Multiplexing NMEA on a router (NMEA over WiFi redux)

[Mollymawk]

Some time ago - over two years, it seems; time flies! - I asked some questions on the forum about broadcasting NMEA data over WiFi for the use of devices such as Android tablets, as well as computers which needn't be at the chart table.
After a very interesting discussion with lots of helpful ideas from forum members I decided I would build my own NMEA multiplexer with an Ethernet interface to my router, using an Arduino or perhaps a Raspberry Pi - but two years later it still hasn't happened. Too many more important but less interesting jobs competing for my time, plus it's hard to get things like Arduinos and RPis in South America.

Anyway, some time back I came across RobbieW's account of getting kplex working on a router, and I realised that of course this is a simpler way to achieve my goal - or at least, it's a way that only uses equipment I already have, since I have a router with a USB port.

I've installed OpenWRT on my router (TP-Link TL-MR3240), but not got as far as cross-compiling kplex yet.
I know RobbieW was planning to switch to OpenWRT from Tomato, so I'm hoping he might have some tips for me - but I thought I'd post here rather than emailing him, so that others can benefit from the discussion.
And maybe others will have useful input, too!

[RobbieW]

I havent (yet) migrated my router to OpenWRT. Mostly because the imperative to do so is no longer so urgent, the conflicting serial device problem I've worked around simply by not plugging the mobile dongle direct into the hub router. There is also the concern that devices with the Broadcom wireless chipset that mine has are not really on the main development branch of OpenWRT and are not supported by older, stable versions.

I used 'tomatoware', a compile environment for Tomato, to build kPlex direcitly on the router having failed to create a cross compile environment that would work. 'tomatoware' was installed on a mountable USB stick on the router and utilised from there.

I found, with the help of tomatoware's creator, that I needed a couple of compile options to get a clean build of kPlex that would run when the compile environment was unmounted. Those options are -zmuldefs and -static; the first seems to deal with multiple definitions of the same variable in the source, the second builds all externally called functions into the kPlex module making it independent. A side effect of using -static was to create a much larger module, c. 600k against about 70 iirc. I used another utility 'upx' to compress the compiled module, which got it down to a more reasonable 200k.

One of the things that made this route feasible is that Tomato has drivers for the FTDI Serial-USB converter built in, you do have to load them with each reboot however as they are not part of the core function. I havent looked at OpenWRT for a while now so cant remember if the same drivers are present. I also found it was possible to use a USB hub to enable multiple USB inputs to a router with limited USB ports, kPlex didnt seem to mind so long as you address each one correctly.

[smac999]

look at moxa stuff. they probably make this stuff already. I have defiantly seen nmea put into Ethernet before.

maybe like this

Moxa Wireless Device Servers NPort W2150A/2250A - 1 and 2-port RS-232/422/485 Wireless Device Servers


once it's in the network you'd need some way to decode it back down though?...

[muttnik]

There's a recently started google group for kplex which might be worth posting to. There's definitely people who've got it running on OpenWRT (on mips) who both Robbie and I have been in contact with. I'll try and dig out some mails and ping them.

[Mollymawk]

Robbie, Keith, thanks for the replies.

I would be inclined to go for the 'easy' route of installing Tomato and following where you have gone before, but I have the opposite problem you have: Tomato is not available for my Atheros-based router. Anyway, I enjoy the challenge and hopefully if I get it going and write about it here it will help others in the future.

Compiling directly on the router certainly seems logical. I haven't found anything useful about compiling on an OpenWrt router, but perhaps I should ask on the OpenWrt forum. What are the build requirements? If I could get GCC installed on the router would that be enough?
On the other hand, I suppose I should at least try cross-compiling first, since that's the standard way to port software to OpenWrt.

Of course, I could use DebWrt, then I wouldn't have to do any of this, but I'd have to build it myself for my router and I'd have to set up the routing aspect of it myself too.

Keith, thanks for pointing me to the kplex Google Group, I will post there later. If you were able to point people who have got this going in the direction of this thread that would be great.

