I won't address hardware issues, but from a software standpoint, in my
experience, DCSs usually use function block programming (and, so I've heard,
boolean logic), and PLCs ladder logic.
DCSs were designed for apps that required a lot of analog monitoring and
control, and PLCs for machine or sequential control apps. These days there
is vast overlap.
In my limited experience, function block programming is well suited to
analog control applications; doing the same thing with PLC software is often
quite a bit more tedious. On the other hand, implementing sequential control
is generally easier to do with ladder logic than the boolean logic I've seen
used in DCS software.
Some PLC and DCS providers allow both function block and ladder logic
programming. Modicon, the venerable PLC co., does, and Moore Process
Automation DCS does, or did. Moore was bought out by Siemens in 2000.
Moore's software allowed programmers to integrate use of function block,
ladder logic, "instruction list" and sequential function charts (SFCs). I
hope I remember this right from 4+ years ago, anyway. I'm sure there are
others doing the same thing these days.
As to the hardware, costs and reliability issues, I leave that to others.
One unfortunate thing that too many users of PLCs and DCSs do is get too
many of brands in their plant, or plants. Too often an accountant's
perspective of the purchase price is what comes into play. Overlooked or
underplayed is all the time and money, that it takes for people to learn how
to use different stuff.
Many providers, like Rockwell Software/ Allen-Bradley, the PLC supplier,
offer a suite of proprietary hardware and software meant to work with each
other, and, of course, give you more reasons to be married to them as a
tom < firstname.lastname@example.org> wrote in message
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