Hardware Comparison: Allen-Bradley vs Siemens PLCS

The Allen-Bradley Logix family of controllers is considered the most popular programmable logic controller family in the US.  It consists of numerous variants but for this comparison we will look at the ControlLogix, CompactLogix and MicroLogix PLCs.

By comparison, Siemens is much more common in Europe where their extensive line of PLCs are used in automation control systems throughout industries.  The Siemens PLC equivalents for Allen-Bradley’s Logix family members above are part of the Siemens SIMATIC controller family and consist of the S7-200, S7-300, and S7-400 as direct comparisons.

General Differences

Before diving into unit by unit comparisons, we can first discuss some general differences between the Allen-Bradley and Siemens PLC lines:

  • Performance – Both Allen Bradley and Siemens PLC groups have similar speeds and reliability.  Their outputs are also close in number as well.  The main differences in performance comes in their ease of use and how they integrate into control systems.

Allen-Bradley PLCs are considered more user-friendly and easier to program, whereas Siemens requires more programming knowledge.  Allen-Bradley PLCs are also considered easier and faster to debug than Siemens PLCs.

  • Hardware – For installation of hardware, there is a considerable difference between the two.  Allen-Bradley PLCs require both an Allen-Bradley rack and an AB power supply as well.  Siemens, by comparison, can be powered by any 24V DC power supply externally and does not require a rack.
  • Software – For software, Allen-Bradley ControlLogix and CompactLogix use the popular RSLogix500 software, while the MicroLogix controllers, made for small or standalone equipment controls, uses the MicroLogix Micro Starter Lite software.  By contrast, Siemens uses a Windows based software called Simatic Manager for its S7-300 and S&-400, while its S7-200 (their equivalent for small or standalone controls) uses a micro Win program.
  • Programming – There is considerable difference when programming Allen-Bradley vs Siemens.  Allen Bradley is considered much more intuitive and user-friendly to program.  This means that programming can be done by more people and it is easier for someone learning PLC programming to come up to speed.  Siemens controllers, on the other hand, are considered more difficult to program and require the skillset of an engineer, technician, or well-trained maintenance staff.
  • Communication Protocols – The biggest differences in communication protocols between the two types of PLCs come from their regional differences.  Allen-Bradley controllers support North American protocols such as DeviceNet, ControlNet and Ethernet IP as well as DH+ and DH485.  Siemens uses European protocols such as Profibus, ASI, MODBUS or MODBUS TCP/IP.

Specific Comparisons

Allen-Bradley ControlLogix vs Siemens S7-400

Introduced in 1999, the ControlLogix series from Allen-Bradley has been a workhorse for many years.  The series is divided between the 5580 and 5570 models, with significant differences in capabilities between the two.  The Siemens equivalent, the S7-400 series controller has been in service almost as long as the ControlLogix and consists of four base models, the CPU 412-1, CPU 412-2, CPU 414-2, CPU 414-3, CPU 416-2, CPU416-3 and CPU 417-3.  Both are designed for large industrial automation systems.

  • Memory – The ControlLogix memory capabilities varies greatly over many base units within the 5580 and 5570.  This memory ranges from 2MB of user memory up to 8 MB on high end base models within the 5570 but ramps up dramatically from 3 MB of user memory through as many as 20MB of user memory plus 6 MB for safety on higher end base models within the 5580.

By contrast, the S7-400 series for Siemens splits memory capabilities into main memory and load memory which delivers a dramatic increase in performance.  There is also a large difference in what this memory is dedicated to, with low end CPU412-1 controllers carrying 96 Kbytes of integrated memory and 32 Kbyte for instruction and higher end CPU 417-4 carrying 4Mbytes of memory integrated with 1335 Kbytes for instructions.

  • I/O – A key difference in the ControlLogix vs S7-400 series controllers is that the ControlLogix series is a chassis-based system with total modularity.  Components can be chosen based on end use requirements.  The Siemens S7-400 is also modular but is not chassis based.  In terms of I/O, because of their modularity, both can support distributed I/O with capabilities of supporting tens of thousands of I/Os on a connected system.

Because of this modularity and ability to support distributed I/O, both Allen-Bradley ControlLogix and Siemens S7-400 controllers can support both analog and digital I/O expansion.  The S7-400 can only accept up to 21 expansions.  However, additional expansion modules can be added to those units as well to achieve higher I/O numbers.  Total digital I/O for the S7-400 is 16384 and total analog I/O is 4000

  • Communication – The ControlLogix again proves more flexible when comparing communication methods.  Both the 5580 and 5570 can use EtherNet/IP, ControlNet, DeviceNet, DH+, Remote I/O, SynchLink and USB client.  The S7-400 series supports connection to the Ethernet.  However, it is for loading websites and email.  The S7-400 also connects to industrial EtherNet using ISO/TCP or TCP/IP.  It can also connect using PROFIBUS.
  • Safety – There was a time when Allen-Bradley controllers had embedded safety features while Siemens safety was only achieved through add on modules.  The S7-400, like the S7-300 before it, now has embedded safety features as well.  Because it is a chassis-based architecture, Allen-Bradley controller users can choose safety control modules as their central CPU.  These are marketed as GuardLogix controllers and are rated for use with many different safety type-approved applications. 

Allen-Bradley CompactLogix vs Siemens S7-300

The Allen-Bradley CompactLogix controllers come in two variations.  There is an all-in-one PLC and a modular system not requiring a chassis.  Like the ControlLogix, the CompactLogix uses the RS-500 software.  There are five base models which include the 5380, 5480 and three versions of the 5370.  Both the Allen-Bradley CompactLogix and the Siemens S7-300 are designed for small and medium-sized industrial automation applications.

