PLC learning series 3: PLC Architecture and wiring of PLC

Architecture of PLC:


A processor or central processing unit (CPU) is a unit that contains a microprocessor that processes input signals and controls, according to the program stored in memory, then communicates the decisions it takes as control signals to the output interface.

The CPU function is to manage all the processes that occur in the PLC. There are three main components that make up this CPU, namely the processor, memory and power supply

Input/Output module:

In general, data information on PLC is expressed in the form of electrical voltages between 5-15 VDC, while the outside voltage system varies between 24-240 VDC and AC. I/O Unit is intended for interfacing between these two quantities.

The input module has several functions including:

  • Detects when a signal is received from the sensor.
  • Convert the input signal into a voltage level that can be received by the processor.
  • Sending a signal to the PLC input indicator so that it can know which input is receiving the signal

The output module has several functions including:

  • The output unit in the PLC also functions as an interface to external equipment.
  • The PLC output acts as a switch to the power supply to operate the output equipment (eg relay, solenoid valve, etc.).
  • Components commonly used by PLC as output units are relay for AC / DC, TRIAC for AC only, and transistors or FETs for DC only.

PLC Data and Memory:

Like computer systems, PLC memory consists of RAM and ROM. The memory capacity between one PLC and another varies depending on the type and manufacturer. Some factories state the size of memory in bytes, there are also kilobytes, and some are expressed by the number of instructions that can be stored

Power supply:

The voltage source needed by the CPU, memory and other circuits is a DC voltage source, generally for digital components a direct voltage of 5 volts is required.

Programming Unit:

Programming unit provide printed suggestions so that users can communicate with programmable control circuits. This allows the user to enter, edit and monitor the program by connecting to the processor unit and allowing access to the user’s memory

PLC wiring:

The wiring diagram demonstrates straight (hard wired) linked inputs and outputs to the PLC. The displayed instruments are on / off or digital in nature, but the PLC signal is analogous.

Many frequently used phones comply with a 4-20 mA standard whereby 4mA and 20mA signals form the minimum and maximum analog signal values respectively.

A separate cable must be run between the end device and the control system with analog devices because on the circuit only one analog signal can be represented. Slowly replacing the 4-20 mA standard with network or fieldbus communications. Fieldbus is a two-way digital multi-drop communication connection between smart devices.

PLC 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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