Make Your DIY Davis Datalogger Work With Weatherlink

Progress comes in fits and starts.  That is why the iPad preceded the iPad 2,  why there had to be light before there was a light bulb, and why there had to be a Martin Sheen before we were fortunate enough to get a Charlie Sheen.

I Need To Come Up With Better Examples

So it is with reverse engineering.  First I figured out how to get a computer connection to a Davis weather station console without buying their overpriced Weatherlink software / hardware combo.  That was all well and good, but my DIY interface didn't have datalogging capability until a tip on a wxforum post gave me the clue I needed to implement that too.  This was my iPad 2.

What's that you say?  The release of the iPad 3 is imminent?  Drat.  Indeed it is.  I need a problem to stretch this dubious metaphor even further.  And that problem is the fact that Davis software products like Weatherlink and the firmware updater don't work with the DIY interface and logger.  The software would scan through all the ports on my laptop and pause at the COM port assigned to the USB to Serial Adapter, but wouldn't initiate communications to it.

I thought that Davis was likely doing some kind of factory initialization on the flash chip that I was not privy to, and I wasn't going to bother disassembling the firmware image to figure out what I needed to do.  Cumulus works great and is free for personal use, and the Weatherlink software has been beaten to death with the ugly stick.  It burns my eyes.

Windows 95 Called - They Want Their GUI Back

Still, this problem bugged me.  Until now.  Enter belfryboy.  He is the Mr. Watson to my Alexander Graham Bell, the Dr. Watson to my Sherlock Holmes, and the supercomputer Watson to my Ken Jennings.  belfryboy is the guy who designed the schematic and PCB layout for a DIY logger.  He's made the Eagle design files publicly available and he'll even build one for you if you want.  He also lives in the UK, where they add a superfluous "u" to many words for no particular reason (the phrase "for no particular reason" is also superfluous in that sentence when you think about it, but I'm not about to start letting "thinking" get in the way).

It turns out this problem was bugging him too.  He took a different approach.  What if it had nothing to do with some factory initialization?  What if it was a handshaking issue?  So he started playing around and actually got it to work!!!  Here is what you need to do, and this applies to both a straight serial or a USB to Serial interface to the console.

• Connect CTS to RTS
• Connect DSR to DTR to DCD
• Make sure to use the 1Mb dataflash chip.  The console will not recognise the datalogger with larger capacity flash chips, even though it will log to it

To illustrate this, I opened up the Sparkfun USB to Serial converter schematic in Eagle.  The dashed lines show the changes that need to be made.

A Pretty Standard RS-232 Hardware Handshaking Dance

This is a pretty easy job to do on the back of an RS-232 DB-9 or DB-25, but you'll need a steady hand and some good soldering skills to do it on the FTDI chip on these USB to Serial converters.  The connections are mostly not adjacent to each other, so it will take more than just a simple solder bridge between pins.  Time to break out the 30 AWG wire.

But maybe you aren't up for this?  belfryboy has got you covered once again, as he has a new version of the DIY datalogger in the works and will build you one if you'd like.  He'll also be making the design files available once again for anybody to build their own.  I'll put up a link to them on this blog once they are available.

I have to take my hat off to belfryboy because this solution wouldn't have occurred to me.  Ever.  The console does not have any lines for hardware handshaking, just TxData and RxData.  Why didn't Davis just ignore hardware handshaking in their software instead of hardwiring it on to the logger board?  It makes no sense, but that is the way it is.

By the way, please spare me any wailing about how this encourages piracy of the Davis software.  There are many legitimate reasons for enabling this functionality in a DIY interface.  Here are a few.  There are others.

• Your Davis branded logger got zapped and no longer works (happened to belfryboy himself)
• You've got a Davis branded USB logger that is dropping out all of the time
• You've got multiple consoles and want a DIY version for the second one
• You've got the Davis branded logger but want to build a DIY wireless version of the interface
• You want to use the Davis updater software to update the firmware in your console

I haven't tried implementing this hardware handshaking fix myself yet.  Unfortunately, it seems that I have somehow fried my console after digging in to its innards one too many times.  I had thought it was just the LCD that was blown, but I got a replacement and that isn't working either.  These are the hazards when you mess around like I have been.  C'est la vie. This is going to put a temporary hold on any progress on my alternative wireless console, but rest assured that I will pick it up again once the new console arrives.

So there you have it.  A DIY datalogger that has 100% of the capabilities of the Davis version, and more than two weeks before the antipated announcement of the iPad 3.  That's how we roll here at Mad Scientist Labs.