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Re: [Discuss-gnuradio] uhd running parallel tx/rx flowgraphs

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] uhd running parallel tx/rx flowgraphs
Date: Sun, 26 Feb 2012 21:09:24 -0500
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On 02/26/2012 08:54 PM, George Nychis wrote:

On Sun, Feb 26, 2012 at 5:01 PM, Marcus D. Leech <address@hidden> wrote:
On 02/26/2012 02:29 PM, Apurv Bhartia wrote:
Its an XCVR2450, but I do *not* start any 'packet' transmissions. All I do, is to start both the flowgraphs, and just listen for packets.

In which case, the TX side is running--even if you aren't sending any *data* bits, it's still transmitting, and blocking the receiver.

You'll have to get more sophisticated about half-duplex flow management, using tagging to tell UHD to turn on/off the TX side.

Josh probably has better words of wisdom on this than I.

Hi Marcus,

I'm working with Apurv, so I'm going to chime in here :)   

I tried doing some searching on the mailing list, but I wasn't really able to find much on this.  I also thought that auto tr would handle this.  I found a post from Josh on the mailing list that said Auto TR is always enabled in UHD.  

Yes, it is enabled in UHD.  But since Gnu Radio is a *streaming* model, you need to take special measures to control TX from within a
  Gnu Radio flow-graph.  YOu need to insert a tag in the stream to control the transmitter, otherwise, you'll be continuously streaming.

What you do is insert a burst-tagger into your stream, and set it to send the appropriate tags for UHD into the stream using the trigger
  input.  I just can't off the top of my head, remember what those stream tags are at the moment.  But the basic issue is that Gnu Radio
  uses a streaming model, and while UHD itself (at the C++ level) has fine-grianed control over transmitter functions, etc, gr-uhd doesn't
  directly expose any of that, because there's just not mechanism within Gnu Radio to expose that stuff.  The stream tagging, however,
  does allow you to control the transmitter state.  In the particular case of the XCVR2450, the hardware is physically incapable of
  TX and RX at the same time.

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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