Hi Kevin,\ \ As far as I know, the \`radio_\*_stb\` signals are strobe signals that are coming from outside of the image_core from the RF Analog HW on the daughterboards, signaling when the radio data can be transmitted/received.\ They can be understood as signals that are similar to tready signals of the AXI-Stream handshaking mechanism.\ \ If you trace the \`radio_tx_stb\` back to where it is assigned in x4xx.sv, it seems to be assigned based on the \`dac_data_in_tready\` signals:
* X410: https://github.com/EttusResearch/uhd/blob/master/fpga/usrp3/top/x400/x4xx.sv#L2685 * X440: https://github.com/EttusResearch/uhd/blob/master/fpga/usrp3/top/x400/x4xx.sv#L2664 As I understand the \`radio_tx_stb\`, it indicates, that on the same clock cycle as \`radio_tx_stb\` is asserted, the data that you pass to the radio via the \`radio_tx_data\` bus is successfully transferred to the DAC and will be transmitted OTA down the line. If \`radio_tx_stb\` is not asserted, the radio_tx_data will not be transmitted. I have not checked all USRP types, and some might not use the strobe signals at all, but at the meaning should be nearly the same for all variants. It looks like e.g. [X300 is not really using it and just constantly asserting it](https://github.com/EttusResearch/uhd/blob/master/fpga/usrp3/top/x300/x300_core.v#L689), as it seems it can always accept TX data. Please someone with a better understanding correct me if this assumption is wrong.\ \ Hope this helps at least a little. \ \ Regards,\ Niels
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