Reverse-engineering RS-485 communication
data bits per transfer (8? 9 bits?)


Looks like 9. Unusual, but this happens sometimes.


  stop bits (1, 1.5, 2?)


This actually looks like 3 or 4 stop bits to me -- but it does not really matter. As long as you only receive data, set your receiver to "1 stop"; any extra bits will be just an idle line.


  and could this be a multi-drop bus communication protocol?


Quite possible -- the last bit does look kinda like address select. But it should not matter that much for packer reception.

It seems that now you have enough information to start collecting the data. You'd want to collect at least 20 data packets, then put it into spreadsheet and stare at them for a while. Here is a good example: Reverse Engineering the Maverick ET-732. It is about RF device, but once you got the bits, it does not matter where they come from.

You'd want to record all 9 bits of data, and depending on your dongle, this can be hard. On Linux, you can set PARMRK,INPCK,PARENB,CMSPAR flags together to get your bit 9 info via special escapes in the input stream (see termios(3) for details). However, this requires support from your usb-serial chipset, and in generally, quite a pain. You may be better off scripting your DSO, making a converter from fast Arduino, or just manually saving lots of traces.