Thanks Niels.
OK, so it seems that isn't my problem on the X310. My deeper issue is that I can't get the USRP to start transmitting in internal loopback mode specifically when I need to combine both rx radios in a custom block. Kind regards, Kevin From: niels.steffen.garibaldi--- via USRP-users <usrp-users@lists.ettus.com> Sent: Thursday, 07 August 2025 17:06 To: usrp-users@lists.ettus.com Subject: [EXTERNAL][USRP-users] Re: "radio_tx_stb" input on radio_tx_core 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 <https://url.za.m.mimecastprotect.com/s/90I9Czm5zzHwQB7I4f3s9BkLy?domain=git hub.com> * X440: https://github.com/EttusResearch/uhd/blob/master/fpga/usrp3/top/x400/x4xx.sv #L2664 <https://url.za.m.mimecastprotect.com/s/Bi8mCAnollflLo8U8hYsGw75v?domain=git hub.com> 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://url.za.m.mimecastprotect.com/s/SWzjCBgpmmiRpmytNixs2Kn08?domain=git hub.com> , 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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