Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!uakari.primate.wisc.edu!indri!ames!pacbell!att!chinet!mcdchg!ddsw1!olsa99!tabbs!martin From: martin@tabbs.UUCP (Martin Walker) Newsgroups: comp.sys.ibm.pc Subject: Re: Interupt conflicts Message-ID: <176@tabbs.UUCP> Date: 18 Aug 89 12:55:30 GMT References: <[1580.2]comp.ibmpc;1@point.UUCP> Organization: TABBS - Totally Awesome BBS Public Access UNIX Lines: 69 From article <[1580.2]comp.ibmpc;1@point.UUCP>, by wek@point.UUCP (Bill Kuykendall): >>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, > The standard design philosophy used in designing interrupt control circuitry is to make all the interrupt line drivers open-collector. This means that the peripheral device can pull an interrupt line low, but cannot drive it high. Thus one can connect as many interrupt lines together as desired. When one of the peripherals wants attention, it merely pulls the interrupt line low. This does not in any way damage any of the other peripherals. This is usually referred to as a "wired-OR" configuration. The device which services the interrupt would then poll the various devices to determine which of them generated the interrupt and act accordingly. On most PC's, however, each peripheral has its own interrupt line, obviating the need for software polling, but the drivers ARE usually still open-collector, for precisely the reasons given above. It prevents damage when people configure their PC's incorrectly! I cannot say for sure if this sensible design philosophy was followed in the PC in question, but I would be rather surprised if open-collector logic hadn't been used. It is after all less complex than fully driven logic.... -- Martin Walker | ..!!ddsw1!olsa99!tabbs!martin @ | "The more people I meet, the more Totally Awesome | I like my dog..." BBS | (:-D D-:)