These circuits can be "port-powered" using the RTS or DTR lines, or powered externally with a 5V regulator like the Critical Specs for Your Build Signaling: 1-wire bidirectional TTL ( ); data line is when idle. Baud Rate:
Here's a more detailed schematic using the FTDI FT232RL chip: icom ci v usb interface schematic top
For a reliable "top-tier" homebrew design, using a dedicated USB-to-TTL serial chip like the is highly recommended. ICOM CI-V - K7MEM These circuits can be "port-powered" using the RTS
Thus, a simple USB-TTL cable will not work. You need a circuit that performs: icom ci v usb interface schematic top
Add a from the combined line to VCC (5V) to ensure the signal stays high when idle.