PLC or PC? If that were the only question facing engineers, then it would be easy. If PC (personal computer) is the choice, then what type of PC? If PLC (programmable logic controller), then should I/O modules be on a backplane or distributed? Add to this the impact of Microsoft Windows CE and Ethernet.

Dave Johnson, vice president and general manager of the controller business for Rockwell Automation (Mayfield Heights, O.), places the discussion in this perspective, "The important thing for the user is to implement real-time control well. We use both commercially available hardware and custom ASICs for our controllers so the customer has a choice. Since distributed control is now accepted with a continued emphasis on smaller platforms with higher performance, networks will become even more important. As the Logix engine [Rockwell's control software and firmware technology] is deployed in more devices, control will become even more distributed."

Mike Sullivan, Schneider Electric (formerly known as the Schneider Automation unit of Groupe Schneider, North Andover, Mass.) global business development manager, says the overall product strategy is using Ethernet TCP/IP for control, diagnostics, and visualization. Modicon Momentum now provides distributed control over Ethernet. Modicon Premium is the first multimaster consumer/producer platform.

Jeff Christensen, GE Fanuc Automation (Charlottesville, Va.) marketing manager, control and I/O products, says, "The new VersaMax product line shows a more integrated approach to networking and to hardware choices. Moving away from the 'PLC' or 'PC' selection as we know them today, the main considerations in controller choice will become development, runtime, and maintenance tools (software)."

"Siemens Energy & Automation's (Alpharetta, Ga.) strategy is to unify hardware and software components into a coherent system based upon the Simatic product line," says Schylon Yates, controller platform product marketing. "Siemens' example of blurring the PLC/PC line is the Simatic M7. It looks like an S7 PLC, but is a PC and responds to increasing demand placed on automation systems requiring computer technologies."

"Even though PC-based control has emerged or many machine control applications, large PLCs are not disappearing," says Mitsubishi Electric Automation's (Vernon Hills, Ill.) product marketing manager, Azam Owaisi. "Many large PLCs are specifically designed to fit particular applications."

PLCDirect's (Cumming, Ga.) strategy has recently expanded in two ways. Its DL05 PLC now sells for less than $100 while the WinPLC and RunTimePC are based on Microsoft Windows CE. WinPLC is a PLC that runs Think & Do (Ann Arbor, Mich.) flowchart control software, while RunTimePC runs Think & Do on an industrial PC platform from Ann Arbor Technologies (Ann Arbor, Mich.).

Omron Electronics (Schaumburg, Ill.) offers a variety of controller form factors. "As users demand smaller products with more functionality," says Jeff Meyers, product marketing manager, "small and mid-size units will incorporate features of the large PLCs."

Stephen Luft, vice president of Entertron (Gasport, N.Y.), notes the company's small size permits it to respond quickly to requests from OEMs for custom PLCs. He sees continued market growth in micro to small controllers.

Advantech (Taipei, Taiwan) manufactures industrial computers for control and HMI. It has joined the Windows CE movement with a series of embedded PC boards that include features like Ethernet, VGA adapter, audio, watchdog timers, and PC/104 sockets.

Opto 22 (Temecula, Calif.) has pursued a strategy of distributed control since 1989. "Time-critical functions such as digital event/reactions, PID loop solving, engineering unit conversion, frequency measurement, counting, and much more, are better handled when moved to the I/O level with a PC-based supervisory controller," says Benson Hougland, director of technical marketing. "The advantage is a system less prone to system-wide failure. In addition, Ethernet offers possibilities considered unthinkable even months ago like configuring I/O systems right from a web browser."

Phoenix Contact (Harrisburg, Pa.) also has a distributed control strategy. It has a controller on a PC board as well as a DIN-rail-mounted controller with Ethernet connectivity to other controllers and an Interbus master for I/O control. According to Mark Knebusch, Interbus Group director, this strategy allows users to distribute control to the point required by the application.

National Instruments (Austin, Tex.) has implemented a PC-based strategy by embedding its LabView RT software on its data acquisition boards. Target markets include machine and process control.

VMIC (Huntsville, Ala.) has a hardware strategy based on application size. Small systems would use a single ISA or PCI computer board. A medium system could use a VME or Compact PCI rack with several hundred I/O points. A very large system would integrate several VME chassis with multiple processor boards.