I thought of another little question last night. My Garmin GPS has a USB port as well as an NMEA connection. But on the USB port it will only talk Garmin's proprietary language. Is there a way to translate that data into NMEA format and get it into kplex? Maybe gpsd can do it?

Cheers, Caesar

[Mollymawk]

smac999, I know about those Moxa devices, but can't afford them! I was planning to build one myself - one day - but here I'm talking about actually installing software directly in the router to handle the NMEA broadcasting over ethernet.

[Mollymawk]

Well, I've got kplex running on OpenWrt, no problem. It took a bit of research, since nothing is documented properly, but was really quite trivial. It installs and runs fine (as a service or normally).
I haven't been able to actually test it yet as I don't have a source of NMEA data handy...

I'm going to be offline for the next few days (or more), but when I get back online I'll document this and maybe upload my code to github.

Keith, have you thought of putting your code online under version control, eg on github?
Then I could just create a repo for the openwrt package and include kplex master as an external...

[Mollymawk]

Oh, and it's only 21kB... :-)

It does depend on an external module; libpthreads. I thought I ought to be able to roll that into the same package but didn't have any luck. But it's available from the standard OpenWrt sources so not a big problem I guess.

[RobbieW]

Excellent, well done. I might tap into your experience sometime in the future!

[muttnik]

Originally Posted by Mollymawk

Keith, have you thought of putting your code online under version control, eg on github?

It is under git version control (I resisted the temptation to use sccs from the outset :-) but not on github. There's a very old version that someone else posted there but last I looked it wasn't maintained. The source is obviously always downloadable but this week when I've got a bunch of fixes but no time to make and test a release has convinced me that I should open up the repository

github doesn't appear to be ipv6 accessible: what's the point of that? :-)

[Mollymawk]

Originally Posted by muttnik

It is under git version control (I resisted the temptation to use sccs from the outset :-) but not on github. There's a very old version that someone else posted there but last I looked it wasn't maintained. The source is obviously always downloadable but this week when I've got a bunch of fixes but no time to make and test a release has convinced me that I should open up the repository

That would be great – the bazaar is better than the cathedral!
Gives everyone the chance to test fixes in trunk before you make a stable release, and everyone can contribute fixes too.

Originally Posted by muttnik

github doesn't appear to be ipv6 accessible: what's the point of that? :-)

Seems like a major oversight on their part, but I don't know much about it as I never have an IPv6-enabled connection. I feel it's probably not a sufficient reason to avoid them... :-)

[RobbieW]

This discussion prompted me to go and look at the status of the currrent, Barrier Breaker, releases of OpenWRT. Seems its now released with formal support for the RT-N16, so I've downloaded 14.07. Whether I install that before leaving the boat or next spring depends on how I get on with putting the boat to bed

[Mollymawk]

Ok, while I've been offline I put together a quick howto and I've now uploaded the OpenWrt Makefile, the OpenWrt-specific init file, and instructions on how to use them to https://github.com/CaesarSchinas/kplex-openwrt.

If anyone has any questions or suggestions let me know.

[RobbieW]

Have you found that you need to run 'modprobe' or something similar to load drivers for whatever Serial/USB device you have ?

For example, I have to include in the startup scripts for Tomato something like 'modprobe -a ftdi_sio pl2303' to load drivers for FTDI & Prolific devices.

[Mollymawk]

No I don't have to do anything special, just have the drivers installed. You'll need the packages kmod-usb-core, kmod-usb-serial, and kmod-usb-serial-pl2303 (for Prolific devices - I don't recall the name for FTDI but it is the same story).

With those installed my USB serial adaptors just automatically show up at /dev/ttyUSB0 etc when I plug them in.

One scenario I haven't tackled yet is connecting the USB serial adaptors after startup. By declaring the serial interfaces as optional in the kplex config file, kplex will load even if the adaptors are not plugged in, but I don't think it will start reading data from them when they are connected. The simple workaround is to reboot the router after connecting the adaptors, if they weren't connected at startup, but ideally I'd like to be able to (re)start kplex when a USB serial adaptor is plugged in.