Path: utzoo!utgpu!watmath!att!tut.cis.ohio-state.edu!brutus.cs.uiuc.edu!wuarchive!wugate!uunet!deimos.cis.ksu.edu!eecea!gordon
From: gordon@eecea.eece.ksu.edu (Dwight Gordon)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Interupt conflicts
Keywords: PC Bus
Message-ID: <795@eecea.eece.ksu.edu>
Date: 17 Aug 89 14:45:55 GMT
References: <1875@uceng.UC.EDU> <[24ea313d:5029.1]comp.ibmpc;1@ddsw1.MCS.COM>
Reply-To: gordon@eecea.UUCP (Dwight Gordon)
Organization: Kansas State University, Manhattan, KS
Lines: 88

In article <[24ea313d:5029.1]comp.ibmpc;1@ddsw1.MCS.COM> 
   karl@ddsw1.MCS.COM (Karl Denninger) writes:
>>Item 5029 (0 resps) by kthompso at uceng.UC.EDU on Wed 16 Aug 89 09:29
>>[ken thompson]    Subject: Interupt conflicts
>>
>>A friend of mine returned his AT clone back to the store for warrenty repair.
>>The owner told him that he would have to buy a new mother board.  He said the
>>mother board had smoked because two boards were set on IRQ 4. He said it also
>
>>destroyed the hard drive.  He now refuses to fix or return the PC to him,
>>says he may need it for evidence.

  I'm not a lawyer, but it sounds like theft to me.  Your state Attourney
General probably has some type of consumer protection division.  (Kansas
even has a toll-free number for us!)  They might warrant a phone call.  The
Better Business Bureau sure does.  When he returned the equipment to the
store, he probably (better have) got a receipt for the equipment.  This
receipt has suddenly become important!

>>
>>I have installed boards with conflicting interrupts before and other than 
>>software problems, there was no damage.  The interrupt chip is just a
>>TTL input and shouldn't care how many highs are put on it's input.  I think
>>my friend is getting the screws.
>>
>>Comments please!
>
>Comments?  Here's some....
>
>First, you CAN damage things by conflicting IRQ settings..... but --

  Agreed (as, you say - Highly unlikely for well-designed boards).

>You're most likely to damage the peripheral boards themselves.  You see, if
>one board is driving the pin high, and the other driving it low, well, you
>get to see how good a fuse some TTL level driver makes... on the peripheral
>card itself.  In this case, you might fry a modem or serial board -- both of
>which are on IRQ 4.

  Wait a second, true TTL doesn't source current (at a logic high), it sinks
current at a logic low.  However, few systems with the C&T chipset are really
using TTL inputs.  So this point is somewhat mute.
  More likely the problem is related to what other things are messed-up.
INT 4 is used for COM1.  If the real problem was two devices connected
to COM1 (INT4 and 03F8h Port address), then any device accessing COM1's
ports will access more than one device simultaneously.  Each of these
devices, since they are on a 3-state bus, are not TTL, but 3-state.  3-state
devices don't necessarily follow the no-source design rule of TTL.  This
could blow out the data bus drives.  This, in turn, could blow out any
other data bus connection (e.g. hard drive controller) which, in turn, could
blow out . . .

>Since the motherboard doesn't drive _anything_ in this situation, it would
>be slightly difficult to damage it with this type of mistake.  As for the
>disk drive, that would be even harder, as it has no direct connection to the
>main board at all; it's driven through a number of buffers and other
>circuitry (the controller card of course).

Agreed, with the above restrictions.

>I think your friend is getting the snow job.  I'd tell Mr. Clone Shop Owner
>that you want the machine back, fixed, right now, or you are going to sue.
>It sounds to me like someone is trying to make a lot of money for doing
>nothing at all -- this clone shop person could simply fix the jumper settings,
>charge your friend for a motherboard, and send him on his way -- lots poorer.

  AGREED!  Call Clone Shop Owner, and, if no satisfaction is received, call
your attourney general's office.  (Note - Most times, I've only had to
threaten to call the attourney general's office in order to get the ear
of a "bad" dealer.)  If you do have to go through the attourney general's
office, record-keeping can either make or break your case.  Make sure
to log all calls/contacts with the shop owner.  Make copies of all 
correspondence/receipts/etc. and keep them in a file.

  I have consulted for almost three years at a company that repairs
IBM desktop computers.  I've seen a whole lot.  The design of the board
is not terribly robust.  Some times a simple mistake will cause an
avalanche of failures.  It is possible (although unlikely) that the
board is dead from your INT4 mistake.  I'm no lawyer, but in my opinion
this does not provide an excuse for Clone Shop's owner to TAKE the
board(s).  At best, he may refuse to provide warranty service, since
the unit was tampered-with.  However, IBM's open bus architecture was
designed for just this such addition of user-purchased boards.

Dwight W. Gordon, Ph.D.  |   913-532-5600    |   gordon@eecea.eece.ksu.edu
Electrical & Computer Engineering Department |     dwgordon@ksuvm.bitnet
Kansas State University - Durland Hall       | rutgers!ksuvax1!eecea!gordon
Manhattan, KS 66506      | {pyramid,ucsd}!ncr-sd!ncrwic!ksuvax1!eecea!gordon