The official programming set is: Yaesu CT-42 which contains CT-28 cable and the CT-29 Interface unit
The resistor inside the radio is for the Clone function between 2 radios, a resistor is absolutely necessary when used with an external programming interface.
When used with a USB to UART device a diode is required, it may sometimes JUST about work on a genuine FTDI chipset as per your case, but not reliably and it will be borderline, it also depends on how the FTDI chipset has been implemented and it will absolutely not work on every system or every radio.
It will definitely not work without a diode or resistor on a CP21xx family of USB to UART IC's.
A diode may not always be necessary with a RS232 to TTL level converter with inversion, whether that is a MAX232 or similar or through transistor level shifting, but it does not cause any harm in that scenario and can help stability on some systems.
The same interface is used for a VX170/FT270, VX6R/E, FT70D/E etc and they will definitely not work without a resistor.
I'm on holiday at the moment and I just did a quick google search for schematics and found the following:
Also, ICOM, Alinco and some Yaesu mobiles use the same type of interface and use a diode and resistor.
For the sake of soldering 2 extremely extremely cheap components it just does not make sense why someone would not fit them and become defensive about it.
Would you put the wrong oil in your engine and risk causing damage? but you would say "well it works", yes for the moment.
