Re: Best way to set USRP Sink value from separate thread

[Marcus Müller]

Hi Guy,

you don#t have to stop a flow graph to set the gain value. In fact, you
shouldn't.

Yes, you'll still have signal passing through your USRP while you're
adjusting the gain, but that's exactly what timed commands are for.

> I've heard about sending commands to the
> USRP Sink, though I couldn't find any documentation on the subject and the
> one page I found has been removed (and was never archived). 

Maybe you mean
https://www.gnuradio.org/doc/doxygen/page_uhd.html
?

That describes the commands you can send in both via asynchronous
message passing, and via tags.

> message I keep receiving from the flowgraph:
> 
> gr::log :FATAL: tagged_stream_mux0 - Missing a required length tag on port
> 1 at item #100
> thread[thread-per-block[10]: <block tagged_stream_mux(14)>]: Missing length
> tag.
>

that means you've set your USRP sink to tagged stream block mode, but
then you sent in data that wasn't part of any tagged stream "blob".

> It seems to happen at random with any tagged block in the flowgraph (the
> point of failure typically changes every time we run it). While we could
> theoretically just catch the error and restart the flowgraph, 

Uuuuh, that sounds like it won't *actually* work.
> I would
> rather find a more permanent solution. We're using two Ettus B200 SDRs for
> our current tests.

Yeah, use timed commands to set the gain. Use tags to add them to your
sample stream. Don't use tagged stream blocks unless you actually want to.

> 
> If anyone has any information on proper dataflow control for blocks or the
> USRP sink message passing API, I'd really appreciate it.

Best regards,
Marcus