This debate always interests me. It brings out some very strong opinions and sometimes even emotionally charged responses
especially when it comes to terms such as "PC Based PLC". This is often a result of one's own definition of "PC".
So what really is a "PC"? I think all of us would agree that a computer or laptop running Microsoft Windows is a PC.
What about an Apple Mac? Some would say it's a PC others would disagree and say it's a Mac.
If there were a piece of software to make a Mac operate like a PLC, would the debate then include "Mac-Based PLC"?
So what about a Raspberry Pi or Beaglebone? These are small microprocessor boards with an ARM processor that no where
resembles your conventional desktop computer, but yet they run Linux with a windows style OS, run user applications,
and are used with a keyboard and mouse.
If a PLC runtime software is installed on these devices, is it a PC based PLC? Now what if a real time embedded operating system
such as VxWorks is installed on the Pi and Allen Bradley's PLC software installed, what kind of controller is it now?
http://en.wikipedia.org/wiki/Raspberry_Pi
http://beagleboard.org/BLACK
Then we get to the Beckhoff hardware, such as the CX2020. By Beckhoff this is considered an embedded PC because it has
either Windows or Windows CE as it's operating system. Windows CE is Microsoft's version of a real time embedded operating system
that directly competes with VxWorks. Then there is the CX8090 which has an ARM processor with Windows CE operating system.
Beckhoff still considers this an embedded PC, but yet it is no different than an Allen Bradley Controllogix if you compare
the hardware and type of operating system.
http://www.beckhoff.com/english/embedded_pc/embedded_pc_series_cx2000.htm?id=97337418839693
http://www.beckhoff.com/english/embedded_pc/cx9020.htm?id=5563118742705
In my opinion, before we can express opinions like "PC based control will never be as reliable as a PLC",
we need to define what we consider to be a PC, or be more specific. So I think most of us could agree on this statement:
"A consumer grade desktop PC can never be as reliable as a PLC"