Electronic Design

Bob's Mailbox

Dear Bob: Re: "What's All This Spicey Stuff, Anyhow? (Part III):"* Spice is a modeling tool. Modeling is necessary because "worst-case" parts are seldom available for use in a test circuit, and because some situations can only be modeled. (Worst-case parts are not usually well represented in Spice models. /rap) Any model is limited by the approximations used, by parasitic components not included, and by pitfalls of the modeling technique. (THERE is a set of understatements! We tend to agree. /rap)

Spice analysis can be exceedingly accurate and can provide insight into circuit operation. (Sometimes... sometimes it can be TOO accurate—in misleading ways. And when it gives what seems to be an accurate answer, that builds up unfounded trust, which sets the stage for disaster when it starts LYING. /rap) It can also be misleading and give impossible results. (There is another understatement! /rap)

The analyst is responsible for the accuracy and appropriateness of the model. (Yeah, but most kid engineers have no way to comprehend this./rap) A Spice model can rapidly provide detailed and accurate answers, easily explaining the unexpected. It's a thinking tool, rather than a substitute for thinking.

Peter Koninsky (via e-mail)
Pease: Most engineers I see use Spice as a substitute for thinking. As I said, a CRUTCH. I prefer real thinking. Best regards and thanks for writing.

Dear Bob: Spice should only be used by someone who already knows the answer to the problem being simulated.

Jerry Steele (via e-mail)
Pease: Nicely put!! I tend to agree!!

Hello Bob: I have been an avid reader of your columns for many years. I especially enjoyed "What's All This Spicey Stuff, Anyhow? (Part III)." Your comments about Spice telling you the wrong direction to go reminded me of the time I used the Mapquest software program to find a small town in Wisconsin. By carefully following the directions given by Mapquest, I ended up 50 miles west of my destination! Call me old fashioned, but give me a map and compass anytime.

John Kessinger (via e-mail)
Pease: One time I had some very good and precise Mapquest directions on where to find National Instruments in Austin, Texas. The instructions put me 1.4 miles southwest of where I wanted to be. Puzzled the hell out of me! Spice can easily do the same. And sometimes, it DOES. Mapquest maps only list the names of a few of the roads in the area. REAL maps list the names of ALL the roads. Viva La Difference!

Dear Bob: I have just one thing to say regarding your column on Spice. BRAVO!!!

Bob Grober (via e-mail)
Pease: Thank you!

Dear Bob: I just can't waste any more time trying to find this answer on the Internet! It seems like such a simple thing, and perhaps that's the problem. What is the meaning behind the symbols now used on electronic power switches to indicate "On" and "Off"—you know, the "I" and "O" symbols? The closest I got to the answer was an indication that they are IEC international standard markings, but no discussion of meaning or origin. Another place said that they represent binary "1" and "0," but that seems too obscure for what is meant to be universally understood symbols. Can you shed some light on this?

John Reynolds (via e-mail)
Pease: Hello, John. I can only guess about the same as your guess. I and O are slightly more international than ON and OFF. I'll ask around.

Comments invited! [email protected] —or:
Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090

*Eletronic Design, see Sept. 1, p. 24.

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