Address Decoding for Dummies! (was: SPDT Dip Switches...

From: Roger Merchberger <>
Date: Sat Sep 22 12:11:07 2001

Rumor has it that Pete Turnbull may have mentioned these words:
>On Sep 15, 14:42, Roger Merchberger wrote:


>(It can be done
> > easily without the switches -- I could use a jumper block without
> > heartburn, but the switches would look neater, and wouldn't have the
> > for floating inputs/outputs...)
>In other words, this:
[snip gfx of my idear...]


>Wouldn't this be easier and do the same thing?

[snip better idear...]

Probably. I did mention that I was primarily a softee-ware guy -- and when
dealing with chips I have a tendency to think of them *as software* --
like: if you try to write to a ROM, what happens? Nothing. (duh...) I
looked at the chip specs for the demux, and you have inputs & outputs... In
my mind (at first), your suggestion was "ramming 5V down an output port"
and sometimes I have to stop & think that "those ports are just a bunch of
gates jammed together on a chip to serve one purpose" and doing stuff like
that is not a "Bad Thing (TM)." :-)

The design continues... :-)

>I've just seen Tony's reply, and his suggestion using a MUX would have been
>my next suggestion too. It's also easier to extend to more address bits
>(fewer switches required).

My board isn't a "complete" board -- it's to be designed to make
experimenting much easier by having flexible on-board address decoding, and
(at least for my purposes) I want "one address per switch", so I'm not sure
if the MUX suggestion is what I want. As of right now (but I'm still
researching CoCo memory maps) I'm looking to have an 8K ROM socket at the
"normal" location ($C000-DFFF) w/jumper as to whether or not it will
auto-start (jumpering the Cart signal to the Q line) and I'm looking at
having the address decoding for my I/O ports at $FF50-3 & $FF60-3, but if I
want *just* $FF51 & $FF53 to be decoded & $FF50 & $FF52 to be ignored, I
want to be able to switch them individually.

(Originally, my design was just going to have 1 address decoded from each
"bank", but then I realized that if I wanted to interface "most anything"
like a PIA, UART or a FDC that it wouldn't work.)

For anyone here who does their own hardware designing, what software do you
use? I've tried "Protel 99 SE" and CircuitMaker 6 (student version) -- and
a few others, and I must say that I'm not keen on most anything I've tried
compared to "good ol' AutoCad".

Anyway, an updated (and large) picture (of both sides of traces - red is
solder-side, lt. blue is component side) of the board is here: (118k, 16color, looks
like crap, but is smaller than the jpg) (180k, 24bit color,
looks decent but is a slow download for modem users) (85k, but only viewable
if you have AutoCad .dwg compatible software...)

As the board I started out with was just a bare board, I seriously doubt
that I'd get into any type of copyright trouble by the time this thing gets
to press; everything you see in the pix stemmed from my own brain...

Oh, I've found a really good site to purchase all manners of circuit board
"stuff" w/really good prices, too:

As always, Thanks everybody for all the help I've received so far...

Roger "Merch" Merchberger
Received on Sat Sep 22 2001 - 12:11:07 BST

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