9-bit multidrop UART
RS-485 multidrop buses decoded: addresses told from data, and a trigger on one slave's address only.
Add-on video
What it does
A character is a start bit, 5 to 8 data bits, the address bit and 1 or 2 stop bits. You set whether a 1 or a 0 in the ninth bit marks an address. Addresses show in blue in the decode lane, and the event table has one row per message: the slave's address, how many bytes followed it, and the bytes.
The scope's trigger hardware compares eight data bits, so on its own it also fires on a data byte of the same value. NovaOS reads the ninth bit of the character it triggered on and re-arms when it marks data, so a capture starts only on that slave's address. Probe both wires of the pair and it decodes A minus B, which keeps decoding through common-mode noise.
How to use it
- Decode > NovaOS > UART9; set the address bit, data bits, stop bits and rate.
- Trigger on B1 with the slave's address in Trig. addr; Addr. only is on by default.
- For an RS-485 pair, set Minus (A-B) to the channel on the second wire.
- Over SCPI: :NOVAos:BUS1:MODE UART9, and :NOVAos:BUS1:DATA? for the table.
On Rigol's firmware
DHO800: not offered · DHO900: not offered · MHO900: not offered.
Where it stands
Built and checked on test signals on both families, and filmed on our MHO934 with its own generator playing a 9-bit exchange. A live RS-485 bus on our bench is next. The request it came from.
A paid add-on; none is for sale yet. Everyone named on its request page keeps it free. Core and add-ons.