PLCs - Programmable Logic Controllers

A PLC (programmable logic controller) is an industrial digital computer designed for the control and automation of manufacturing processes or robotic devices. Programmable Logic Controllers are used where reliability control, simple programming and fault diagnosis is necessary. They were originally built to replace electromechanical relay systems in industrial automation.

A CPU (central processing unit) forms part of the PLC system. PLC CPUs are a microprocessor-based control system and act as the electronic circuitry within an industrial computer. They are usually built-in carry out instructions from the computer program via programming languages, performing basic mathematical, logical, control, input and output operations.

PLC Applications

PLC controllers are incredibly useful and have many applications across several industries such as manufacturing, food & beverage, and building control. PLC's control the automation process in the machinery used for:

  • Production plants
  • Traffic light controls
  • Packaging machines
  • Lifts and escalators
  • Medical applications
  • Automatic gate systems
  • Heating control systems

How do PLC's work?

PLC Controller's respond to inputs from programming software and give a desired output. Programmable Logic Controllers can also include human-machine interfaces.

Different models use varying PLC Programs based on the programming languages used in the software. Programming languages that are used in PLC programming include:

  • ST: Structured Text (text-based)
  • FBD: Function Block Diagram (graphic-based)
  • LAD: Ladder logic (graphic-based)
  • STL: Statement List (text-based)
  • SCL: Structured Control Language (graphic-based)

PLC programming can be quite complex but there are simpler, image-based options available such as Ladder Logic that has user-friendly Ladder diagrams to understand and improve processes. Laptop and software are usually used for Ladder Logic and Ladder Diagrams, however, some older models use handheld controllers.

Types of PLC's

Programmable Logic Controllers can be categorised in multiple ways, each with different functions, features and capabilities. For example, Soft PLC's are software based and use their own operating system but do not have their own CPU. Whereas, Hard PLC's are hardware based and typically only include the essential control functions. The full range of programmable controllers include:

  • Hard PLC's
  • Soft PLC's
  • Compact PLC's
  • Slot PLC's
  • Unitary PLC's
  • Modular PLC's

Programmable Controllers can also be made more efficient by using equipment such as, HMI displays, DIN Rails, and Communication modules (such as MODBUS) which can be enhanced into a highly functional automated control system with PLC accessories. More information can be found our complete PLCs guide


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Description Price For Use With Manufacturer Series Number of Inputs Number of Digital Inputs Output Type Input Type Number of Analogue Inputs Voltage Category Number of Digital Outputs Supply Voltage Number of Analogue Outputs Interface Kit Included Number of Outputs
RS Stock No. 917-6370
BrandRS PRO
€102.24
Each
units
- - 8 - Digital, Relay Analogue, Digital - - - 12 → 24 V dc - - - 4
RS Stock No. 917-6373
BrandRS PRO
€86.98
Each
units
- - 8 - Digital, Relay Digital - - - 110 → 240 V ac - - - 4
RS Stock No. 917-6361
BrandRS PRO
€101.57
Each
units
- - 8 - Digital, Relay Digital - - - 110 → 240 V ac - - - 4
RS Stock No. 917-6377
BrandRS PRO
€86.84
Each
units
- - 8 - Digital, Relay Analogue, Digital - - - 12 → 24 V dc - - - 4
RS Stock No. 917-6399
BrandRS PRO
€14.62
Each
units
RS PRO Logic Modules - - - - - - - - - - - - -
RS Stock No. 917-6389
BrandRS PRO
€53.24
Each
units
RS PRO Logic Modules - 8 - Digital, Relay Digital - - - 12 → 24 V dc - - - 4
RS Stock No. 917-6383
BrandRS PRO
€39.52
Each
units
RS PRO Logic Modules - 1 - RS485 Protocol (D-), RS485 Protocol (D+), RS485 Protocol (Two Wires) TTL Level - - - 110 → 240 V ac - - - 1
RS Stock No. 917-6392
BrandRS PRO
€42.55
Each
units
RS PRO Logic Modules - 1 - RS485 Protocol (D-), RS485 Protocol (D+), RS485 Protocol (Two Wires) TTL Level - - - 12 → 24 V dc - - - 1
RS Stock No. 917-6386
BrandRS PRO
€51.69
Each
units
RS PRO Logic Modules - 8 - Digital, Relay Digital - - - 110 → 240 V ac - - - 4
RS Stock No. 468-4444
Mfr. Part No.SR2A101FU
€172.90
Each
units
- Zelio Logic Smart 6 - Relay Discrete - - - 120 V ac, 240 V ac - - - 4
RS Stock No. 468-4242
Mfr. Part No.SR3B261BD
€293.55
Each
units
- Zelio Logic 16 - Relay Analogue, Discrete 6 24 V dc - 24 V dc - - - 10
RS Stock No. 862-4461
Mfr. Part No.6ES7212-1BE40-0XB0
BrandSiemens
€336.77
Each
units
SIMATIC S7-1200 Series S7-1200 8 (Digital Input, 2 switch as Analogue Input) 8 Digital, Relay Analogue, Digital - 85 → 264 V ac 6 85 → 264 V - - - 6 (Digital Output, Relay Output)
RS Stock No. 468-4450
Mfr. Part No.SR2B121FU
€189.05
Each
units
- Zelio Logic 8 - Relay Discrete - 100 → 240 V ac - 120 V ac, 240 V ac - - - 4
RS Stock No. 864-4541
Mfr. Part No.0660-0600
BrandBARTH
€165.26
Each
units
STG-600 lococube mini-PLC 10 - Mosfet, Solid State Analogue, Digital - - - 7 → 32 V dc - - - 8
RS Stock No. 821-1769
Mfr. Part No.CP1L-EL20DT1-D
BrandOmron
€501.35
Each
units
CP Series CP1L-EL 12 - PNP DC - 20.4 → 26.4 V dc - - - - - 8
RS Stock No. 795-7975
Mfr. Part No.FX3S-14MR/ES
BrandMitsubishi
€229.32
Each
units
FX3 Series FX3S 8 - Relay, Transistor DC - 100 → 240 V ac - - - - - 6
RS Stock No. 750-8602
Mfr. Part No.2080-LC10-12QWB
€114.88
Each
units
- Micro810 8 - Relay Analogue, Digital - 24 V dc - - - - Yes 4
RS Stock No. 750-8605
Mfr. Part No.2080-LC10-12AWA
€114.88
Each
units
- Micro810 8 - Analogue, Digital Analogue, Digital - 24 V dc - 120 → 240 V - - Yes 4
RS Stock No. 862-4471
Mfr. Part No.6ES7214-1AG40-0XB0
BrandSiemens
€508.13
Each
units
SIMATIC S7-1200 Series S7-1200 14 (Digital Input, 2 switch as Analogue Input) - Digital, Transistor Analogue, Digital - 24 V dc - - - - - 10 (Digital Output, Transistor Output)
RS Stock No. 825-1681
Mfr. Part No.6ED1055-1MB00-0BA2
BrandSiemens
€80.92
Each
units
LOGO! 8, LOGO! 8.2 LOGO! 4 4 Relay Digital - - 4 12 → 24 V dc - - - 4