Computing stuff tied to the physical world


Relaying SMA data as RF12 packets

In Hardware, Software on Nov 7, 2012 at 00:01

Yesterday’s post shows how to read out the SMA solar PV inverter via Bluetooth. The idea was to install this on the Mac Mini JeeLabs server, which happens to be in range of the SMA inverter (one floor below). But that brings up a little dilemma.

Install a potentially kernel-panic-generating utility on the main JeeLabs server? Nah…

I don’t really care whether this issue gets fixed. I don’t want to have the web server go down for something as silly as this, and since it’s a kernel panic, there’s no point trying to move the logic into a Linux VM – the problem is more likely in Apple’s Bluetooth / WiFi stack, which will get used no matter how I access things.

The alternative is to implement a little “SMA Relay” using a JeeNode with a Bluetooth module attached to it, which drives the whole protocol and then broadcasts results periodically over RF12. That way I can figure out and control it.

I tried to use the SoftwareSerial library built into the newer Arduino IDE releases, but ran into problems with lost bytes – even with the software UART speed down to 19200 baud.

So I ended up first debugging the code on an Arduino Mega, which has multiple hardware UARTs and allows good ol’ debugging-with-print-statements, sending out that debug info over USB, while a separate hardware UART deals with all communication to and from the Bluetooth module.

Once that worked, all debugging statements were removed and the serial Bluetooth was switched to the main (and only) UART of the JeeNode. The extra 10 kΩ R’s in the RX and TX lines allow hooking up a USB BUB for re-flashing. The BUB will simply overrule, but needs to be removed to try things out:

DSC 4236

(the Bluetooth module I used in this setup is Sparkfun’s BlueSMiRF Silver)

Next step was to add a little driver to JeeMon again, the aging-but-still-working Tcl-based home monitoring setup at JeeLabs. Fairly straightforward, since it merely needs to extract a couple of 16-bit unsigned ints from incoming packets:

Screen Shot 2012 11 06 at 10 39 10

And sure enough, data is coming in (time in UTC):

Screen Shot 2012 11 06 at 10 32 35

… and properly decoded:

Screen Shot 2012 11 06 at 10 33 08

The ATmega code has been added as example to JeeLib on GitHub, see smaRelay.ino.

I’m still debugging some details, as the Arduino sketch often stops working after a few minutes. I suspect that some sort of timeout and retry is needed, in case Bluetooth comms get lost occasionally. Bluetooth range is only a few meters, especially with the re-inforced concrete floors and walls here at JeeLabs.

Anyhow, it’s a start. Suggestions and improvements are always welcome!

  1. I http://www.on4akh.be/SMA-read.html found this article that explain how to decode the SMA BT recs. http://www.on4akh.be/SMA-read.html Fot me this can be nothing … but for you maybe can help. I also use 2 X SMA 3000TL for my home PV sys. Thanks for this articles. Denis

    Denis 7 November 2012 at 12pm

  2. in this tool ( http://code.google.com/p/sma-bluetooth/ )there is a hard coded 300 second interval maybe for a reason

    Henk 7 November 2012 at 1pm

  3. sma-bluetooth: The 300 second delay is a PVOutput thing, they only allow a few updates per hour. The code unintelligently fires data at the inverter and waits for a reply – it needs to be tweaked for each different inverter. The code posted on my site and on JeeLabs.org won’t need this.

    SMA-read: very old protocol, not used over bluetooth anymore.

    Stuart Pittaway 7 November 2012 at 2pm

  4. sma-bluetooth is based on sma-read. They both use the same “logic” (if you can call it like that ;-) I use Stuart’s work to build something for Windows, Linux and Android You can find the open source projects on Google Code: http://code.google.com/p/sma-spot/ and http://code.google.com/p/sma-rt/

    sbf 8 November 2012 at 9am

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