Logic Structure
Rung Composition
Input Conditions
Contact instructions
Output Instruction
Coil symbol
Logic Flow
Scan Direction
Left-to-right only
Execution Order
Top-to-bottom
Logic Continuity
True Path
Complete logic connection
Output Activation
Energizes coil
Branching & Internal Control
Branch Instructions
Input Branching
OR logic paths
Output Branching
Multiple outputs
Internal Relays
Internal Bits
Stored in memory only
No Field Wiring
Software-only control
Advantages
Hardware Reduction
Fewer output modules
Program Clarity
Easier maintenance