PLCs Aren’t Just Older, ‘They’re Better’

What functionality/features are today's programmable logic controller (PLC) users seeking? How are they applying PLCs? Is any other technology, such as personal computers, taking away market share from PLCs? Control Engineering wanted to know, so we asked a random sampling of 1,500 readers to participate in a survey about today's PLC.

By Staff February 1, 1998
  • Open systems

  • Communications

  • Meeting PC competition

  • Increasing analog capabilities

Filling the Void between PLCs and DCSs

What functionality/features are today’s programmable logic controller (PLC) users seeking? How are they applying PLCs? Is any other technology, such as personal computers, taking away market share from PLCs? Control Engineering wanted to know, so we asked a random sampling of 1,500 readers to participate in a survey about today’s PLC. Twenty-three percent responded. Results were interesting.

Survey: Micro, medium, large

CE’s survey categorized the PLC market into three segments: micro (under 128 I/O points), medium (128 to 512 I/O points), and large (greater than 512 I/O points). Many of the results reported total more than 100% due to multiple responses.

The survey asked, “What types of PLCs do you now use at your company locations?” MicroPLCs accounted for 75.8% of responses, medium PLCs, 73.1%, and large PLCs, 42.7%.

Originally PLCs were intended for machine control applications. These applications still accounted for 77.3% of the survey’s responses. However, 61.5% respondents reported using PLCs for process control applications. Motion control came in third at 36.1%. In decreasing order were batch control (27.8%) and diagnostics (13.7%).

Are PLCs used mainly in networked applications or as stand-alone controllers? Over half of the respondents, 55.7%, reported that their PLCs are used in stand-alone applications. Networked applications with other PLCs account for 35.6%, 26.3% are linked with distributed control systems, and 32.2% are networked with PCs.

Future plans

Will users buy more, fewer, or the same numbers of PLCs within the next 12 months? Excluding the respondents that reported “not certain,” the adjusted average outlook is a 10.5% increase in purchases.

PLCs with more plug-in I/O modules are expected to account for 38.7% of the new purchases and 48.1% of the new PLCs are expected to ship with more remote I/O subsystems.

The OEM view

Many “mature” technologies grow staid and fade from the marketplace. One might think that PLC technology, around more than 25 years, would be prone to evolutionary and market stagnation. Nothing could be further from today’s PLCs. Original PLCs were applied strictly to incremental (digital based) machine tool applications, but today’s PLC offerings bristle with features embodying motion control, PID, and other analog functionality.

“We are seeing the disappearance of the completely proprietary PLC system of CPU, power supply, and I/O modules on a rack,” says Bill Cummings, senior product marketing specialist, Omron Electronics Inc. (Schaumburg, Ill.). “In its place, is the rise of unbundled systems with open architectures and standard networks.” As open systems continue to evolve, Mr. Cummings suggests PLC manufacturers will be able to offer a wide range of form factors along with expertise in specialty I/O functionality.

Mr. Cummings has also notes that most customers now demand more seamless connections between third-party devices and PLCs. To accommodate this trend, Omron’s C200H Alpha midsize PLC, introduced last year, supports communications were previously processed through an ASCII/Basic module.

According to Trayton Jay, director of marketing for PLCs and related software at Mitsubishi Electric Automation Inc. (Vernon Hills, Ill.), demand for I/O subsystems is increasing because of the wider availability of unbundled products, such as distributed “rack,” “block,” and “direct” (sensor/actuator) I/O modules that are mated to alternative “brains,” such as a PC.

MicroPLCs are considered by Kevin Colloton, senior applications engineer at Rockwell Automation/Allen-Bradley (Mayfield Heights, O.), to be viable alternatives to more traditional relay-based or single-board controller solutions. Their small size and low cost have brought microPLCs to the point where they provide an efficient means to increase automation on simple machines and less complex processes.

Eddie Prince, marketing manager, PLC Products at Siemens Energy & Automation Inc. (Alpharetta, Ga.), says, “It simply doesn’t make sense to replace microPLCs or the vast majority of small PLC systems with a PC-based solution. Even though the reliability of the PC hardware has come a long way over the years, the high rate of technology turnover, operating system software changes, service, and spare parts access issues are barriers yet to be overcome.”

