PLC CPU Symbol
Definition: The PLC CPU symbol represents the central processing unit module of a Programmable Logic Controller, designated PLC or CPU in schematics, that executes the user's ladder logic or function-block program by scanning digital and analogue inputs (In 1, In 2) and setting digital outputs (Out 1, Out 2), per IEC 61131-3 and ANSI/NEMA ICS standards.
Also known as: PLC processor, programmable logic controller CPU, PLC central processing unit, PLC processor module, PLC controller block, logic controller.
What the PLC CPU symbol means
The PLC CPU symbol denotes the brain of a Programmable Logic Controller system: the module that contains the microprocessor, program memory (RAM/Flash), I/O bus interface, and communication ports. It executes the stored control program in a cyclic scan loop, reading input states, processing logic, and updating output states on every scan cycle (typically 1–100 ms).
In system-level schematic and block diagrams, the PLC CPU symbol represents the entire controller's processing capability. Its input pins (In 1, In 2) symbolically represent the data bus connections to input modules (field sensors, switches), while output pins (Out 1, Out 2) represent connections to output modules (actuators, drives, indicators). The symbol signals to the reader that programmable automatic control logic is executed at that block.
How to identify the PLC CPU symbol
The PLC CPU symbol is drawn as a rectangle (the module body) with the label 'PLC', 'CPU', or 'PLC CPU' inside, often with a processor or chip graphic to indicate computing function. Input terminals labelled In 1 and In 2 connect on one side (typically left), and output terminals Out 1 and Out 2 connect on the other side (typically right). Some representations include a communication port (Ethernet, RS-485) terminal and a power supply terminal. The symbol is larger and more structured than a simple relay or logic gate, reflecting its role as a complex programmable module.
Function in a circuit
The PLC CPU performs a repetitive scan cycle: (1) Input scan — reads all connected input module states into an input image register; (2) Program execution — executes the control program (ladder logic, FBD, ST, SFC, or IL per IEC 61131-3) using the input image to determine output states; (3) Output scan — writes the output image register values to connected output modules to activate actuators; (4) Communications — services communication port requests (HMI, SCADA, programming terminal). Scan cycle time is typically 1–100 ms for standard PLCs, and under 1 ms for high-speed motion control CPUs.
Standards: IEC vs ANSI
| IEC 60617 | IEC 61131-3 defines the five PLC programming languages (Ladder Diagram LD, Function Block Diagram FBD, Structured Text ST, Instruction List IL, Sequential Function Chart SFC) executed by the PLC CPU. IEC 61131-1 covers general hardware definitions. IEC 61131-2 specifies PLC equipment requirements, electrical characteristics, and environmental ratings. |
|---|---|
| ANSI/IEEE 315 | ANSI/NEMA ICS 1 and ICS 2 cover industrial control equipment including PLCs. NFPA 79 Industrial Machinery Electrical Standard specifies PLC installation requirements for machine control. UL 508A covers industrial control panel standards including PLC installations in North America. |
| Key difference | There is no distinct IEC vs ANSI schematic glyph difference for the PLC CPU symbol; both use a labelled rectangle block. IEC 61131 is the universal PLC programming standard adopted globally, including in North America. The primary difference is in panel installation codes: IEC 60204-1 (Europe) vs NFPA 79 (North America) for machine electrical installations. |
Terminals / pins
| Pin | Name |
|---|---|
| in1 | In 1 |
| in2 | In 2 |
| out1 | Out 1 |
| out2 | Out 2 |
Typical values
Supply voltage: 24 V DC (typical DIN-rail PLCs) or 120/240 V AC (older rack PLCs); program memory: 16 KB – 32 MB (depends on model); scan time: 1–100 ms typical; I/O capacity: 8–8192+ I/O points via expansion modules; communication: Ethernet/IP, PROFINET, Modbus, EtherCAT, RS-485; CPU clock: 16 MHz – 1 GHz (varies widely by controller class).
Where the PLC CPU symbol is used
- Industrial machine control: PLC CPU executes the program that controls conveyor motor start/stop sequencing, safety interlocks, and position feedback processing
- Process automation: in chemical, oil/gas, and water treatment plants the PLC CPU runs PID control loops for flow, pressure, level, and temperature
- Building automation (BAS): HVAC, lighting, and access control sequences executed by PLC or building controller CPU modules
- Robotic systems: PLC CPUs with high-speed I/O and motion control extensions coordinate multi-axis robot arm movement
- Panel automation block diagrams: the PLC CPU block sits at the centre of system architecture diagrams showing connections to HMI, I/O modules, drives, and field devices
- Safety systems: safety-rated CPU modules (SIL 2/3) execute safety programs for emergency stop, safety gate, and machine guard monitoring
Example
In an automated conveyor system block diagram, the PLC CPU symbol is placed at the centre of the diagram. Its In 1 and In 2 connections link to a PLC Input Module that reads photoelectric sensors and push-buttons; its Out 1 and Out 2 connections link to a PLC Output Module that drives the conveyor motor starter relay and a pilot light. The CPU executes a ladder logic program on each scan cycle to start the motor when START is pressed and stop it when STOP is pressed or a jam sensor activates.
