Internal Modem and AT&T SV/386 R3.x

[Bill Mayhew]

I'm not sure if just changing the address and IRQ of sdevice.d and
cf.d entries for the standard asy port driver will work for other
than the standard COM1 3f8 and COM2 2f8 addresses and IRQs 4 and 3
respectively.  There is also a problem in that COM3 shares IRQ 4
with COM1.  You may experience either hardware, software or both
conflicts with the interrupts.  The problem is that in the standard
MSDOS ISA bus architecture interrupts are edge triggered.  Almost
all cards are stupidly designed so that they hold the IRQ lead low
until their hardware is serviced.  This means that any subsequent
interrupts generated by other boards in the system while the first
card is holding the IRQ lead low will not be noticed by the 8259
controller.  This is a really p*ss poor design.  I know why board
designers do that, it allows an IRQ service routine to psuedo poll
devices, but really violates the intended way of making things
work.  The other problem is the &**#@! boards themselves.  You
could share the interrupt if the boards had open collector outputs,
but I've seen quite a few that have regular totempole TTL outputs
driving the bus because they were too cheap to use one extra open
collector driver part.

Anyway, it seems that although the Sys V r 3.2 documentation hints
that you can share IRQs, that it doesn't work.  The idbuild process
will bitch if you have two drivers with the same IRQ.  I don't know
if that is a "bug" in the idbuild process or if that is a
limitation of the asy driver.  I'm not an expert at writing
drivers.  I agree that the FAS 2.06 driver that Peter recommends is
a good idea.  I've installed it on my system.  I also rigged my
COM3 port to use IRQ 7 so that I could still have the built-in
ports.

At least the newer designs fix the quick and dirty design of the
original IBM PC.  Both the EISA bus and MCA bus have level
triggered interrupts.

Bill


-- 
Bill Mayhew      NEOUCOM Computer Services Department
Rootstown, OH  44272-9995  USA    phone: 216-325-2511
wtm at uhura.neoucom.edu   ....!uunet!aablue!neoucom!wtm
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