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PLC HARDWARE COMPONENTS - Coggle Diagram
PLC HARDWARE COMPONENTS
The I/O Section & Chassis
Chassis System: The physical frame with 4, 8, 12, or 16 slots; contains the backplane for power and data.
Local vs. Remote: Local I/O is in the CPU chassis; Remote I/O is located near field devices to save wiring.
Connectivity: Fiber optics (20+ miles/noise immune) or Coaxial (2+ miles).
Addressing Logic: How the CPU finds a device.
Rack/Slot Addressing: Fixed physical locations (e.g., $I:3/8$).
Tag-Based Addressing: Alphanumeric naming used in modern systems (e.g., Conveyor_Start).
Analog I/O Modules (Word-Based)
Signal Conversion
A/D Converter: Turns varying voltage/current (0–10 V, 4–20 mA) into a digital word.
D/A Converter: Turns a program value into a signal to control valves or motor speeds.
Signal Integrity
Resolution: The precision of the signal (e.g., 0.3 mV increments).
Shielding: Use of shielded twisted-pair cables to prevent signal degradation over distance.
Discrete I/O Modules (Bit-Based)
Input Circuitry: Converts field signals (switches, buttons) to logic.
Optical Isolation: Uses light to protect CPU logic from high-voltage field spikes.
Filtering: Removes "contact bounce" and electrical noise (9–25 ms delay).
Output Switching
Relay Outputs: Mechanical contacts; handles both AC and DC but operates slower.
Solid-State: Triacs for AC loads; Transistors for DC loads.
Current Flow Logic
Sinking (NPN) vs. Sourcing (PNP): Defines whether the module or the field device provides the positive voltage source.
Special I/O & Intelligent Modules
Intelligent Processing
PID Module: An on-board processor that handles complex math for temperature or flow control.
High-Speed Counter: Tracks pulses up to 100 kHz, bypassing the standard PLC scan time.AC
Specialty Interfaces
Motion Control: Modules designed specifically to drive Stepper or Servo motors.
Communication Modules: Connects the PLC to peer-to-peer networks or office computers.
Central Processing Unit (CPU) and Memory
CPU
Scan Cycle: Reads inputs- Executes logic - Updates outputs
Processor: Contains the microprocessor, memory, and support logic
Memory Structure
Memory Structure: Divided into the Executive (OS) and User (Ladder Logic) areas.
Memory Types
EEPROM/Flash: Non-volatile; used for permanent storage of programs.
RAM: Volatile; fast for program execution; requires battery backup.
Interface & Data
HMIs
Function: Graphical touchscreens for operator monitoring and control
Data Entry: Adjusting setpoints and viewing real-time process status.
Programming Terminals
Recording: Methods for storing, retrieving, and backing up programs.
Devices: Laptops or handheld terminals used for code download
I/O Specifications
Discrete Output Specifications
Leakage Current: The small current flow ($< 5$ mA) present even when the output is logic "0."
Short Circuit Protection: Fusing (individual or grouped) to protect the module from wiring faults.
Inrush Current: The momentary surge capacity for starting inductive loads like motors or solenoids.
Discrete Input Specifications
Threshold Voltages: Specific ON/OFF voltage levels that distinguish valid signals from noise.
Response Time: The delay (9–25 ms) required for the circuitry to filter contact bounce.
Nominal Voltage: Defines the magnitude (24V/120V) and type (AC/DC) the module accepts.
Analog I/O Specifications
Common Mode Rejection: The module’s ability to ignore electrical noise shared by both signal wires
Input Impedance: The resistance the module places on the signal loop, affecting signal accuracy
Resolution: The bit-level precision of the A/D converter (e.g., how many mV per digital increment).