Date: Fri, 19 Jul 96 17:35:10 EDT From: gtc@aloft.att.com (gary.corcoran) To: Brett_Glass@ccgate.infoworld.com, alex@fa.tdktca.com, bde@zeta.org.au Cc: E00114@vnet.atea.be, freebsd-hardware@freebsd.org Subject: Re: Multiple COM ports with same IRQ Message-ID: <9607192135.AA15984@stargazer>
next in thread | raw e-mail | index | archive | help
>> maybe you can tell me how an EISA/ISA box solves the IRQ sharing problem >> for ISA devices. > >It doesn't. Only EISA devices or E-ISA devices can share IRQs. To clarify, (correct me if I'm wrong): EISA devices, which by definition only plug into an EISA motherboard, can only share IRQs if they have "open-collector" (i.e. open-drain) drivers on the IRQ line. This allows any device on a particular IRQ to pull the line low without "fighting" any other device's IRQ driver. This capability, combined with the fact that EISA motherboards allowing changing the PIC operation to be level sensitive instead of edge-triggered, is what potentially allows EISA devices to share IRQs. One question though: The "open-collector" scheme described above (which is what is commonly used for interrupt sharing) works quite well for ACTIVE LOW interrupt signalling. But the IRQ lines on EISA (and ISA) are ACTIVE HIGH signals. Do EISA PICs also allow the active state of the bus IRQs to be changed from high to low? (I don't think so). If not, then how *is* IRQ sharing implemented on EISA devices? Gary Gary.Corcoran@lucent.com
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?9607192135.AA15984>