Key facts
- The PLC CPU symbol (designator PLC or CPU) represents the central processing unit of a Programmable Logic Controller, with input pins In 1, In 2 and output pins Out 1, Out 2.
- IEC 61131-3 defines the five programming languages for PLC CPUs: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC).
- The PLC CPU operates in a continuous scan cycle: input scan → program execution → output scan → communications, repeating every 1–100 ms (standard PLCs).
- IEC 61131-2 specifies the electrical and environmental requirements for PLC hardware including CPU modules, covering operating temperature, humidity, vibration, and supply voltage tolerance.
- PLC CPUs communicate with expansion I/O modules, HMI panels, SCADA systems, and other PLCs via industrial protocols such as PROFINET, EtherNet/IP, Modbus TCP, and EtherCAT.
- In schematic block diagrams the PLC CPU symbol represents the entire controller logic; actual I/O field wiring appears on PLC Input and PLC Output module symbols.
- Safety-rated PLC CPUs (Safety PLCs) meet IEC 61508 and IEC 62061 requirements for SIL 2 and SIL 3 safety integrity levels, enabling use in machine guarding and process safety applications.
- Typical PLC CPU supply voltage is 24 V DC (small/medium PLCs) or 120/240 V AC (older rack-style CPUs); the I/O bus connecting the CPU to expansion modules operates at 5 V or 3.3 V internally.
Frequently asked questions
What does the PLC CPU symbol mean in a schematic or block diagram?
The PLC CPU symbol in a schematic or block diagram represents the Programmable Logic Controller's central processing unit — the module that runs the control program. It shows where the programmable automation logic is executed, reading inputs from field sensors (via input modules) and setting outputs to control actuators (via output modules) on every scan cycle.
What does the PLC CPU symbol look like?
The PLC CPU symbol is drawn as a labelled rectangle with 'PLC', 'CPU', or 'PLC CPU' inside, sometimes with a processor or chip graphic. Input terminals (In 1, In 2) connect on one side and output terminals (Out 1, Out 2) on the other side. It is typically larger than individual logic gate or relay symbols, reflecting its role as a complex programmable module.
What are the terminals (pins) on the PLC CPU symbol?
The PLC CPU symbol has four representative terminals: In 1 and In 2 (inputs representing connections from input modules and field sensors) and Out 1 and Out 2 (outputs representing connections to output modules and actuators). In real PLC systems, the CPU connects to I/O modules via an internal backplane or I/O bus rather than individual terminal wires.
What is the difference between a PLC CPU and a PLC input/output module?
The PLC CPU is the processing module that executes the control program; it does not directly connect to field devices. PLC Input Modules (with terminals I0–I5) read voltage signals from field sensors and switches and pass the states to the CPU. PLC Output Modules (with terminals Q0–Q5) receive commanded states from the CPU and switch power to field actuators. The CPU, input modules, and output modules are separate physical units that communicate via the backplane bus.
What standard defines PLC programming languages?
IEC 61131-3 (Programmable Controllers — Programming Languages) defines the five PLC programming languages: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Instruction List (IL), and Sequential Function Chart (SFC). This standard is globally adopted and supported by all major PLC manufacturers including Siemens, Allen-Bradley/Rockwell, Mitsubishi, Schneider, and Omron.
What is the scan cycle time of a PLC CPU?
Scan cycle time is the time for the PLC CPU to complete one full scan: input scan, program execution, and output scan. For standard PLCs it is typically 1–100 ms, depending on program size and CPU speed. High-speed motion control PLCs achieve sub-millisecond scan times. The scan time directly determines the fastest process change rate that the PLC can reliably detect and respond to.
What supply voltage does a PLC CPU require?
Most modern DIN-rail PLCs (Siemens S7-1200/1500, Allen-Bradley Micro820, Mitsubishi FX5) operate the CPU from 24 V DC. Older rack-style PLCs (Allen-Bradley SLC 500, Siemens S5) used 120 V or 240 V AC for the power supply module. The 24 V DC internal logic supply is generated by the PLC's own power supply module for the CPU and I/O backplane.
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