CH340 UART - USB Chip - Cable Length Problems?

You might try adding some bulk C to the +5V-ISO line... say a 10uF ceramic or so.

Also, for whatever reason the datasheets I've seen mention bypassing the 3.3V pin with 4700pF to 0.02uF.
I wouldn't think 0.1 would be a problem but maybe there's a stability issue with the internal regulator.

Interesting thought. I'll try a lower value cap.

Come to think of it, I too have some USB extension cables that cause problems.

Tumbleweed was on the right track I'm sorry to admit. When I got to thinking about the Vusb bypass cap, I didn't really remember soldering it on. Before I tried a lower value I looked at the board to see where it was....and it wasn't! I inadvertently left it out. More when I bung a resistor in!

As expected,adding a 0.1uF bypass caps from Vusb to ground solved the problem. I had successful communications with a crappy thin 8' cable.

I don't know how I skipped the cap on the board. It's especially bad - I should know better since I received a Microchip demo board missing the Vusb cap and I had to install one to get the board to work.

Now to apologize to my client who had a lot of trouble making this circuit work with different computers. I don't think all of his troubles were caused by this but it certainly added to his frustrations!

A satisfactory outcome- Tumbleweed was on the right tracks.

spec