There are ten base models of the S7-300, with the distinctions broken down between size application and communication protocol port access.  In increasing capability and functionality, these include:

  • CPU 312
  • CPU 312C
  • CPU 313C
  • CPU 314
  • CPU 314C
  • CPU 315-2-DP
  • CPU 315-2 PN/DP
  • CPU 317-2 DP
  • CPU 317-2 PN/DP
  • CPU319-3 PN/DP
  • Memory – Like the ControlLogix series, the CompactLogix memory capabilities vary greatly over its base units within the.  This memory ranges from a 0.6 to 10 MB of user memory for the low end to as much as 348 KB to 10 MB of user memory on the high end 5370 L1.

By contrast, the S7-300 series for Siemens ranges as low as 32KB of memory for the low-end CPU 312 and as high as 2560 for the upper end model CPU 319-3 PN/DP.  The S7-300 also uses a Micro Tested Memory Card for data backup.  Because of this, programmers can make firmware updates and program updates without need of a programming device. 

  • I/O – The CompactLogix system is not confined to finite I/O as it is designed to support distributed I/O through multiple nodes, each capable of system expansion as well.  Local I/O modules on the controller itself may range from 8 to 31.  But the support of the nodes on CompactLogix controllers may range from 4 to 8 on the 5370 L1 to as many as 250 for the 5480. 

The Siemens S7-300 modularity gives it a total digital I/O of 1024 and a total analog I/O of 256. 

As they are both modular systems designed for distributed I/O, both Allen-Bradley CompactLogix and Siemens S7-300 controllers can support analog and digital I/O expansion.  The S7-400 can only accept up to 32 expansions.  Like the S7-400, additional expansion modules can be added to those units as well to achieve higher I/O numbers.

  • Communication – The CompactLogix is flexible in its communication methods.  But unlike the S7-400, the S7-300 by Siemens has a wider availability of communication protocols available to it.  While still predominantly European used protocols, this increase in capability brings it somewhat close in overall performance to the CompactLogix series.

The CompactLogix uses less available communication protocols than the ControlLogix with the most versatile being the 5370 using EtherNet/IP, DeviceNet and USB client.  The S7-300 uses Profibus and ProfNet as well as Industrial Ethernet.

Allen-Bradley MicroLogix vs Siemens S7-200

At the basic end of the spectrum are the Allen-Bradley MicroLogix and the Siemens S7-200.  These are both used for smaller applications or standalone equipment that only needs basic control and motion function.  There are five base models in the Allen-Bradley MicroLogix family including the 1000, 1100, 1200, 1400, 1500.  Likewise, there are five base models in the Siemens S7-200 family including the CPU 221, CPU 222, CPU 224, CPU 224XP and CPU 226.

  • Memory – The MicroLogix family memory runs from a low of 1K of user program and data space on the 1000 to up to 10K user program and 4K data space configurable.  The Siemens S7-200 family runs from a low of 4K/4K user to data space on the CPU 221 up to 16K/24K on the CPU 226. 

The MicroLogix family also has data logging memory that starts at 128kB on the 1100  and 1400 1400 (not available in the 1000 and 1200 and only 48kB on the 1500) while the data logging memory for the Siemens CPU 221 and 222 begins at 2048 bytes, with 8192 bytes for the 224 and 10240 for the CPU 224XP and 226.  All Siemens PLCs in the S7-200 family have a battery backup, while battery backup for the MicroLogix family is only available on the 1100, 1400 and 1500.

  • I/O – The MicroLogix family generally has more digital I/O compared to the Siemens S7-200 family.  I/O for MicroLogix includes:
  • 1000 – Up to 32
  • 1100 – 16
  • 1200 – Up to 40
  • 1400 – Up to 32
  • 1500 – Up to 28

The Siemens S7-200 family includes

  • CPU 221 – 6 In/4 Out
  • CPU 222 – 8 In/6 Out
  • CPU 224 – 14 In/10 Out
  • CPU 224XP – 14 In/10 Out
  • CPU 226 – 24 In/16 Out
  • Analog I/O – There is much greater capabilities in the Allen-Bradley MicroLogix family for analog I/O.  These include:
  • 1000 – 5 embedded
  • 1100 – 2 embedded and up to 32 expansion
  • 1200 – Up to 24 expansion
  • 1400 – 6 embedded and up to 56 expansion
  • 1500 – Up to 256 expansion

The Siemens S7-200 by contrast only allows analog through more limited expansion modules that allow 4 In/2 Out, 8 In/4 Out or 4 and 8 thermocouple inputs.  This means the Allen-Bradley MicroLogix is more flexible and can be utilized for grater I/O.

Pulse Outputs – For Pulse outputs, both Allen-Bradley and Siemens offer similar functionality on almost all base models starting with 2 at 20kHz through 2 at 40kHz for Allen-Bradley and a consistent 2 at 20kHz for Siemens (except for the CPU 224XP with 2 at 100 kHz.

Real Time Clock – Allen-Bradley MicroLogix base units all have embedded real-time clocks except for the 1000 while Siemens only offers them embedded in the CPU 224, 224XP and 226 with the CPPU 221 and 222 available through cartridge add on.

Communication Ports – Allen-Bradley MicroLogix also exceeds flexibility compared to the Siemens S7-200 on its communication ports.  The Siemens S7-200 offers only a single RS-485 port on the CPU 221, 222 and 224 while Allen-Bradley offers RS 232, DeviceNet Peer to Peer, Ethernet/IP, DH485 DF1 Half Duplex/Radio Modern on all family members and RS 485, Modbus RTU and Modbus TCP/IP on all base models except the 1000.


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