I have tested the ttl (waveshare) with loopback at 1500000 and it works.
Tried pins 8 & 10 as stated in another post and they are dead.
We are trying to get our yocto image to boot and need ttl access to see what is wrong.
The board is up on debian image, and that does not have any debug output on 8 & 10.
That is where I found the information and it does not work. console is turned in extlinux. What adapter are you using?
Current link for UART should be: https://docs.radxa.com/en/rock5/rock5b/radxa-os/serial
wiki seems to be deprecated and not updated anymore.
I think the best one UART adapter is CH343:
or same in tiny package:
Those supports up to 6Mbit and already have clock oscillator built in chip. Also they work with no issues in most systems. You can find them for about 1$ 
Okay, thank you for that information. I will order that one.
Try swapping your RX and TX pins, just in case. The RX of the cable needs to go to the TX of the board and likewise for TX cable RX board.
That is what I thought was wrong on the first attempt. It did not light up after that, it seems odd.
At least some noise should surface. Also launched gtkterm with gtkterm --speed 1500000 --port /dev/ttyUSB0 & since it did not have a drop down for 1500000. Tried minicom as in the docs and that did not work… Only thing I have not done yet is to probe the bus with the decoder.
Running 22.04 Ubuntu.
Oh nice ones, with RTS and CTS. Great to program ESP8266, thanks for the info!
It depends on your kernel. With the original rockchip kernel, the serial port is called ttyFIQ0. With mainline, it’s called ttyS2. For several months I thought that all my mainline kernels were hanging hard after “decompressing linux” but actually it was due to the serial port having another name (and it hanged later). Note that I’ve read on another thread somewhere that apparently on some recent 5B they had disabled console output on u-boot, something I find absolutely crazy. So it might be possible that you don’t get any output from the boot loader and that when the kernel starts you’re on the wrong tty.
My 5B (on rockchip kernel) has: console=ttyFIQ0. The speed defaults to 1500000 though if u-boot doesn’t initialize it you might need to force it. My Rock 5 ITX on mainline has console=ttyS2,1500000n8.
You need to make sure your USB adapter supports 1.5 Mbauds. I’ve had some jokes in the past with some adapters not supporting that speed and falling back to the closest supported one (e.g. 1 Mbauds or 921600). I seem to remember that PL2303 doesn’t support it for example. I’m now always using bootterm (bt) that I developed after being pissed off by many terminal emulators, and it turns out that bt shows you the effective speed after setting it, and that’s how nowadays I detect the ones that don’t work since I’m reading a different speed from the one I configured.
What image are you booting. I am using the out of the box debian, typically all we use is Yocto. We have been patching the bsp to get the Rock 5B to run with systemd and without a debug port its causing headaches here.
Thank you very much on the u-boot tip, I will look at defconfig and see if that is the problem.
I got even smaller ones from here: https://pl.aliexpress.com/item/1005006851154405.html
They have DTR and RTS pins on the bottom:
Expand description for full pinout.
For me main difference between waveshare bigger brother is that small version will disconnect when SBC is power off big one remains com-connected even if nothing is attached to pins.
If You need any tests for those two CH343 adapters just let me know.
Prolific UART adapter are problematic, I always got problems with mine, recently found that driver for it has weird name, something like on this screenshot. Tried to replace driver but sooner or later windows update will decide for me.
I think CH343 is the way to go now. 
I also found your adapter above this morning, but it has less connections (no CTS/DSR) compared to the first ones you showed. In case one only wants a tiny adapter with just TX/RX, the usual CH340G/CH340E are so small they can fit in a cable. And they work fine as well (I’m often using that and that’s what I’ve glued inside my rock5itx).
BTW if you have the link for the “same in tiny package” above (the one with all the pins in small format), I’d be very interested, as I couldn’t find it.
Here it is: https://vi.aliexpress.com/item/1005005775551705.html?gatewayAdapt=glo2vnm
have You seen any CH343 as complete cable?
I think that something like USB-C to UART + USB-pd would be awesome to connect sbc with one cable to laptop and get UART and power to SBC on the other side.
Thanks for the link! CH343 in a cable, no, not seen yet but not searched either. I’m fine with the tiny modules with dupont wires hanging at the end. I get your point regarding USB-pd + UART. It could make sense for some use cases. Preferably with an ON/OFF switch to ease rebooting without having to unplug the UART though 
Thank you for helping out.
I did get it up and running using the waveshare ttl-usb adapter. Found the problem, I was starting gtkterm cli it does not like to be started at cli with an arg. I just typed in the 1500000 in the gui box and it ran. However mini-com does not work on ubuntu 22.04??
Also found out the .deb packages for u-boot install are broken. They are depending on different version than what is currently installed.
Minicom does work Ubuntu 22.04, what you may find is that by default it has hardware flow control enabled and it’s not always obvious. You want to disable flow control (be it SW or HW).
Thank you, I will check that.
