Is there some relatively standard (or at least, de-facto Yaesu/Icom/Kenwood standards) way for a transceiver to directly announce its desired frequency to something like a connected external coax switch or antenna tuner? Offhand, I can think of a few good ones: second (output only) serial port... god forbid, multidrop RS-485, so multiple things like an antenna tuner & coax switch could all passively watch it for broadcasts. Maybe even get fancy, and implement an additional line that's normally pulled high, but another device can pull low to indicate that the radio should NOT transmit, because something is unready. I2C Dallas 1-wire low-frequency low-power signal directly mixed into the antenna feed to communicate the desired transmission frequency. Maybe even something like Bell-103, modulated onto a VLF carrier for downstream observers to see, before it ultimately got filtered out by high-pass filter before the antenna Ethernet. Ok, I'm dreaming, unless it's a SDR, in which case it would be absolutely trivial to do... ;-) Example use case: Arduino-controlled coax switch with trio of cascaded SPDT RF relays to switch coax between 3 antennas (40m, 30m, and 20m) and a dummy load. When radio is tuned to something like 7.070mhz, the switcher notices & sets the relays to connect the radio to the 40m antenna. If the frequency gets changed to something like 14.071 or 10.138, it changes the relays to connect the 20m or 30m antenna. If the radio gets changed to something like 3.560 or 28.180mhz, it connects to the dummy load, and pulls the error line low to tell the radio it shouldn't transmit. The same station might have an automatic antenna tuner connected the same way. It sees that the radio was tuned to 7.070, checks to see whether the error line is low or high, pulls it low *itself*, temporarily switches the radio's output to a dummy load & takes over the antenna itself using another RF relay, acts like a headless NanoVNA to recursively adjust itself to optimize SWR within a second or two, then gives the antenna back to the radio & lets the 'unready' line go high again. Does anything like this exist? Or, is CAT strictly a single-client polled interface, and doing something like this would require the host pc (or software running on it) itself to act as a proxy & communicate the radio's state to interested observers (like the coax switch, antenna tuner, etc)?