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From: dennisg@felix.UUCP (Dennis Griesser)
Newsgroups: sci.electronics
Subject: Re: cheap address decode ideas?
Message-ID: <152086@felix.UUCP>
Date: 6 Oct 90 01:17:45 GMT
References: <28711@nuchat.UUCP>
Sender: daemon@felix.UUCP
Reply-To: dennisg@felix.UUCP (Dennis Griesser)
Organization: FileNet Corp., Costa Mesa, CA
Lines: 27

In article <28711@nuchat.UUCP> steve@nuchat.UUCP (Steve Nuchia) writes:
>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
>3->8 demux chips and a jumper field.  This would be ok, except that
>the jumper field would take 160 pins to permit truly arbitrary
>addressing using regular shorting blocks.

How about an exclusive-OR gave for every address bit to be compared.  Feed
one input from the address and the other from a DIP-switch.  The XOR output
goes low when the address matches the DIP-switch.  OR together the XOR
outputs.  If it's high, one of the address bits didn't match.  Depending on
whether the rest of the circuit needs active-high or active-low enable, you
can invert the output (or use a NOR).

This might use more chips than you want...

The address-recognition RAM sounds like more trouble than it's worth.  You
will need some way to initialize it, and that probably means some form of
dual-porting, with some well-known location (so you can find the address to
tell the port its address...).

If you really want the demux/field/select like you mentioned, it might be a
better idea to replace the jumper field with a matrix switch - prettier and
uses less space.