Competing against PCs

PC-based control has matured into a viable control solution for a wide variety of industries and applications. Many users see PC-based solutions as meeting and expanding traditional capabilities.

Are PLCs being phased out? More than 31% of the respondents to CE’s survey reported that they intend to replace PLCs with PCs and soft logic software within the next 12 months.

But, PC-based control isn’t the right solution for every application. For some users, new software-based control technologies bring benefits, such as improved information flow, integration, and in some cases, reduced cost. Others find traditional control handle the application task equally well, or in some cases, better.

Mitsubishi’s Mr. Jay suggests certain PC-based control applications leverage the PC’s strengths. Commercial PCs offer potential for reduced hardware cost compared to most large PLC CPUs, and they offer potential for wider-ranging, lower-cost options from multiple vendors. Commercial PC hardware is not as rugged as PLC hardware, so, among the other parameters, one must consider the transportation requirements and installation environment carefully. In cases where industrial PCs are used, some or all of the basic hardware cost advantages vs. PLCs may be lost. In these cases, other beliefs about PC-based control will support its use.

Many sources emphasize caution before switching from PLC to PC-based systems. When a PC graphics board dies, or a hard disk fails, is another or similar one available? Who is responsible to ensure that the new one works in the application? What if a third party PC card is installed, who takes responsibility if or when the system dies? If an existing PC is replaced with a newer model, and the software doesn’t work properly, who is responsible, and who can fix it? In the case of a PLC system, where all parts and programming tools are from a single manufacturer, the answers to these questions are clear.

Proponents point out PLC systems are designed specifically for industrial control, with integrated watchdog timers, benchmarked performance, and other safety features. The knowledge base of major controls manufacturers is mature and specific, and PLCs implement current semiconductor and manufacturing technologies.

According to Omron’s Mr. Cummings, “Another key to the PLC’s success in the face of competition from PCs is the PLC’s ability to be closely scaled to the application. Increased price/performance of the small, microPLC with increased functionality previously only found in large, rack-mounted PLCs, will make this category of PLC very attractive over PC solutions for small applications.” To achieve this flexibility, users can replace many large PLCs by distributing control to smaller PLCs and networking them together.

Networking—increasingly important

Control system users are demanding more openness and commonality from their components. Commonality in components and architecture increases integration capabilities, and reduces the learning curve for implementation.

The customer’s insistence on open systems has resulted in manufacturers including various mixes of industry-standard interface ports, such as RS-232 (connection to serial devices), RS-485 (for networking), and RS-422 (allows operator interface and Microsoft Windows programming software to be connected simultaneously).

The table shows the 10 communication protocols most-cited by CE’s survey respondents. It illustrates what’s in use, and what’s planned in the next 12 months. An additional 10 protocols (not shown on table) were also cited as being planned for use within the next 12 months or in use.

According to Dave Johnson, vp of Controller Business, Rockwell Automation/Allen-Bradley, reports, “During the past several years, distributed control and increased communication trends have continued to play a major role in shaping control technology. The latest technology trend in both PC-based and PLC-based systems leans toward improved information flow.”

Within the PLC marketplace, common architecture and improved network technology provide unparalleled flexibility for sequential, process, motion control, communications, and I/O functionality.

Mr. Johnson continues, “Conventional PLC systems and their backplanes (VME, PCI, etc.) are based on master/slave network topology, which requires all information to be funneled though or directed by a single controller (PLC) or system arbiter.”

To fulfill perceived needs of future control systems, A-B engineers developed ControlLogix using “producer/consumer technology.” In operation, the backplane removes the bottleneck caused by funneling all data through a single point. It treats the controller backplane as a network, giving all modules (input, output, communications, and processors) the ability to broadcast information. Producer/consumer technology allows information to be sent simultaneously to multiple controllers in a system, reducing the latency caused by traditional techniques.

ControlLogix looks like a PLC, but that’s where the similarity ends. This model for A-B’s future control developments is a rack-based system with ControlBus, a control backplane directly embedded with the ControlNet producer/consumer network. ControlNet is an open, high-speed networking option combining I/O points, peer-to-peer interlocking and messaging on the same cable.

