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Date:      Thu, 03 May 2001 11:49:37 -0600
From:      Brett Glass <brett@lariat.org>
To:        Terry Lambert <tlambert@primenet.com>, dchulhan@uwi.tt (Dale Chulhan - Home)
Cc:        chat@FreeBSD.ORG (chat@FreeBSD.ORG)
Subject:   Re: Modem Woes
Message-ID:  <4.3.2.7.2.20010503114324.0459d260@localhost>
In-Reply-To: <200105031734.KAA20670@usr02.primenet.com>
References:  <3AF14814.B259D186@uwi.tt>

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At 11:34 AM 5/3/2001, Terry Lambert wrote:

>In FreeBSD, COM ports can not share IRQs.

Terry, this is not correct. I have several multiport cards 
in which several UARTs share IRQs. They all run fine
under FreeBSD. Here's an excerpt from the kernel config of 
one of them:

device          sio2    at isa? port "IO_COM3" tty flags 0x585 vector siointr
device          sio3    at isa? port "IO_COM4" tty flags 0x585 vector siointr
device          sio4    at isa? port 0x1f8 tty flags 0x585 vector siointr
device          sio5    at isa? port 0x1e8 tty flags 0x585 irq 9 vector siointr

All of them share IRQ 9.

>If your modem is COM4: in Windows, it uses IRQ 3, which is
>already allocated to COM2:.

Attempting to share IRQs between ports can cause an electrical 
problem on the bus IF the ports are on two separate
ISA cards, because the IRQ lines on ISA are tri-state, not open 
collector. But if the UARTS are on EISA or PCI cards, or
on the same card, an IRQ can be shared.

>Note that Windows can "share" IRQs 4 and 3 (COM1: IRQ 4,
>COM2: IRQ 3, COM3: IRQ 4, COM4: IRQ 3) because it does not
>open both COM1: and COM3: or COM2: and COM4: simultaneously.

I've had comm ports that shared IRQs open simultaneously under
Windows.

--Brett


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