Allen-Bradley vs Siemens PLCs

Overview

In the global industrial automation landscape, two players dictate control standards across discrete, batch, and process manufacturing: Rockwell Automation (with the Allen-Bradley brand) and Siemens AG. Historically, geographic preferences dominated this division, with Allen-Bradley controlling the North American market and Siemens retaining market dominance across Europe and Asia. However, globalized supply chains and multi-platform standards mean modern control engineers must frequently design, integrate, and maintain systems across both ecosystems.

Rockwell Automation’s primary controller line leverages the Logix control engine, split between the large-scale ControlLogix (chassis-based) and medium-scale CompactLogix (distributed) systems, alongside the Micro800 series for standalone machinery. Siemens utilizes the SIMATIC identifier, driven by the flagship S7-1500 (modular) and S7-1200 (compact pocket-style) controllers, alongside the distributed ET 200SP CPU family.

Understanding the structural differences in execution style, communications alignment, and development approaches between these two brands is essential to maximizing system reliability and reducing engineering hours.

Key Differences at a Glance

While both platforms run standard industrial processes, their architectures differ fundamentally in database management, tagging, and physical module construction.

Specifications Comparison

To provide a precise technical comparison, the following table matches the flagship mid-to-high level platforms of each provider: the Allen-Bradley ControlLogix 5580 (specifically the 1756-L85E) and the Siemens SIMATIC S7-1500 (specifically the CPU 1518-4 PN/DP).

Performance & Capabilities

The Allen-Bradley 1756-L85E and Siemens CPU 1518 represent top-tier processing units. However, they approach execution loops differently:

• Processor Architectures: Allen-Bradley’s ControlLogix L8 series utilizes a high-speed multi-core ARM processor that shifts communication processing away from the primary logic engine. This ensures logic execution loops are not degraded by heavy network traffic. Siemens S7-1500 models use highly optimized proprietary ASICs designed to process binary instructions in as little as 1 nanosecond (on the 1518 model), providing rapid execution of complex floating-point calculations and math instructions.
• Motion Control: Under Rockwell, Integrated Motion on EtherNet/IP (CIP Motion) allows synchronization of dozens of high-performance servo axes directly inside the logic controller. Real-time scheduling is managed inside the controller's task structure. Siemens relies on dedicated "Technology Objects" (TO). These virtual software tools represent physical axes and run execution loops natively synchronized with the PROFINET Isochronous Real-Time (IRT) subgrid, resulting in deterministic synchronization down to sub-millisecond intervals.

Programming & Software

The programming experience highlight's the largest operational divide between the two systems.

Allen-Bradley: Studio 5000 Logix Designer

Studio 5000 uses a unified, direct tag-based database. System tags are created using descriptive text (e.g., Conveyor_Start_PB), entirely eliminating traditional register mapping (such as %M or %I). Allen-Bradley excels in direct Ladder Diagram (LD) execution, mirroring North American electrical schematics. Editing code online—without requiring a CPU stop—is seamless on the Logix platform, making it a favorite for 24/7 run-time operations where downtime is costly.

Siemens: TIA Portal (Totally Integrated Automation)

Siemens TIA Portal integrates PLCs, HMIs, drives, and safety in a single software workspace. It uses a structured database approach, dividing program architectures into Organization Blocks (OBs), Function Blocks (FBs), Functions (FCs), and Data Blocks (DBs).

While TIA Portal supports Tag variables, it maps them directly to strict, optimized memory addresses. This structure encourages highly reusable object-oriented code, particularly when using Structured Control Language (SCL), which is Siemens' version of Structured Text, and Statement List (STL). TIA Portal provides superior diagnostics, simulation, and hardware-configuration tracing, though online code manipulation is more structurally rigid than Rockwell's system.

Communication & Networking

Communication protocols dictate network topology, physical medium selections, and device integration.

EtherNet/IP (Allen-Bradley)

Built on the Common Industrial Protocol (CIP), EtherNet/IP utilizes standard TCP/IP and UDP packet framing. This means standard off-the-shelf industrial Ethernet switches (e.g., Stratix or Cisco) can route control traffic. However, without managed switch architectures utilizing IGMP Snooping, heavy multicast traffic from distributed I/O can flood non-essential devices on the network.