Internet extends PLCs’ reach

According to GE Fanuc Automation North America Inc., (Charlottesville, Va.), one significant trend emerging on the PLC marketplace is remote programming and diagnostics of PLCs over the Internet. The Internet and intranets are increasingly viable and secure ways to program and troubleshoot PLCs remotely. Truly open PLCs connected to Ethernet TCP/IP are the best way to fully exploit capabilities of PLCs and the Internet. Taking advantage of this ability offers numerous benefits, especially in time and travel savings. For example, accessing remote PLCs over the Internet allows information processing when & where needed. In addition, PLCs embedded in OEM products offer a competitive advantage through lower support costs and a product differentiator with timely support.

With customers worldwide, an OEM can upgrade its programs in the PLC over the Internet. While there must be an Internet access at both ends, improvements in communications with 100 Mbyte Ethernet and recent reductions in connection costs make Internet access a very realistic approach to customer support.


Even though PC and soft-logic-based solutions are growing in popularity, the PLC remains the right choice for many applications. Manufacturers continue to advance and enhance PLCs, with more memory, faster CPUs, and Internet capabilities to meet today’s manufacturing needs and hopefully, tomorrow’s perceived needs.

Micro Network Controller Weds PLC

Schaumburg, Ill.— Offering up to 256 distributed I/O points, the SRM1 micro network controller provides users the power and flexibility that results from combining a high-speed distributed I/O network and a fully functional PLC in an ultra compact package. Based on the networking system rather than on the controller’s fixed I/O ports, the SRM1’s I/O is distributed via Omron’s high-speed (750 kbps) CompoBus/S network. This distributed configuration reduces wiring and offers machine builders the flexibility to distribute I/O points anywhere on the machine, rather than designing machines around the controller.

Omron Electronics Inc.

MicroPLC Offers Faster Performance

Vernon Hills, Ill.— The “FX2N microPLC” is rated by Mitsubishi Electric Automation to be 600% faster than and about half the size of its existing Enhanced FX PLC. Boasting such speed enhancements as standard 60 kHz high-speed counters and 10 kHz pulse output with motion-control functions, the speed of the FX2N is comparable to that of Mitsubishi’s high-performance large rack systems, offering potential for significantly increased machine speeds. It provides PLC-to-PLC networking capabilities which operate automatically over multiple stations. Seamless modem integration for networks covering wide geographic regions, programming and monitoring may be performed remotely.

Mitsubishi Electric Automation Inc.

Memory, Communications Options Added

Alpharetta, Ga.— Siemens has expanded its Simatic S7-200 microPLC product offering with several new CPU options and communications capabilities. The Simatic S7-215 and -216 PLCs join the -212 and 214 to increase the line’s range of internal memory to 8 kBytes and built-in I/O ports from 14 to 40. The -215 unit has integrated point-to-point(PPI)/FreePort (serial ASCII protocol) and Profibus DP communications ports, while the -216 has two PPI/FreePort connections for interfacing to a LAN or peripherals.

Siemens Energy & Automation Inc.

PLC Has Built-In HMI

Medina, N.Y.— The Model Might-11 PLC combines a PLC and intelligent human-machine interface (HMI) into one integrated panel-mountable unit. The design eliminates the need to purchase a separate HMI, saves space, and labor costs. The HMI has a 2-line by 20-character LCD, a 20-key keypad, and three programmable indicator lights. The unit can be programmed with up to 160 messages with embedded data display or editing. Two operating modes are available for the messages: either menu-driven or controlled by user’s ladder diagram program. Password protection adds security.

SKH Systems Inc.

D3-350 Expands DL305 Family

Cumming, Ga.— With 14.8 kBytes total memory and 368 local I/O points, expandable to 880, the D3-350 CPU from PLCDirect is compatible with the original GE Series One and Siemens Simatic TI305 bases and I/O subsystems. This allows customers to upgrade to a state-of-the-art instruction set with over 160 instructions while taking advantage of communications capabilities such as an operator interface/programming port, built-in remote I/O, and networking. A real-time calendar/clock, 4 PID loops, floating point math, and drum timers are also supported.


