On Tue, May 16, 2006 at 02:15:44AM +0000, Terry Hancock wrote:
>
> 1) Do you need to know the entire engineering process behind each
> capacitor or resistor on your circuitboard? NO: because capacitors
> and resistors are replaceable commodity components; you only need
> to know their ratings (3 or 4 numbers); there are loads of suppliers;
> and there is no danger of them becoming unavailable.
You'd be surprised. About 10 years ago Allen-Bradley quit making
carbon composition resistors, because sales were petering out in the face of
surface mount assembly, and the production machinery was completely worn
out. The Defense Department went nuts and tried to put a stop to the
shutdown, even offering to buy the machinery, because carbon comp resistors
have the unique property of being able to handle heavy short-term surges
without a lot of parasitic inductance. That's key to building electronic
subassemblies that can handle ESD, lightning surges, and EMP. No luck,
Allen-Bradley dropped the line anyway. Eventually a couple of companies in
the far east started making carbon comps, but they're not nearly as well
characterized, and nobody is sure what their quality levels are.
> 2) Do you need to know the design of chips? Well, maybe. It depends
> on how commoditized they are. 74xNN chips are ubiquitous; there
> are free-licensed designs for them; and loads of substitutable
> components -- so who cares?
Not necessarily. Some competing 74HC designs with the same part
number have different internal logic designs, which can affect propagation
delays, setup times, and the test vectors you need to use if you want to
detect hardware failures. That's important in some safety-critical designs,
where the regulatory agency requires that a failure result in a safe
shutdown within some hard time limit, like 4 seconds.
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