Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!sdd.hp.com!ucsd!celit!silogic!markd
From: markd@silogic.UUCP (Mark DiVecchio)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: 8250 and 8259 chips
Keywords: 8250 8259 interrupts serial
Message-ID: <263@silogic.UUCP>
Date: 5 Nov 90 03:49:48 GMT
References: <1210@vaxeline.COM> <751@csource.oz.au>
Reply-To: markd@silogic.UUCP (Mark DiVecchio)
Organization: Silogic Systems, San Diego, CA
Lines: 39

In <1210@vaxeline.COM> gordon@vaxeline.COM (Gordon Lee) writes:

> My Interrupt Service Routine (ISR) is currently defficient in that it 
> can miss a transmit completion, thereby wedging my outgoing data stream.  

I had this problem here is a summary of what I found (take directly from
the source file of my program PC-VT :

;	There is problem with the 8250-A USART chips in the
; PC and PC compatible communications ports. The problem is that if
; there are several stacked interrupts, only the highest priority
; one is ever processed. PC-VT will just hang up waiting for the
; others. The 8250 and 8250-B work fine in the PC.
;
; I have rewritten PC-VT to only use one interrupt, the Data Ready
; interrupt. This should reduce the problem considerably.
;
; The problem is not "in" the 8250 but is in the interface between the
; 8250 and the 8259. The 8259 is operated in edge-trigger mode, but some
; of the versions of the 8250 do not drop their interrupt line if several
; interrupts are stacked up. Thus the 8259 never "sees" them.
; Specifically, the 8250 and 8250-B do "glitch" their interrupt lines.
; The 8250-A does not. Therefore, if you have a PC with an 8250-A, PC-VT
; would regularly hangup.
;
; In the PC AT, the problem compounds. The AT uses the new Serial
; chip from National, the 16450. All I know at this point, is that
; with only one interrupt, PC-VT works ok on the AT
;
; I would like to thank Marty in the applications department at
; National Semi in Santa Clara for his help. Two relevant documents
; from NSC are B01361 and B01511-1
;

-- 
Mark DiVecchio, Silogic Systems, 619-549-9841                       K3FWT
-----   9888 Carroll Center Road, Suite 113, San Diego, CA 92126    -----
markd@silogic.uucp                             BBS 619-549-3927
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