According to Cutler-Hammer/Eaton's (Westerville, O.) product line manager, Gene Stovall, users are choosing distributed control with intelligent devices and small PLCs. PCs provide supervisory control and intensive data handling chores. Look for Cutler-Hammer to continue developing small PLCs and increasingly powerful PCs like its new Intel Pentium II machine.

Amy Archer, director of marketing for Xycom Automation (formed by the merger of Xycom and ASAP, Saline, Mich.), sees customers moving toward open solutions—programming, controllers, I/O devices, and networks. A new family of controllers introduced at National Manufacturing Week offers products for differing applications from a fully open computer with many options to a shoebox PC controller all bundled with ASAP soft control.

Jay Vierling, CTC Parker Automation (Milford, O.) general manager, says hardware presents challenges to PC-based control. Specifically, the failure rate of hard drives is a great concern to users. "We move mission critical software to compact flash RAM, which has no moving parts and provides easy data transfer."

"Hardware for PC-based control will have to satisfy two customer bases, those who are coming to PC-based control primarily for ease of integration, and those because their application is especially demanding," says Mike Tschantz, hardware product manager at Nematron (Ann Arbor, Mich.). "We currently see the Wintel [Microsoft Windows/Intel chips] platform as the preferred solution to both of these customers. "

Inova Computer's (Cotuit, Mass.) control hardware strategy revolves around industry standard buses, I/O devices, and networking. Says president, Paul Gaudreau, "We have seen CompactPCI become the perfect solution for process control on the factory floor due to availability, MTBF, MTTR [mean time between failure, and…to repair], and industrial quality at desktop PC prices."

Wayne McGee, vice president of SBS Embedded Computers (Raleigh, N.C.), notes that CompactPCI and VME are found in many machine control applications. SBS has recently introduced "or" industrial computers. This manufacturer's DIN-rail PCs include Pentium processors; 64 MB EDO RAM; hard disk or flash RAM; VGA adapter; and Ethernet, CANbus, serial, and parallel communications. An optional temperature range is -40 to 70 °C.

PEP Modular Computers (Pittsburgh, Pa., Kaufbeuren, Germany) offers computers in the VME and CompactPCI form factors including multiprocessing capability. The latest offering is the CP610, which features a Socket 7 microprocessor and supports up to 14 I/O modules. An optional temperature rating of -25 to 75 °C is available.

"As networks become more powerful, distributed architecture technology, based on commercial standards supporting off-the-shelf applications, will be the driving factors behind machine control technology," states computer manufacturer Texas Micro (Houston, Tex.) president Michael Stewart.

Bob Burckle, vice president of PC/104 computer maker WinSystems (Arlington, Tex.) cites the compatibility of this form factor to PC software. PC/104 is a low power device with extended temperature ratings. It is designed for industrial use with a long product life cycle.

For engineers that prefer PC-type languages the PC/104 format with add-on cards for motion control and sensor input are available from Micro/Sys (Glendale, Calif.). The latest release, Netsock/100, is a low-cost controller with digital and analog I/O points optimized for distributed control with on-board Ethernet adapter.

Dave Medin, director of technology at Crystal Group (Hiawatha, Ia.), reports a strategy to provide space-efficient PCs with PCI and ISA expansion slots. "We target up to 32 PLCs in a 7-foot tall, 19-in. EIA rack. We also have a scheme that allows a computer to be removed and replaced after a maintenance action, in about 10 seconds."

Compaq Computers, Digital OEM business unit (Shrewsbury, Mass.) provides CompactPCI single board computers as well as Digital Alpha processor motherboards.

Micro Industries (Westerville, O.) supplies CompactPCI and NLX format computers and allows multiple CPUs in the PCI and CompactPCI factors with asynchronous bridge technology.

Revealing the wealth of choices provided by PC suppliers, Technoland (Sunnyvale, Calif.) manufactures single-board computers, rackmount chassis, toolbox chassis, special backplanes, CompactPCI, and panel-mount PCs. A new technology developed by SBS and MEN Mikro Elektronik (Nürnberg, Germany) is PC-MIP, a low-cost, high-density mezzanine board for PCI systems. The new products from MEN are an Ethernet interface card and an Ultra2 SCSI interface card.

This diversity of hardware certainly provides an option for engineers to find the best hardware fit for the application problem.