# Loopback flowgraph

**URL:** <https://www.ruby-forum.com/t/loopback-flowgraph/239443>\
**Category:** GNU Radio\
**Created:** [February 26, 2014, 2:35pm UTC](https://www.ruby-forum.com/t/loopback-flowgraph/239443 "2014-02-26T14:35:46Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\
**Post date:** [February 26, 2014, 2:35pm UTC](https://www.ruby-forum.com/t/loopback-flowgraph/239443/1 "2014-02-26T14:35:46Z")

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Hello all GnuRadio users,  
I wanted to have a flowgraph without an USRP source and USRP sink, only  
by using the software blocks. The objective is to transmit and receive  
the BPSK modulated packets.  
I attached a thread at the receiver to watch if the packets are  
demodulated. When I run my flowgraph no packet has been received. After  
that, to see if the packets have been decoded in my packet sink, I  
changed a CC code by adding a fprintf function. When i launched the  
python flow graph, I started to print a received packets.  
Can you explain me why I have got this probleme ? Is it a packet queue  
of my watcher in python or there are a specific problem ?  
Thank you for your replay.

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**Author:** ![raf\_raf](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/6bbea6/32.png) [@raf\_raf](https://www.ruby-forum.com/u/raf_raf)\
**Post date:** [February 26, 2014, 2:48pm UTC](https://www.ruby-forum.com/t/loopback-flowgraph/239443/2 "2014-02-26T14:48:09Z")

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Hi Raf Raf,

I’m not quite sure this is what’s happening here, but:  
Threading with python and GNU Radio can be a bit tricky, and even more  
tricky when you use a GUI framework.  
Maybe you should elaborate on what you mean with “I attached a thread”,  
so we can understand your architecture better 🙂

Greetings,  
Marcus

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**Author:** ![raf\_raf](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/6bbea6/32.png) [@raf\_raf](https://www.ruby-forum.com/u/raf_raf)\
**Post date:** [February 26, 2014, 2:49pm UTC](https://www.ruby-forum.com/t/loopback-flowgraph/239443/3 "2014-02-26T14:49:55Z")

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Remove the printf. Put in “tag debug” blocks at different points in your  
flowgraph to see what’s going on.