LZ: Project Status Update: THE RAM EXPANDER WORKS!

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LZ: Project Status Update: THE RAM EXPANDER WORKS!

After about 10 hours of total anguish, sweating and swearing, I
finally figured out why my design didn't work, and more importantly, I
fixed the problem!!!  I fixed the design by adding a single capacitor!
I asked around on sci.electronics on how to use a resistor-capacitor
time delay circuit (which I wasn't sure how to use), and someone
responded.  The capacitor sets up an RC time delay, which delays the
output of the buffer by a few microseconds.  That way, the "race"
problem is corrected, allowing the latch to properly output the data.


I was able to manually send and receive commands to the chip for the
first time today.  I haven't tested all the commands, but there's no
reason why they shouldn't work.  It's weird knowing that I have the
worlds first TI-85 with a whole megabyte of storage on it! =20


The value of the timing capacitor will have to be tweaked, because if
the delay is too long, this will limit the maximum transferr speed.
However, I don't think that the speed will be noticeably diminished.
>From here on out, I'm going to be glued to Dos Edit, writing the
(currently unfinished) send/receive software.  I've got to also
rewrite the chip command send/receive routines, too, but that won't
take very long.


I'll release the schematic within a day or two, but I'm really busy
with some Java programs and a report that have to get done, so be
patient.  Now that I know the reason why my design didn't work, I
think it can be simplified even more.  A suggestion from Ed Plese Jr.
got me thinking, and I now believe that the whole thing can be built
with just a single 74HC126, the memory chip, a voltage regulator and a
few capacitors.  This method is a bit slower, but then whole thing
could then be built on a 1" by 2" pc board (my current design fits on
a 1" by 3" pc board)!


I also got an idea for another (totally different) way to interface to
the chip using an XOR gate and a clock divider, but since my design
now works, I'm not going to investigate that possibility.


I've probably forgotten to mention something important...  oh well...


-Mel