Modicon Line Expands with TSX Premium

North Andover, Mass.— The Modicon TSX Premium’s backplane is based on a high-speed bus known as BusX that enables up to 16 racks to be distributed up to 270 meters without remote I/O interfaces, bridges, repeaters, or networks. The Premium is based on the consumer-producer model with ASIC intelligence on each module that enables communication between modules without CPU overhead. With bus speeds exceeding 12 MB/ sec—I/O latency be-comes a nonissue. The possibility of multiple processors, and parallel processing within each CPU maximizes performance and flexibility. For users, this means plug-and-play configuration, and low-cost, compact, high-performance distributed control.

Schneider Automation

Program PLCs using Windows

Charlottesvile, Va.— Programmers of GE Fanuc PLCs and I/O subsystems can now enhance productivity with Cimplicity Control Version 2.0. This Microsoft Windows-based PLC programming software can save users development time, improve project time frames, and reduce overall costs. Using Microsoft’s Win32 technology, Cimplicity takes full advantage of Windows 95 and NT architectures. With Cimplicity 2.0, users can program Series 90-70 and Series 90-30 Model 351 and 352 PLCs. Support is planned for additional Series 90-30 CPUs, including the new 350 and 360 models, in early 1998.

GE Fanuc Automation North America Inc.

PLC Has Advanced Communications

Sunnyvale, Calif.— The Micro3C PLC features advanced communications which allow it to “talk” to any intelligent device with a protocol. This new functionality targets applications using modem communication, user-defined communication, or monitoring data. Two communications ports are standard, RS-232 and RS-485. Either port can be used to communicate with other devices using an 8-bit protocol.

IDEC Corp.

Architecture for Next-Generation PLCs

Mayfield Heights, O.— Described above, ControlLogix is compatible with existing A-B PLCs, I/O sub- systems, and communications networks. The ControlLogix gateway consists of a chassis and communication modules. Available components include the Logix5550 controller; 36 analog and discrete I/O modules; a 2-axis servo motion module; and RSLogix 5000, the system’s common programming software.

Rockwell Automation/Allen-Bradley

Profibus DP Master for GE Fanuc

Indianapolis, Ind.— The HE693PBM100 is a Profibus DP network master for Series 90-30 GE Fanuc PLC backplane. This product controls up to 64 network slave devices. A network slave module, the HE693PBS105 is also available. The HE693PBS100 module allows Series 90-30 I/O to be made available on the Profibus DP network.

Horner Electric

A-B SLC 5/03 Married to Modbus

Bakersfield, Calif.— The RTU 5/03 processor for the A-B 1746 platform brings together standard Allen-Bradley SLC 5/03 technology and the Modbus protocol in one package. Targeted at SCADA applications, the RTU-5/03 is available in 8 kByte and 16 kByte RAM versions. All features of the SLC 5/03 are available in the RTU-5/03, including the complete 5/03 instruction set, built-in math, analog I/O, and PID capabilities.

ProSoft Technology Inc.

Filling the Void between PLCs and DCSs

The original PLC was created to replace racks and racks of relays that had to be rewired every time an auto manufacturer changed model years. Because, for the most part, discrete parts manufacturing processes rely on simple ON/OFF signals, they can be quantified. So, the PLC’s digital design and programming was a “natural” for such applications.

But, what about motor control and other processes that are functionally analog? One answer has been for PLC manufacturers to offer PID, motor control, and other types of analog-based modules, enabling the digital PLC to operate in the analog world.

Continuous processes are analog in nature and most are controlled by distributed control systems (DCSs). However, there’s a gray area between the DCS and PLC.

Why not simply produce an “analog” PLC?

Enter the RTP 2000 Analog PLC from RTP Corp. (Pompano Beach, Fla.). RTP 2000 supports a multivendor environment, enabling it to accommodate a wide range of data acquisition and control products without costly investments in proprietary software or customized programs. For example, the unit features high-speed DDL interfaces to Intellution’s FIX, TA Engineering’s AIMAX, SYSECA’s APMS, and CiTechnologies’ Citect. It also offers DDE connections to virtually all standalone HMIs.

The RTP 2000 is positioned to offer users a choice between limited-performance PLC systems and expensive proprietary DCS architectures.

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