From: Tracy (
Date: Sun Oct 11 1998 - 20:20:03 EDT

Thanks to you who responded to my request for assistance with the EEPOT data
sheets. It seems the data is ONLY available in their PDF files, not on the
HTML part of the site.

For reference, I am using a part numbered X9418WP24-2.7 Dual CMOS EEPOT, 10k
Ohms. It supposedly has 'low noise' characteristics and has 64 wiper
positions. Information on these parts can be found on Xicor's web site at .

These little buggers are just what the Dr. ordered for controlling 'regular'
radios and circuits via computer. They have several packages, and some have
'low noise' characteristics which makes them useful in radio projects. The
part number noted above are in DIP packages which makes working with them
easy ...

I smell variable bandpass audio filters, digital tuning, etc. They (Xicor)
have schematics online that show how to digitally control op-amps, voltage
regulators, etc. Everything you used to use a physical (variable resistor)
control for you can now utilize a serial device to control digitally. I'm

They use an addressing scheme that allows you to chain 16 of these chips on
a single serial line. The same line is used for transmitting and receiving.
(You can READ the current value of the wiper!) There are several 'registers'
that can contain pre-set data that can easily be switched between, and the
data in register 0 is automatically loaded when the unit powers up, so it
can conceivably 'remember' the last setting if you do your coding correctly.

I'm working on a source for small quantities of these chips. Right now the
only source I have (for a reasonable price ...) has a minimum order quantity
of 97 for this part. Go figure. But that brings the price down to just under
$5 each. I'll let you know how that goes.

Now down to the dirt. (I think this is QRP, it is to me because I'm building
a radio with it and ALL the radios that I build are QRP ...) I'm going to
control this with an 8255 PPI (Parallel Peripheral Interface) chip. I'm
planning on using one bit to 'clock' the data, one bit for the actual serial
data, and one bit for the hardware write-enable. This leaves 5 of 8 bits on
one port of the 8255 for other functions.

Does this sound logical/practical to those of you who have used parallel
devices to control serial devices?? Or is there a part that I could put
between the parallel controller and serial device to serialize the data
automatically?? The first method seems like more complicated coding, but
with less parts. (cheaper ...) The second method seems much easier with less
code overhead. Oh, those engineering choices ...

Still trudging toward a totally computer-controlled homebrew radio ...
Tracy N4LGH
QRP-L #1453

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