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Replacing DCS PLC Cards
Why are DCS and PLC spare parts very expensive?

Replacing faulty DCS and PLC cards such as processor cards, IO cards, communication cards, etc. can be a costly affair to the company. This applies not only to DCS/PLC cards, but any electronic item used in an industry for instrumentation and control. But what makes them so expensive? What makes them different from commercial electronic products that generally have much greater processing power than a typical DCS? Is it due to their higher precision and reliability? Or is it just a simple matter of supply and demand?

Industrial products have features not found in commercial or home products, such as:

Reliability (24/7 operation)
Extreme temperature range.
Burned-in for longer life.
Designed for use and abuse (extended operation ranges)
Low volume production runs.

This holds true even if some specs look better for home products (often exaggerated). I hope my 2 cents of knowledge clarify the why.

I recall many years ago, a mechanical engineer asking if he could control the machine in front of us, using his Sinclair Z80. The answer was, you have the computing power (and speed if using machine code) to make it work but you wouldn't would you?

You would need a great deal more hardware to automate the energising of contactors for motors and solenoid valves for hydraulics; and would you be happy (and safe) with all this mechanical movement reliant on the edge connector contacts of a Z80.

At the time We were using a modular rack mounted Allen Bradley PLC2.

What 'commercial electronic products' do you have in mind? Recent developments have seen tablets/mobile phones being used in conjunction with HMI's, but giving very limited access.