Date: Tue, 23 Jul 1996 18:06:04 +1000 From: Bruce Evans <bde@zeta.org.au> To: schofiel@xs4all.nl, tweten@frihet.com Cc: hardware@freebsd.com Subject: Re: The multiple COM ports discussion Message-ID: <199607230806.SAA24356@godzilla.zeta.org.au>
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>schofiel@xs4all.nl said:
>>4) In the PC/ISA scheme, interrupts are POSITIVE-GOING, EDGE triggered.
>> In the EISA scheme, interrupts are by the default compatible to this,
>> but can be configured to be ACTIVE LOW, LEVEL triggered.
>>5) TRistate interrupt line drivers are not neccessary in this scheme.
>What do you mean by "this scheme?" If you mean that "ACTIVE LOW, LEVEL
>triggered" is "this scheme," fine. All that is needed then is OPEN and
>ACTIVE LOW -- two states in ths strictest sense (though people often use
>"tristate" when all they really mean is that one state is OPEN). If "this
>scheme" is "POSITIVE-GOING, EDGE triggered," see my hardware concern below.
I think many of the points only apply to EISA.
>Both ISA and EISA wire all interrupt "pins" on their motherboard connectors
>together. The only ISA standard I have any familiarity with is the 8 MHz
>IBM PC/AT, and the IBM-supplied boards that went with it. Those boards
>drove their interrupt lines with two states, ACTIVE HIGH and ACTIVE LOW.
>There was no OPEN state. There was no output resistor to mediate between
>"dualing" bus driver circuits. Therefore, there was no possibility of
>getting predictable results from putting two boards on the same interrupt.
>For each of the four combinations of two boards asserting or not asserting
>an interrupt you could get board-one dominance, board-two dominance, or you
>could get the privilege of replacing fried parts. It all depended upon the
>variable details of the driver circuits on both boards. Most of the time
>the hardware survived this kind of abuse because the board designers
>accounted for the possibility of people accidently installing two boards at
>the same interrupt.
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)
Now all these devices except the Monochrome display are usually in one
chip, but the interface hasn't changed (:-(). There would be no point
in leaving out the tri state enables which probably cost 0.1% as much
as the jumper block for manual enables.
Bruce
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