From: wong@cs.tu-berlin.de (Wolfgang Jung) Subject: Re: Can COM1/2: and COM3/4 be used simultaneously? Date: 7 May 1993 09:02:48 GMT
Theodore Ts'o (tytso@athena.mit.edu) wrote:
: From: rmarson@rp.CSIRO.AU (Ralph Marson)
: Date: Wed, 28 Apr 1993 04:09:03 GMT
: What I want to know is, can Linux be configured to do something clever when
: an interrupt occurs on IRQ3 say, so that it polls both serial ports to
: see which one has the data?
: The serial driver is *already* polling both serial ports to see which
: one has the data. The problem is a hardware problem --- if you have two
: ISA boards trying to use the same IRQ, only one, or possibly none, of
: the boards will actually be able to send an interrupt to the CPU. So,
: the serial driver, never gets the interrupt from one of the boards, and
: here's nothing you can do in software to fix this problem.
: The reason why the serial driver polls all of the serial ports on a
: particular IRQ is to support multi-port serial boards, which are
: *designed* to support 4, 8, or 16 serial ports using a single IRQ.
: These Linux will support fine --- as long as you get the interrupt to
: the serial driver, it should deal appropriately. Examples of multi-port
: serial boards which Linux will support include the AST Fourport (and its
: clones), the Usenet Serial Board II, and the Boca 4-port, 8-port, and
: 16-port (BB-1004, BB-1008, and BB-2016).
The problem (Feature, or whatsoever) is the DESIGN of the ISA Bus.
from all responses I got & observing my hardware (look at a Card which holds
16450(16550) and follow the IRQ lines) that A CArd whcih wants to generate IRQ
has to DRIVE the IRQ Line, so If two Cards try to drive this line, either
One wins or the IRQ Controler of the Board will find a Level Outside the
the allowed range, and won't announce an Interup to the CPU.
I would't declare this a Bug, it 's a feature, which shows up to be a
BAD on. But on the 16BIT ISA there are enough IRQ Lines for each Card (assuming each Card uses ON(except the COM1,2 Cards)
there are on a normal 16 Bit ISA Bus IRQ's from 1-16
(2 is Cascade and 6 is used by the FLOPPY Adapter IRQ 3,4 by COM1,2
7 BY the Printer 1 is used internally.) I Think there are 6 IRQ's
free for every One.
many VGA Cards seem to use IRQ 9 for something. but I have this turned
of, and hadno Problem :-)
Gruss
Wolfgang