Whilst there are plenty of USB-to-CI-V interfaces available at very low cost, e.g. on ebay, there is no evidence that I am aware of that they correctly pull down the combined TX/RX data line with an open-collector (or open drain) drive. This seems to be required because of the possibility of contention on the bidirectional data bus. In fact one supplier that I contacted specifically stated that the drive is not open-collector.
So what to do? I didn't want to rick killing my IC-718, and existing circuits almost universally use an MAX232 level converter from RS232. I chose to combine the open-collector drive of one such circuit with a cheap USB-to-TTL serial adapter from an ebay supplier. This was a few pounds from China, and is based on the CP2102. The drive chip is a 7417 hex buffer.
So far I have constructed this on a breadboard. Next I'll commit it to stripboard and put it in a box.Back to G4TJC