[arm-allstar] GPIO vs Nano

[Barry Buelow]

All,

I originally built a pcb that plugs into the GPIO of the RPi. It had 
room for 2x FOBs (minus plastic cases) and buffers for PTT-COS. I have 
experience with complex board design (avionics) and expected some 
coupling from the RPi to the board, but it was significantly higher than 
I expected. The audio always had "worms" and it was quite difficult to 
knock down the spikes on the +5 and +3.3. The short answer is that the 
IO board was too close and there was no simple way to get more 
isolation. It was also clear that if there were some undesirable event 
on the radio, the path to Gnd was via the RPi. An aside comment for now: 
building a system with several floating ps is a really bad idea.

Over the past few months, I've shown Dave and Doug my work and received 
comments. The NanoDB is not a bullet proof interface. Doug always argues 
for cheap :-), but he wants the features of a more expensive unit.  The 
NanoDB is rather minimalist in design. I had a 74HC14 buffer between the 
inputs (COS, CTC) and the Nano, but took it out. The amount of 
protection it offered was inadequate if there were a big event (as was 
re-enforced at an IEEE EMI class a few months ago). The 74HC14, with 
jumpers, also allowed the IO to invert signals to have LEDs indicate the 
correct state regardless of COS=1 or COS=0 from the radio. I still like 
that idea, but it can be accomplished in the Nano Sw.

What drives my designs is the desire to have no external ugly blight of 
wires and components necessary in normal installs.  Why would anyone 
want a design that requires external 2N2222 + bias resistors hanging in 
midair off of a DB25?  My goal was to have the NanoDB work for 90% of 
the simple nodes. It allows use of the potted FOB to build a cheap 
system.  I'd be very interested to hear about cases where it did NOT 
work as a future rev might address some issues.

I have also contemplated a more elaborate mechanical design which 
accommodates IO for 2 radios, 2 FOBs, and integrates into a nice 
extruded metal case with room for a pair of DB-9s on the rear. Think 
bigger $.  Dave suggested an integrated USB HUB. Those are fine pitch 
parts which exclude home bench soldering and small quantity builds are 
more expensive.  Maybe someday.

I think the NanoDB will be a contribution to Allstar, which was my 
original intent.

Barry w0iy
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