Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!cpqhou!randys
From: randys@cpqhou.uucp (Randy Spurlock)
Newsgroups: comp.os.msdos.programmer
Subject: Re: U8250 Questions
Message-ID: <1991Jan15.160612.7474@cpqhou.uucp>
Date: 15 Jan 91 16:06:12 GMT
References: <2535@sparko.gwu.edu>
Organization: Compaq Computer Corporation
Lines: 61

in article <2535@sparko.gwu.edu>, viraf@seas.gwu.edu (Viraf Bankwalla) says:
> 
> Hi,
> 
>	***** 8250 Interrupt Description Deleted *****
> 
>   Say I'm in my isr, processing Receive Data Available,
>   and the modem status changes, am I interrupted, or
>   will I notice the change the next time I poll the
>   IIR in the loop, as I havn't issued the End Of Interrupt ?

	It depends on the order of things done in your interrupt routine,
if you have already issued the EOI to the interrupt controller the 8250
will still NOT generate an interrupt until you clear the receive data
interrupt by reading the data register. Also I have noticed that more 
than one interrupt condition can occur at the same time and this condition
will *****NEVER***** generate more than ONE interrupt to the interrupt
controller, therefore you should ALWAYS loop back to the start of the
interrupt handler and check the 8250 for more interrupt conditions pending
or you could wind up hanging your 8250, I know it happened to me!

> 
>   I've also noticed, that on turning the modem OFF, the DCD,
>   and other bits in the Modem Status Register get set, resulting
>   in me returning a false result.
>   Assuming that DCD always reflects the true state of the
>   carrier, how could I avoid the above problem ?
> 

	The DCD is Delta Carrier Detect, not actual Carrier Detect (CD)
this bit in the modem status is used to indicate that the carrier detect
signal has changed state and should be checked.

>   Any help, and references would be greatly appreciated.
> 

	You should get a data sheet on the Intel 8250 or any other 
company that made and 8250 compatible device, i.e., Motorola, Western
Digital, etc. The data sheet, although not the best written or easy to
understand usually has the most complete information.

>   Thanks in advance.

	I have a very old (Vintage 1985) MS-DOS serial device driver 
that supports interrupt driven serial communications that I could e-mail
to you if you are interested in examining an 8250 interrupt routine.

> 
>   viraf bankwalla
>   viraf@seas.gwu.edu
>   ...!uunet!gwusun!viraf


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          - Randy Spurlock -	      |      Compaq Computer Corporation    
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These opinions are mine...all mine... | He fired his hyper-jets and...  
just ask anyone who's heard them!     | blasted into the 5th dimension!
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UUCP: ...!uunet!cpqhou!randys         |                     Space Man Spiff
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