If nothing else I am thinking about this (similar to this post, but with proceed line connected):   * Arduino 6PIN FTDI USB TO TTL RS232RL 5V cable (e.g.  ebay) - key parameters: FTDI chip, USB to TTL 5Volt variant, 6pins * SIO PLUG (https://lotharek.pl/productdetail.php?id=103) * 2x Shottky diode Connect TxD to "data in" (SIO PIN 3) via diode (for example 1N5817) this way: Connext RxD to "data out" (SIO PIN 5) Connect CTS to "command line" (SIO PIN 7)