PROFINET (Siemens)

PROFINET uses three distinct communication channels: normal IP traffic for configuration, Real-Time (RT) bypass packets for high-speed I/O, and Isochronous Real-Time (IRT) for low-jitter motion control. Because IRT bypasses the standard TCP/IP stack, it requires specialized Ethernet chipsets (ASICs) in the switches, meaning Siemens networks frequently require PROFINET-certified managed network hardware to ensure deterministic performance.

Pricing & Lifecycle

Both brands sit at the premium end of the industrial market. However, their total cost of ownership (TCO) models diverge:

• Hardware and Software Licensing: Allen-Bradley software licensing uses an annual subscription model (Studio 5000 and FactoryTalk View), often priced per developer license features (such as Lite, Full, and Professional). Siemens TIA Portal typically uses a perpetual licensing model with optional annual upgrade contracts, though they have introduced subscription options under the "SaaS" model.
• Lifecycle and Availability: Both companies maintain reliable active support windows. Hardware typically transitions from "Active" to "Active Mature," then "End of Life/Obsolete" over a 10-to-20-year window. Rockwell’s legacy SLC 500 and PLC-5 families maintain a massive aftermarket despite their obsolete designation, while Siemens' legacy S7-300 and S7-400 product catalogs remain highly sought-after as plants transition to S7-1500 architectures.

When to Choose Each

Select Allen-Bradley (ControlLogix / CompactLogix) if:

1. Your plant floor is primarily in North America, where local electrical standards, system integrators, and plant engineers are deeply trained in Rockwell ecosystems.
2. Your production processes require continuous run-time updates, online tag modifications, and live logic corrections.
3. You are standardizing plant automation on a single, massive tag database with a highly flat, human-readable hierarchy.

Select Siemens (S7-1500 / S7-1200 / ET 200SP) if:

1. The machine or automation line is being fabricated in Europe/Asia or is part of a complex global OEM footprint.
2. Your control architecture demands highly structured, object-oriented code blocks designed for rapid replication and deployment.
3. High-speed motion integration using PROFINET IRT synchronization is required for precise labeling, printing, or packaging applications.
4. You want a single software footprint (TIA Portal) to manage the PLC execution layer, HMI screens, variable frequency drives, and local network diagnostics.

Migration & Upgrade Path

Both brands have structured migration frameworks to guide users from legacy lines to current hardware:

• Rockwell Automation: Legacy PLC-5 and SLC 500 controllers migrate toward ControlLogix and CompactLogix. Rockwell provides physical wiring conversion kits—allowing existing I/O swing-arms to dock directly into new 1756 card frame styles—limiting physical field rewiring. Code converters can parse older RSLogix 5/500 code bases into Studio 5000 XML file structures, although manually auditing indirect addressing and hardware configurations is always required.
• Siemens: Legacy S7-300 and S7-400 physical systems migrate directly to the S7-1500 series. Siemens provides S7-300/400 direct-fitting adapters for fast physical swing-arm transitions. Software-wise, TIA Portal features an integrated migration utility that converts Step 7 V5.x logical programs directly into modern S7-1500 program structures, highlighting incompatible system function blocks (SFCs) and system function calls (SFCs) for manual correction.

Frequently Asked Questions

Q: Can a Siemens PLC talk directly to an Allen-Bradley PLC?
A: Yes, but not out-of-the-box natively without a translation layer. You can bridge them using industrial protocol gateways (such as ProSoft modules or Anybus devices), by implementing Modbus TCP communication on both platforms, or by configuring OPC UA servers/clients on both the ControlLogix and S7-1500 CPUs.

Q: Which PLC has better cybersecurity features?
A: Both offer robust control security. The modern Allen-Bradley ControlLogix 5580 firmware utilizes CIP Security to provide data integrity and encryption across communicating nodes. Siemens S7-1500 controllers utilize TLS-based encryption for communication protocols directly in TIA Portal, including password-protected access levels and proprietary hardware protection to block unauthorized read/write access to specific code structures.

Q: Is structured text better supported on Allen-Bradley or Siemens?
A: Structured Text is highly viable on both platforms. However, Siemens' SCL (Structured Control Language) is a fundamental, native pillar of TIA Portal programming and is deeply integrated into standard library function design. In older Studio 5000 versions, Structured Text execution was slower than Ladder, though modern Logix 5580 platforms now execute Structured Text at parity with other languages.

Related Articles

• Understanding Industrial Ethernet Protocols: EtherNet/IP vs PROFINET
• Migrating Legacy Allen-Bradley SLC 500 and PLC-5 Systems to CompactLogix
• Key Differences Between Modular and Compact PLC Architectures