[aprssig] Quick APRS hardware survey (APRS Hotspot)
scott at opentrac.org
Thu Jun 22 17:47:09 CDT 2017
> Could you expand on this? I did the calculations, and a reasonable SD card should have enough write cycles for several years. Why would the system need to be read-only?
It's not really about the write endurance. With a writeable file
system, you always run the risk of corruption due to software problems,
power failures, and so on.
All of the things I'm trying to do, you could do with a Raspberry Pi and
a suitable add-on board to handle the radio interfaces and such. It'd
mean a lot less development effort, but also vastly greater system
The SR2's power consumption is a lot lower because it doesn't have all
of the overhead of a bunch of dynamic RAM, a GPU, and a full-fledged
operating system. It boots in a fraction of a second, and can be on the
network inside of 10 seconds. The file system can be corrupted or
totally wiped and it'll still boot and retain your settings. Software
updates are monolithic - when it's time to update it needs one file,
about 2 MB max.
It's not prone to memory leaks because there's almost no dynamic memory
allocation. It won't ever get infected with a worm. You don't have to
worry about the processor load ever getting so high that it can't keep
up with the radio channels because the resources for everything are
basically reserved in advance. There's also a hardware watchdog monitor
that makes sure that if the software safety checks on things like
transmitter timeouts fail to run for more than a quarter of a second,
the PTTs are all disabled by an external circuit.
The down side is that it's a lot of work for *me*. I've had to do stuff
like write a super-lightweight telnet client and server from scratch.
It has a screen-oriented text editor with syntax highlighting that
occupies less than 5k of code space. This is stuff you'd never have to
worry about on a *nix box, because all of that code was written 30 or 40
years ago. I kind of enjoy that sort of programming, though. Whether
it'll turn out to be worth it commercially remains to be seen.
> FWIW, at Burlington Amateur Radio Club, we’ve had a couple of Pis at our remote location (one for APRS, one for D-Star) for about a year now, with few actual Pi-related problems. We did have to replace a cheap SD card with a name-brand card, and we’re currently having trouble with a cheap Chinese USB-Serial interface. In general, though, the Pi’s have been pretty solid.
Something like this is never going to replace single-board computers for
everyone. It's intended for places you don't want or need to put in
that kind of effort. It'll work as an appliance for common use cases,
and I'm trying to make the BASIC interpreter powerful enough to easily
handle most custom application requirements with small amounts of easy
to learn code. If for some reason you want to wait for a particular
APRS station to come within a pre-defined area and then play a voice
message on one radio, send an APRS message on another with some sensor
readings, turn on a relay, send a text message via Twilio using their
REST API, and establish an Echolink connection with a particular node,
it'll do that.
You could do all of that on a Pi, of course. But unless someone wants
to develop and support a software distribution to do all of that, and
the interface hardware to go with it, it's going to be a lot of effort
to put it together.
Ideally you'd all buy a bazillion of these things so we could get the
cost down to the Raspberry Pi range, but even if that doesn't happen I
can still beat the price of a Pi and Rigblaster.
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