Analysing pv’s attempt to work with CMS-5000

There’s a library a wrote a little while ago that was designed to interact with some photovoltaic inverters via the serial port. It was written based on captured communication between the official ProControl software

Reader contribution

Simon’s tried out my code to see if it’ll work on a newer inverter, the CMS-5000. Unfortunately the library as it stands doesn’t quite work. So let’s take a closer look at the packets:

First we reset the inverter:

SEND -> aaaa 0100 0000 0004 00 0159

Discover inverters:

SEND -> aaaa 0100 0000 0000 00 0155

RECV <- aaaa fe00 0000 0080 0a 52353634363033303631 04fd OK

Mostly OK so far, but something that strikes me is that the first byte (0xFE) of what I assumed to be the source address seems to have some other significance (it’s not 0×0000). Moving on…

Register the inverter with address “0001”:

SEND -> aaaa 0100 0000 0001 0c 523536343630333036310001 0384

The inverter acknowledges

RECV <- aaaa fe01 0000 0081 01 06 02db OK

So even though the first byte of the address 0xFE is incorrect, things still seem to be working.

Now we try to find what status fields the inverter gives:

SEND -> aaaa 0100 0001 0101 00 0158

RECV <- aaaa fe01 0000 0181 01 15 02eb OK

This bit here is the interesting bit. First, (if my assumptions about the comms protocol were correct), the inverter only gives 2 fields: 0×01 and 0×15. That’s really few compared with the ones I’ve seen before. Not only that, but we don’t yet know what the field 0×15 is for. Basically I assumed that the reply I get is a listing of the status fields, but instead I get something else.

So let’s query the inverter for its status:

SEND -> aaaa 0100 0001 0104 00 015b

RECV <- aaaa fe01 0000 0184 12 05dc003c071c0abe1290139a009900e8034a 080f OK

So here we have the full status update from the inverter. We can’t decode what it’s saying, because it doesn’t quite match the status field query. This is why the pv library is failing to interpret this packet based on the received structure that was supposedly just 2 fields long.

What does this mean?

In the most basic level, there are a few assumptions + generalisations that I’ve made which turned out to be inaccurate, hence it’s not working for a different model inverter.

What we’d need now is a capture of the raw communication between the official monitoring software and the inverter to get a better understanding of what’s going on.

pv – a Python library to monitor/control PV Inverters

After having the solar electricity system installed, I can monitor the energy output of the inverter using Pro Control. However, because the software runs on Windows, I’d have to have a PC turned on just for that purpose wasting quite a bit of energy. So, my goal is to use a low-power embedded Debian box (which is usually on) to do the monitoring. Instead of just writing an application to do the job, I’ve decided to write a library others can use to write their own custom logging software.

pv can be found at http://pv.codeplex.com/, and should work for you if you have a Carbon Management Solutions CMS-2000 or Schneider Electric SunEzy 600E (judging from a sample protocol exchange).

I’ll have to give full credit to the folks at http://www.solarfreaks.com/cms2000-inverter-rs232-serial-port-hack-cms-2000-rs232-t271.html who did the initial work on reverse engineering the communication protocols. Building on what has already been done on working out the protocol, and doing a bit of reverse engineering myself, the library supports sending arbitrary data frames for further exploring, as well as the known protocols.

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Mini-Project: Command line speech synth tool for Windows

The story behind this mini-project is, a friend of mine mentioned how he would SSH into his Mac at home and use “say” to send a short message to his parents, just like in a past XKCD webcomic:

Mac users, lucky you. You get a Terminal utility called “say” that converts text to speech. Windows doesn’t come with this tool, so I thought, it couldn’t be that difficult to make my own command line speech synth tool for Windows.

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DreamSpark for High School Students Too!

Finally! More students can get the share of the good stuff offered by Microsoft. I’m writing an email to my high school computing teach as I write this post. More free software!

Source: Bill  Gate’s announces DreamSpark availability for High Schools

Different computing degrees are similar yet distinct

I have been thinking about the different computing undergraduate degrees offered at Sydney University: software engineering (SE), computer science (CS) and information technology (IT). Each of these are offered by a different department: School of Electrical and Information Engineering, Faculty of Science, and School of IT respectively.

So here’s what I think are the difference between them:

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