I'm trying to develop a flow graph for a system that will support half-duplex communications. I'm using an Ettus X300 and gnuradio 3.10.6. I want to set up a python script that will take in data packets from users and then send them over to the flowgraph for transmission using a zmq-PUB connection. The system should be in receive mode until a message is sent to the ZMQ Sub Source, at which point the UHD: USRP Source should be active and transmit the message before going back into RX mode. I have a basic version of the flowgraph working but the USRP source is reporting 1 underflow on every packet because it's expecting a constant stream of samples and stopping when it doesn't get it (I think). I looked into the documentation here https://wiki.gnuradio.org/index.php/USRP_Sink and it seems like I will need to add "tx_sob" and "tx_eob" tags to the stream to let the USRP know that this is a burst transmission with a limited amount of samples available.
The tutorial on ZMQ blocks ( https://wiki.gnuradio.org/index.php/Understanding_ZMQ_Blocks) for gnuradio says the following about tags "Next, if sending tags is enabled, any tags within the window of the data to be sent are encoded in a special format and prepended to the payload data. If tags are not enabled, this header is elided." Is there any more information or an example of how I can manually tag the incoming data stream in my python script and then send that over the ZMQ PUB-SUB connection? The tutorial mentions a "special format", what exactly is that? I looked through the examples in the gr-zeromq but it doesn't seem like those deal with adding or modifying tags in a ZMQ stream. - Conner
