Academy introductionSiemens, Rockwell and Unitronics controller families

Understanding the PLC families

Different platforms.
The same control principles.

The fundamental principles of automation remain the same whichever manufacturer is chosen: the controller reads its inputs, executes the program and updates its outputs. Siemens, Rockwell Automation and Unitronics organise their controller families differently.

These are practical market comparisons showing where each family broadly fits. They do not suggest that the controllers are technically identical.

Application level	Siemens	Rockwell Automation	Unitronics	Typical use
High-performance control	SIMATIC S7-1500	ControlLogix	UniStream	Complex machinery, extensive communications, motion control and large automation systems
Compact machine control	SIMATIC S7-1200	CompactLogix	Vision	Standalone machinery, production equipment and expandable machine-control systems
Graphical compact control	Separate PLC and HMI normally required	Separate PLC and HMI normally required	Samba	Smaller machines requiring an economical PLC with an integrated colour touchscreen
Compact automation	LOGO! 9	Micro800	Jazz and M91	Small machines, pumps, ventilation, lighting, sequences and applications with limited I/O

Siemens

Where LOGO! 9 fits

LOGO! was originally developed as a logic module that could replace combinations of timers, counters, auxiliary relays and contactors.

The new LOGO! 9 generation has developed considerably beyond the traditional programmable-relay description. It supports up to 800 function blocks, extensive I/O expansion, Modbus TCP/IP, MQTT and browser-based monitoring through its integrated web server.

LOGO! 9 is therefore best described as an advanced compact logic controller. Applications requiring TIA Portal engineering, PROFINET IO, fail-safe control, advanced motion or larger automation networks would normally use an S7 controller.

Rockwell Automation

How the Rockwell range compares

Micro800 provides conventional micro PLCs for small machines and standalone automation, programmed using Connected Components Workbench.

CompactLogix is designed for more advanced machine control and is the closest broad Rockwell comparison with the Siemens S7-1200.

ControlLogix provides Rockwell’s high-performance platform for large machinery, integrated production systems, motion control and plant-wide automation. CompactLogix and ControlLogix share Rockwell’s tag-based Logix architecture in Studio 5000.

The integrated alternative

What makes Unitronics different?

With Siemens and Rockwell systems, the PLC and operator HMI are commonly selected and installed as separate devices. Unitronics has built much of its range around combining the PLC and HMI within one integrated control platform. Machine logic, operator controls, status displays, settings and alarms can be developed as parts of the same project.

High-performance control

UniStream

Programmed in UniLogic, UniStream is a modern, tag-based platform covering PLC logic, HMI screens, hardware, communications, VFDs and motion. It is available as an integrated PLC and touchscreen, a modular system or a standalone PLC using a Virtual HMI.

It supports facilities including EtherNet/IP, OPC UA, MQTT, REST API, SQL, web servers and remote I/O. It is the closest Unitronics comparison with S7-1500 and ControlLogix, but not an exact architectural equivalent.

Compact machine control

Vision

Programmed in VisiLogic, Vision combines ladder logic, hardware configuration and graphical HMI development. It uses conventional memory operands and supports PID control, positioning, data logging, communications and expandable I/O.

Its integrated graphical HMI makes it practical for applications commonly handled by an S7-1200 or CompactLogix with a separate operator panel.

Graphical compact control

Samba

Samba combines a full-function PLC, onboard I/O and a colour touchscreen in one compact unit. It is intended for smaller, cost-sensitive machines that still require a proper graphical operator interface.

Its onboard resources are more limited than Vision or UniStream, although compatible models can communicate with remote I/O when additional signals are required.

Compact automation

Jazz and M91

Jazz and M91 combine micro PLC control with a two-line text display and physical keypad. Available models provide digital, analogue, temperature and high-speed I/O options.

They are programmed in U90 Ladder, which brings PLC logic, text displays, hardware and communications into one software package. Jazz offers up to 40 onboard I/O points, while M91 models can use expansion modules.

Three learning paths

Three manufacturers, one set of principles

The addresses, terminology and software change, but the underlying control principles remain the same.

SiemensLOGO! 9, S7-1200 and later S7-1500
Rockwell AutomationMicro800, CompactLogix and later ControlLogix
UnitronicsJazz/M91, Samba, Vision and UniStream

Continue to Lesson 01 ↓

Choose the PLC platformSiemens S7-1200 · TIA Portal · absolute operands shown for teaching

Lesson 01 · Input and output

Press a button. Light a lamp.

Hold the pushbutton and watch the PLC input, ladder contact and green output energise together.

Meaningful names. Flexible wiring.

The CompactLogix ladder uses descriptive tags such as Start_PB instead of long hardware addresses. An alias tag points directly to a physical input or output bit.

Why do this? If the Start button wiring moves from Slot 1 to Slot 3, configure the replacement module and change the Start_PB alias link from Local:1:I.Data.0 to Local:3:I.Data.0. Every rung still uses Start_PB, so you do not have to rewrite or search-and-replace the ladder logic.

See the alias setup and this lesson’s tag mapping

In the tag editor, enter the descriptive name, select Type: Alias, then select the input or output bit under Alias For. The alias inherits the bit’s BOOL data type.

Tag used in the ladder	Type	Alias For / storage

These slot addresses are examples; your actual addresses depend on the configured hardware. Internal state tags such as Green_State are Base BOOL tags stored in controller memory, without a physical I/O link.

Similar principle in an SLC 500: you could use an input-mapping routine to copy physical inputs into internal bits such as B3:0/0, then use those bits throughout the program. Moving an input meant changing that mapping. A CompactLogix alias provides the same separation from the physical location, but points directly to the source bit rather than copying its state.

Rockwell: creating an alias tag

Working simulation● PLC ONLINE

Program OB1Cycle 0STOPPED

IEC 61131-3: Ladder Diagram (LD) and Structured Text (ST) are standard PLC programming languages. Siemens presents its Structured Text implementation as SCL (Structured Control Language). IEC 61131-3:2025

TON

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