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From: resnicks@netcom.UUCP (Steve Resnick)
Newsgroups: comp.os.msdos.programmer
Subject: Re: problems with com3 & com4
Keywords: com3 com4
Message-ID: <22167@netcom.UUCP>
Date: 28 Jan 91 18:35:30 GMT
References: <1991Jan25.054228.157@frey.newcsatle.edu.au> <1991Jan27.034726.15865@meadow.uucp>
Organization: Netcom- The Bay Area's Public Access Unix System {408 241-9760 guest}
Lines: 38

In article <1991Jan27.034726.15865@meadow.uucp> py@meadow.UUCP (Peter Yeung) writes:
>In article <1991Jan25.054228.157@frey.newcsatle.edu.au> kaos@frey.newcsatle.edu.au (Iain `Kaos' Holmes) writes:
>>
>>	cause i can write to the ports and they react but i think the interupts
>>
>>	are screwy...
>>
>
>Which interrupt are you using? Since the original PC does not have provision
>for com3/4, the interrupt to be used is implementation dependent.
>Most implementations chose to share the interrupts with those being used
>by com1/2. Also, typically, those async cards supporting com3/4 also allow
>the user to choose the interrupt/port address etc. So, make sure you are
>catching the interrupt same as the one setup on the card!
>

COM3 & COM4 have defined, standard addresses and IRQ settings. 
	COM3 Uses a base address for the UART at 3E8 and IRQ 4
	COM4 Uses a base address for the UART at 2E8 and IRQ 3

The interesting thing is that if you are using interrupt driven communications
on, say, COM1 and COM3, you will need to determine the source of the interrupt
based on the IID register on both UARTS. I don't know what happens if 
interrupts happen simultaneously.

Hope this helps,
Steve



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