Path: utzoo!attcan!uunet!ginosko!usc!rutgers!att!mcdchg!ddsw1!ddsw1!karl From: karl@ddsw1.MCS.COM (Karl Denninger) Newsgroups: comp.sys.ibm.pc Subject: Re: Interupt conflicts Keywords: PC Bus Message-ID: <[24ecdcf4:5029.5]comp.ibmpc;1@ddsw1.MCS.COM> Date: 19 Aug 89 05:00:06 GMT References: <1875@uceng.UC.EDU> <5832@tekigm2.MEN.TEK.COM> Lines: 86 >Response 4 of 4 (5029) by timothym at tekigm2.MEN.TEK.COM on Fri 18 Aug 89 >14:27 >[Timothy D Margeson] > >Hi, > >Having read some of these discussions of how setting two add-in cards to the >same interrupt will cause damage..... Not will, can. But it's very unlikely (as in I've never actually seen it happen, but have looked at some boards where it is _possible_ to have it occur). >Have any of you ever seen electronics??? A TTL gate is designed to take >short circuits to ground indefinately (so long as the dissipation spec of the >chip is not exceeded). So long as the dissipation spec is not exceeded..... :-) Now, for a few minutes or so, ok. For a number of _hours_, at 100% duty cycle, and with little or no cooling to speak of (check the so-called 'fans' in some of those cheap PCs as well as chip placement) you could have a slight problem. It all depends on the specifics of the situation. Again, I've never seen a board damaged in this fashion. Note that many of the cheap boards are designed with little or no safety margin beyond normal operating parameters. These are the ones which stop working when you do things like this. >Also, for your info, most PC's with four com ports share interrupts 3 & 4, >and somehow these never blow up!!! Right, because most com boards aren't designed _too_ stupidly. But there are some boards, especially the $50 specials you see in a lot of the foreign clones, which aren't carefully put together..... and who said anything about both of the boards which were 'fighting' being COM boards anyways? I've seen more than one card which didn't have buffers on the I/O and/or IRQ lines....saves a few pennies, 'ya know. >Any other responses about blowing the chips are wrong, and I don't care if >on your mothers PC you know of a case where they did 'cause what probably >really happened is some non-static knowledgable person got into the PC with >pliers or hands and started pulling this part or that part trying to fix the >problem he cause to the card he had just installed. Ask yourself this: > > The last time your worked on your computer, did you wear an anti-static > wrist strap connected to your PC and anti-static desk top? > > If you answered no, then you HAVE damaged your PC, perhaps not in a way > that is immediately noticable, but the damage is real non the less (there > are ways to minimze this damage using good handling techniques but one > minor slip will take out many IC's). Well, actually there are several ways to prevent that problem. If you are grounded to the PC's circuit ground physically, and remain so at all times, you cannot damage the componets. It's potential _difference_ across the board/chips/whatever that does the damage -- if you are at the same potential as the chassis, regardless of what that is referenced to "ground", you can't hurt anything (assuming, again, that the chassis is tied to logic ground -- many aren't for safety purposes. In that case you need a jumper or two, and the power darn well better be OFF AND DISCONNECTED). Besides, these days _most_ chips are quite static tolerant. Nothing like the old 4000-series CMOS stuff that would blow up if you looked at it wrong (yes, I worked with that stuff too. Gave ICs a bad name it did....) We generally do use a stat-strap, but not always. I have _never_ damaged a customer or my own system(s) with static discharge, but then again, I am meticulously careful and paranoid (again, those memories of 4000-series CMOS chips do it to 'ya :-) Oh yeah -- do remove your jewelry/watch/whatever before you go poking around in the system, especially if the power is on. Sparks flying don't tend to help things much, and 20+ amp +5V rails can be nasty, and are found in most AT-class systems! Finally, remember that every electronic component is made with an amount of smoke inside, and that is an _essential_ ingredient. Once you let out the smoke, it won't work anymore. Smells bad too. :-) -- Karl Denninger (karl@ddsw1.MCS.COM,!ddsw1!karl) Public Access Data Line: [+1 312 566-8911], Voice: [+1 312 566-8910] Macro Computer Solutions, Inc. "Quality Solutions at a Fair Price"