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Electronic Design

Bob's Mailbox

Dear Bob: I once had a problem identical to what Jeff Slefske mentioned (ELECTRONIC DESIGN, Feb. 16, p. 18). The way he described it, his circuit did not originally have the power-supply common connected to the line cord green wire.

Often supplies have 0.01 µf or so from each side of the ac line connected to the common to reduce conducted EMI. If the common is not grounded it floats to 60 V rms, with enough current available to give a nasty shock and also blow RS232 inputs. At 0.01 µf, about 0.5 mA is available. (That sounds like"only 60 V," but that is 84 V peak. If you have a 0.01- µF cap charged up to 80 V ac peak, the momentary surge will be larger than 0.5 A, which is capable of blowing a lot of computer inputs—and semiconductors in general. You can make inputs that will survive such an 80-V surge, but you'd have to add resistors and diode clamps. And it takes some real engineering, some significant expense, and some extra board size to protect the inputs. /rap) Another good reason to not pull out the third pin on your power cords!

  • Jim Harman (via e-mail)
  • Pease: Uh, yeah! Or, you might avoid this by not doing hotplug connections, if you don't know what's going to happen. Thanks for the comments, Jim.

Dear Bob: I have a story you might be interested in about"Autosave Stuff" (ELECTRONIC DESIGN, March 2, p. 20). Several years ago, a coworker and I were writing a document (using MS Word) and had autosave turned on. We were using many of the features of Word, including drawing and autoshape options for many complex drawings. Autosave worked nicely, saving everything every 30 minutes. This went on for two days. At the end of the second day we tried to load from the autosaved file and found it corrupted.

(Ouch! And you trusted it to work, so you never checked it before you trusted it. Presumably, it had worked in the past but started to go bad when you needed it. If I have an important document, I will "save to file" and also occasionally "save to template," which is less likely to get scrambled or erased. I also print out important pages on paper, where it's easier to proofread with full accuracy—and slightly less likely to burst into flames. /rap)

We never saw any warnings. And it did not save each new autosave in a separate file. Ouch! Now when I see that an autosave is engaged, I manually save to the file name + a sequence number.

It gives me the feel of being back in 1969 and resaving every 10 lines entered when logged onto timesharing on a mainframe. All that time and technical progress, and we still can not depend on the hardware/software beyond the keyboard and monitor, be it a mainframe or a PC.

  • Charles Ryan (via e-mail)
  • Pease: Yeah, I guess you can hardly trust any computers. Not even analog computers... or punched cards?

Dear Bob: Autosave is one of those things I keep turned off in MS Word. Consider this scenario: I have just done one of those stupid things where I somehow delete a block of text (maybe click on delete instead of cut). The phone rings and I am on one of those long support calls. So autosave soon saves the corrupted document. Of course there is a .bak copy, isn't there? Sure. But 10 minutes later, while I am still on the phone, autosave again saves my corrupted document and saves a backup copy of it as well.

(Ouch! I never had that problem. /rap) Sure, I could still come back and undo the changes. But I am less likely to remember to do this than I was to manually save my work at regular intervals. What is needed is a popup reminder.

  • Frank Deits (via e-mail)
  • Pease: Hello, Frank. PC Write Lite used to provide that, too, on request.

Here are the answers to the brainteasers in my "What's All This Mental Math Stuff, Anyhow" column (ELECTRONIC DESIGN, March 30, p. 18). For more details, see

  1. 25.0400 miles, or 132,211.087 feet
    1b. I don't know, but maybe soon
  2. 80 or 80.05 mph
  3. Three or four double-pole double-throw switches are fine (see my Web site for a full explanation)
  4. 12.48999600
  5. 14 or 13.97 milli-inches

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

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