The PLC vs the PC
Original issue: November 1997.
Have you heard that there is another alternative to the Programmable Logic Controller (PLC)? No it’s not a DCS. It’s a PC. A Personal Computer (PC) like the one on your desktop. Well, it could be but it tends to be more like an Industrial PC. There are a number of controversial issues because of the hardware and software required to perform the required functions.
The software issues tend to be the most controversial, especially operating systems and real time control. We all know that PLC’s have dedicated proprietary operating and software systems. An Allen-Bradley PLC program does not run on a Modicon PLC and vice versa. The PLC programs run in a loop where the program executes from step one to the last step and back to step one and over and over. This is not the case with PC’s. The operating system may run multiple parallel processes (threads) that may not all share equal processor (CPU) time.
The PC operating system (OS) may be the creation of Microsoft or another software company. Examples of Microsoft OS’s are DOS, Windows 95 ® and Windows NT ®. These OS’s all have different characteristics. According to Microsoft, "Windows NT Workstation is the most powerful desktop operating system for the most demanding business needs. Users of mission-critical applications can take advantage of new 32 bit workstation solutions". Windows NT runs on the latest generation of microprocessors including Intel ®, Digital Alpha and MIPS ®. It is a preemptive multi-tasking, multi-threading 32 bit operating system. BUT is it a real time operating system? What about the UNIX operating system that has been around for many years?
What is a real time OS? There are many definitions for real-time. The user has to determine what real-time means and define it to suppliers of hardware and software.
Examples of other software companies that make PC operating systems are QNX and SoftPLC.
Why is this an issue now? The price of PC hardware is falling, performance is rising and users are looking for standardized hardware and software that is easier to use and flexible. PLC programming software is also being standardized by meeting the IEC-1131-3 standard.
Online Articles
There have been many recent articles and information pages on the Internet about this subject.
Crossing PC/PLC Lines by
Industrial Computing
PC’s in Control by Industrial Computing
PC-based control systems break the information barrier by letting you access and distribute data across the enterprise.
NT up Close by Industrial Computing Magazine.NT’s strengths and weaknesses come to light when faced with the demands of industrial, real-time and embedded system development.
Beyond Soft PLC's by Industrial Computing Magazine.
Control systems users need to look beyond the soft PLC engine and examine all the system variables that make up a complete hardware and software solution.
Plugging NT’s real-time holes by Intech Magazine.
Approaches differ for empowering Microsoft’s NT operating system so it can handle time-critical processes.
Yes! Windows NT runs real-time control by Intech Magazine.
Windows NT can provide users with deterministic real-time control at scan rates typical of PLC applications.
Open, dedicated, or blend -- the choice is yours by Intech Magazine.
A controller should meet requirements for interoperability, reliability, application flexibility, support and cost. It may blend both open and dedicated systems.
PLC's Giving way to Smart Control by Intech Magazine History's second control logic evolution is in full swing.
DCS, PLC or PC The System Decision by Control magazine.
CONTROL Magazine November 1997 Cover Story : PLC: Still the Best Mousetrap?
Advances in Software and Network Connectivity Are Keeping PLCs Competitive With PC-Based
and Open Control Systems. PLCs Often Remain the Best Option for Applications Where High
Speed and Robustness Are Critical.
PC vs.
PLC Debate Goes Mainstream When a
mainstream, big-time business magazine addresses a technical controversy, either the
problem is more important than we thought or somebody made a mistake.
In this case, the answer could be both.
PC's vs PLC's: It's Not an Either/Or Situation Both Systems can coexist peacefully and enhance the control solutions available to your plant by Plant Engineering Magazine.
The PLC versus the PC by Forbes Magazine
Who’s in Control of the Factory Floor? By Industry.net
Software turns PC’s into PLC’s by Control Engineering Magazine.
Real-time Control Systems: NT versus UNIX is the wrong question by Chemical Processing Magazine.
There are several factors to consider when choosing an operating system, of which hard real-time performance is the major one. A Real Time Operating System (RTOS) must provide preemptive multitasking. Preemptive means that the OS can interrupt the execution of a task at any time and does not need the cooperation of the task itself. Multitasking means that it may choose to support multiple threads or processes. This article also addresses PC Reliability and Software fault Tolerance.
Windows NT for
Real-Time Control: Whch way to Go? by ICS magazine.
Windows NT can provide a range of control solutions - from soft to hard real-time.
But you need to do a little homework before you make a final decision.
Real Time Encyclopedia web site at : www.realtime-info.be
provides you with all the information you need about real-time issues.
Web Sites
The following company web pages contain product information:
Allen-Bradley
www.ab.comASAP Software
www.asapinc.comControl Technology Corp.
www.control.comCuttler-Hammer
www.cuttler-hammer.comGE Fanuc Automation
www.ge.com/gemis/gefanuc/IO Works by VMIC
www.vmic.comM & R Automation
www.mnrcan.comNematron
www.nematron.comPLC Direct
www.plcdirect.comPLC Open (IEC 1131-3)
www.plcopen.orgQNX
www.qnx.comRockwell Software Rockwell Software
OpenAutomation
www.openautomation.com/SoftPLC
www.softplc.comSteeplechase Software
www.steeplechase.comTaylor Software
http://198.161.96.162/main.htmWizdom Controls
www.wizdom.com
References
"Is PC-Based Control For You?" by Cynthia M. Hollenbeck, presented at ISA
Tech’97.
"IOWorks Base Package –Software Application Guide" by VMIC, Jan. 14/97.
IOWorks ™ is a suite of software components, compliant to IEC-1131-1 and Microsoft ®
new key technologies such as OLS and ODBC, that is available from VMIC to support the
configuration, implementation programming, monitoring and control of data acquisition and
control products. The IOWorks Base Package is a Windows NT ™ operating system product
designed to support the development of soft logic systems.
"Steeplechase
Handbook : A Practical Guide to PC-Based Control and Flow Chart Programming" by
Steeplechase Software.
As PLC veterans, the authors of this Handbook experienced the frustrations of using
proprietary PLCs. They understand your concerns about moving from PLC to PC-based control.
• How did the PC come to replace the PLC for machine control?
• What are the real benefits of PC-based control?
• What are the differences between soft logic and hard real-time control?
• How do I retrofit an existing PLC control system?
• Why has flow chart programming become the standard language for PC-based control?
Tradeoffs to Consider for
PC based Control a whitepaper by Steeplechase Software.
Hard Real-Time Control and
Windows NT a whitepaper by Steeplechase Software.
"Softlogic: Overcomming Funnel Vision". How and why IEC-1131 based SoftLogic frees the enterprise to become agile and profitable.
Using Windows NT for Soft Real-time Control - Seperating the fact from fiction a whitepaper by Rockwell Software.
Windows NT for Automation
a whitepaper by Rockwell Software.
Updated by Brian Smith on July 14, 1998.