Date: Tue, 23 Jul 1996 10:06:37 -0700 From: "David E. Tweten" <tweten@frihet.com> To: Bruce Evans <bde@zeta.org.au> Cc: schofiel@xs4all.nl, hardware@freebsd.com Subject: Re: The multiple COM ports discussion Message-ID: <199607231706.KAA09614@ns.frihet.com>
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bde@zeta.org.au said:
>I think most ISA boards do have tri-state drivers, but there is no
>standard for this. I'm only very familiar with (have a manual for
>:-() pre-AT designs. These devices in the IBM PC Technical Reference
>Manual gate the IRQ through a 74LS125:
> IBM Monochrome Display And Parallel Printer Adaptor (Logic 11 of 12)
> Parallel Printer Adaptor
> Asynchronous Communications Adaptor
>These devices are connected more or less directly:
> 5 1/4" Diskette Drive Adaptor (Logic 4 of 6) ('765 through MC3487 to
> bus)
The confusion seems to have set in when IEEE came out with the P996
standard. The D2.02 draft of it that I have says (in section 2.5.1) that
IRQ signals "are Active HI open collector signals."
Unfortunately, as pointed out by Edward Solari in his book, "AT Bus Design"
(copyright 1990, Annabooks), "The IBM AT and ISA platforms have defined the
INTERRUPT signal lines' operation as unshared edge triggered; consequently,
add-on cards have been designed with totem pole outputs. If these add-on
cards are installed in an E-ISA platform, they must be the only interrupt
resources on the line."
EISA Standard version 3.12 also says (section 1.8) the following about
EISA's two interrupt line modes:
"Edge-triggered operation provides full compatibility with existing,
interrupt-driven, ISA devices. Level-triggered operation facilitates the
sharing of a single system interrupt by a number of devices. Level-
triggered interrupts might be used, for example, to share a single
interrupt between a number of serial ports."
I no longer have access to the IBM schematics for AT era add-on boards. I
did find a schematic for a Zenith parallel port board of the day. Though
it used a tri-state buffer chip on the IRQ line, the tri-state control line
was wired into the active state.
It is evidence like this that leaves me convinced that ISA (as opposed to
IEEE P996) boards cannot share interrupts.
--
David E. Tweten | PGP Key fingerprint: | tweten@frihet.com
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