Critical forum software updates are running this morning to be followed by some performance tuning (no, really!). The site should remain fully available but while tuning is on going performance may be negatively affected and warnings may appear on some pages.
If you want to buy a cable the best thing would be getting the proper rj11 serial cable https://efi-parts.co.uk/product/megajolt-rj11-serial-cable-lead/
and then buying a serial to usb cable with the FTDI chipset like this one https://amzn.to/3gWh9tI
Some heads don’t have the raised section of the seat which makes them a straight fit. A suitable size holesaw will remove the raised section of the seat and allow them to fit and work.