Pokit Pro's view of 19.2 kb RS232 (remember serial ports?)

I recently upgraded a 20 year old Davis Vantage Pro weather system with a serial port add-on.  While a lot of hardware changed, the protocol hasn't, but I figured it would be interesting to see how the serial data looked, especially since a 50 foot DB-9 cable is involved.

Every 20 minutes my Linux system runs three programs that update different things with recent data.  The command requests are all short, the responses range from a few bytes to over 400.

In the image, the top trace is the Linux -> VP2 (weather station) line, the bottom is the VP2 -> Linux line

The long responses include a dump of that last few 10 minute samples, High and low data that can be displayed in a graph on the VP2's console, and a single pass loop command that returns current conditions.  That second one is redundant.

The RS232 spec has idle lines putting out a negative voltage.  Data transmission involves both positive(0) and negative(1) pulses.  Something like a 3-15 volt range, but don't quote me.  I've long since forgotten what it says about zero volts, but the only sensible thing to do is to treat as the negative idle signal.

To keep battery life good on the VP consoles, Davis is quick to shut down its transmit line when it has nothing to say, I marked that with the arrow and "RS232 off".

That funky pattern in the middle of the HILOW response is for data that hasn't been logged yet, Davis sets it to be all 0xff, hence that block of negative data.  The spikes are the RS232 "start" bits.