Re: [Discuss-gnuradio] Tx & Rx in one USRP

[David Tisza]

Hi William,

William Sherman wrote:
> I want to use one USRP to transmit and receive. I tried setting up two
> flowgraphs (top_blocks), one that will be a receiver "thread" and
> another a transmitter "thread". However the program complains:
> RuntimeError: gr_top_block_impl: multiple simultaneous gr_top_blocks not
> allowed
>   
Yes, thats correct you should only have one top block, that's why it's 
called a top block, because it supposed to be on top of a hierarchy 
tree, and as everyone knows "There can be /only one/! - Highlander" :)
But seriously, the trick is that you can have separete distinct chains 
in your flowgraph, eg imagine two simple chain
Think of it as you have a big graph which has multiple unconnected parts.

chain1: usrp->some_processing->packetizer sink
chain2: 
packet_to_stream_source->some_different_processing_than_before->usrp_sink

And you create these blocks in one top block, connect them. Clearly the 
two chains do not have a common point, but the scheduler will take care 
about them, so they will opeate paralelly.
BTW the default scheduler is a thread per block scheduler, so every 
signal processing block has it's own exacutor thread, but you don't need 
to deal with this.
> Is there no way to have a receiver flowgraph in operation while
> transmitting on a different daughterboard?
>   
There is as I mentioned above, even you can have a flowgraph using the 
same daughterboard for receiving and transmitting (not at the same time 
of course)
> In order to implement a TX+RX in one USRP do I have to shut down the
> receiver flowgraph every time I want to start up a transmitter
> flowgraph?
>   
No.
Consider viewing the tunnel example: 
(/gnuradio-examples/python/digital/tunnel.py)
where there exist simultaneously a sending chain towards the usrp which 
gets data from a virtual ethernet device and a receiving chain from the 
usrp which is fed to a virtual ethernet device.
But there is only one top block.

understanding this example can be a little tricky if you just started, 
but it worth it.

Hope it helped

--
David Tisza

University of Notre Dame
Department of Electrical Engineering