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Subject: Re: Avoiding PLCs
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Thu, 17 Oct 2002 12:16:27 +0100
NNTP-Posting-Date: Thu, 17 Oct 2002 12:16:30 BST
Organization: ntlworld News Service
As I originally said:
"The reasons users want you to use 'the make of controller already in use by
them' are sensible - they want to reduce their training and maintenance
costs. Using another type of controller will not help those users."
That has been confirmed I think by many responders.
I went on to say:
"I suggest that you produce a detailed but system independent functional
specification that can then be implemented on any suitable PLC. It is good
practise anyway, and you can use an IEC-61131 type approach for this
functional specification so that it can be easily implemented on an
IEC-61131 type PLC."
Let me explain this a bit more. The big problem that I have experienced over
and over with equipment supplied with embedded controllers (PLC, PC,
Chips-R-Us or whatever) is poor design documentation. This problem is
particularly acute for the pharmaceutical industry due the the need for
validation. There is an army of people working to reverse engineer the
design documentation from the PLC software provided by such suppliers in
order to be able to validate the applications. This should not be happening,
but it is.
I really believe that given good enough design documentation (And I mean
System Independent design) the generation of the actual software is not a
big issue. And of course there are many systems integrators who know AB,
Siemens, Modicon or whatever extremely well.
So I suggest that equipment suppliers should concentrate on producing system
independent Functional Requirements specifications and then get SI's who
know the particular PLC do the implementation. It will probably even reduce
the cost in the long term.
PS Sorry if this is repeated, I thought I had sent it but it has not
appeared so this is a resend.
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