Date: Thu, 18 Jul 1996 11:08:33 -0500 (CDT) From: Alex Nash <alex@fa.tdktca.com> To: E00114@vnet.atea.be Cc: freebsd-hardware@freebsd.org Subject: Re: Multiple COM ports with same IRQ Message-ID: <Pine.BSF.3.91.960718105535.7070q-100000@fa.tdktca.com> In-Reply-To: <vines.mmb7%2BgqYvlA@vnet.atea.be>
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On Thu, 18 Jul 1996 E00114@vnet.atea.be wrote: > ISA boxes can > have hardware installed (accidentally?) with devices sharing the same IRQ, but > the drivers cannot deal with it. The sharing of interrupts between ISA devices is an electrical constraint. When two devices are attempting to use a common IRQ line, the devices compete with each other (electrically) to put the line in their desired state. Needless to say, the results are less than optimal. It is possible for the devices to share an interrupt if both devices can tri-state their IRQ lines. That way when you're operating device A, device B's IRQ line has no pull on the line, and vice versa. Naturally this precludes you from using both devices simultaneously (and even if the hardware does support tri-stating the IRQ line, the software may not activate it). [This only deals with ISA cards, the operation of on board serial ports are implementation specific.] Alex
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