Path: utzoo!mnetor!uunet!husc6!cmcl2!nrl-cmf!ames!amdahl!nsc!voder!wlbr!pete
From: pete@wlbr.EATON.COM (Pete Lyall)
Newsgroups: comp.sys.m6809
Subject: Re: Problems with /t2 & /t3, hardware not software
Message-ID: <9714@wlbr.EATON.COM>
Date: 8 Apr 88 23:36:59 GMT
References: <4757@pucc.Princeton.EDU> <2617@tekgen.TEK.COM> <494@flatline.UUCP> <564@adds.newyork.NCR.COM>
Reply-To: pete@wlbr.UUCP (0000-Pete Lyall)
Organization: Eaton IMS, Westlake Village, CA
Lines: 37

In article <564@adds.newyork.NCR.COM> kieran@adds.newyork.NCR.COM (Joe Cabana) writes:
>In article <494@flatline.UUCP> phaedrus@flatline.UUCP (james hartman) writes:
>>In article <2617@tekgen.TEK.COM>, jonh@tekgen.TEK.COM (Jon Howell) writes:
>>
>>[Description of data going out to one pak going to both on his L2 system]
>>
>
>Actually the reason characters appear on both ports is because of a hardware
>limitation in the RS-232 packs themselves, not a software problem.
>If you look at the schematics in the RS-232 pack tech. ref. manual you'll find
>the RS-232 packs are setup to use the slot select lines to enable thier ROMS
>but NOT the UARTS. 
>[text deleted]..................................The reason for two
>drivers (as far as I can tell) is to allow you a choice of slots in
>case of conflict. (OS-9 still needs to know which slot has the ROM).
>
Actually, the ROM is never mapped in at all. It contains an
exceptionally primitive terminal program that was in the same address
space ($C000-$DFFF) as the Coco's disk controller rom. The roms are
mapped out under any 64k RAM scenario anyway, an are consequentially
invisible to OS9 as well. The only purpose for juggling slot select
was to be able to allow passage of the 6551's interrupt - in this case
passed as a CART interrupt (FIRQ) - through the multipak multiplexor
and into the coco where it could be software mangled (stack frame
diddling) into looking like an IRQ. On the coco3 they simply program
the GIME to make the arriving CART post an IRQ. In the latter case
though, the method of IRQ handling is improper and characters are
frequently dropped (even at lower baud rates). This has led to several
folks physically connecting the 6551's IRQ line to the 6809's IRQ
line. I can provide installation details if interest merits.



-- 
Pete Lyall (OS9 Users Group VP)|  DELPHI: OS9UGVP  |  Eaton Corp.(818)-706-5693
Compuserve: 76703,4230 (OS9 Sysop) OS9 (home): (805)-985-0632 (24hr./1200 baud)
Internet: pete@wlbr.eaton.com      UUCP: {ihnp4,scgvax,jplgodo,voder}!wlbr!pete