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It may work for you, but you do risk bricking the radio if even one byte fails when writing to the radio. I was just being helpful by giving additional information on how it is done officially and to stop someone from potentially bricking their radio.

If you try to use it without a diode and resistor on a VX170 or FT70D and other Yaesu radios it wouldn't even work at all.

The way it reads and writes is by pulling the TX and RX lines low, without the resistor the MCU is "Floating" it's neither high or low. There can't be a pull up resistor inside the radio as the same connector on the radio is used for audio input and output. So the pull up resistor is put inside the data interface/cable.

If something is "Floating" it's very easy for a bit/byte to be interpreted incorrectly, there needs to be a large enough voltage swing to determine whether it is a 1 or 0.

If even one of those 1/0 get interpreted incorrectly the whole bit/byte is wrong and if that gets written to the radio then the programming is corrupt and will either stop the radio working completely or cause strange behaviour with the operation of the radio. In both cases it might be impossible to re-programme the radio again to get it working properly and the EEPROM or MCU may need to be removed from the board and reprogrammed on an external programmer or even replaced. The calibration data for the radio is also stored in this location.

The diode is used so the RX and TX lines do not influence each other when reading or writing. If they are just tied together, when you read from the radio the TX line on the USB to UART is getting pulled Low and when you write the RX line gets pulled Low with data. Some USB to UART chipsets don't like those lines tied together at all and will not work or crash. The CP2102 is one of those.

But you think well I can tie RX/TX together to do a loopback test, yes you can but you are not pulling them both High or Low at the same time on an external device.

Hopefully this helps and makes sense.

The specification requires a pull up resistor and a diode end of discussion. To say it's not needed is careless and could damage someone's radio. There is simply no need to be ignorant and defensive.

I was typing my long winded reply outside in a hot tub. I work with 1000s of different models and do not know the radio pinout off the top of my head. Regardless of what line it uses the radio requires an external pull up resistor. I had to say something before others copy you and make the same mistake.

What do I know, I only run a business that manufactures programming cables and interfaces, repair amatuer and professional radio equipment, amongst many other things.

The specification requires a pull up resistor and a diode end of discussion. To say it's not needed is careless and could damage someone's radio. There is simply no need to be ignorant and defensive.

I was typing my long winded reply outside in a hot tub. I work with 1000s of different models and do not know the radio pinout off the top of my head. Regardless of what line it uses the radio requires an external pull up resistor. I had to say something before others copy you and make the same mistake.

What do I know, I only run a business that manufactures programming cables and interfaces, repair amatuer and professional radio equipment, amongst many other things.