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From: wdp@dukee.egr.duke.edu (William D. Palmer)
Newsgroups: sci.electronics
Subject: Address decoding circuitry
Message-ID: <1115@cameron.egr.duke.edu>
Date: 9 Oct 90 18:39:25 GMT
Sender: news@egr.duke.edu
Distribution: usa
Lines: 35

Tuesday, 09 OCT 90

Fellow Net Ones:

I mailed a reply to the post below.  Judging from the followups in 
sci.electronics, I thought there might be enough general interest for
me to post my reply.  Here it is:

> From: steve@nuchat.UUCP (Steve Nuchia)
> Subject: cheap address decode ideas?
> Date: 29 Sep 90 06:46:04 GMT

> Given a desire to have N (=5 in this case) I/O port groups
> decoded at arbitrary addresses without using a whole boatload
> of parts?

> My current best, decoding 6 bits (sufficient) is a pair of 74ls138 ...

My advice would be to use the 74LS688, which compares two eight-bit words
and produces an active low output when the words are equal.  One word is 
the appropriate eight lines from your address bus, the other is set by an
eight position dipswitch.  Then you need only one dipswitch and one '688 
per port group.  I have used this on several cards which work in 
IBM PC/XT/AT computers.

> Thanks.

No problem.

Dev Palmer
wdp@dukee.egr.duke.edu
Duke Electrical Engineering
Room 309 Old Engineering Bldg.
Durham, NC  27706
(919) 660-5282