Re: Confusion about 'ports'

[Marcus D. Leech]

On 2022-03-14 19:02, Chad Spooner wrote:

All:

I've got a simple flowgraph with two USRP Source blocks and a single USRP Sink block:

This flowgraph is running on an x310. The upper Source block is supposed to stream data coming from another SDR, and the lower Source block is supposed to stream data from an antenna. Then I add these together and output the sum with the Sink block.

When I run it I get this error:

gr::log :ERROR: thread_body_wrapper - ERROR thread[thread-per-block[2]: <block usrp_source
(1)>]: RuntimeError: On node 0/DDC_0, output port 0 is already connected.

If I disable one of the Source blocks, things work as expected, so it is the presence of both that is the problem.

I can't tell from the documentation what a 'port' is, exactly. And there isn't any error message in the companion itself, so I guess it thinks this is OK.

Can somebody please point me to a clear discussion of how 'ports' enter here or please suggest a way to modify the flowgraph?

Thanks,

C

    In this case, it's complaining that the output port on the DDC 0 in
    the FPGA is already connected to some other stream.
    
    
    You appear to have two source blocks talking to the same piece of
    USRP hardware, which isn't allowed--if you need multiple channels,
    you use a *SINGLE* USRP source
      block configured for multiple channels coming from your single
    X310 device. The reason for this has to do with the considerable
    extra computational and housekeeping
      expense that would be required for multiple source blocks (or sink
    blocks) "share" a single USRP hardware device.  It would add another
    layer on the host side,
      and add considerable extra FPGA real-estate on the hardware side,
    which is better used for other things, like user FPGA functions, or
    just the usual DSP
      bits and pieces that are in the hardware.
    
    You haven't said what daughtercard(s) you have on your X310, and the
    phrase "coming from another SDR and from an antenna" isn't very
    clear to anyone.   What is
      your hardware configuration?   Are you configured to have one RX
    input on the X310 on an antenna, and another input directly wired to
    another SDR that's "out of the picture"?