Re: Can't seem to get serial to work

On 03/07/2026 14:28, Polarian wrote:
> After inspecting the motherboard, I found a JCOM header, this allows me
> to connect serial. So I connected a startech plate adapter to it [1]
> and then plugged in a nullmodem to it [2] and then connected a through
> cable to that.
>
> Server side I see the dmesg log for COM0 [3], and the interfaces. [4]
>
> However whenever I plug the DB9 to USB through cable into any device,
> there is no interface which is created, and dmesg shows nothing. I am
> honestly clueless of what more I can try.

As this has been around for a while...

I don't know exactly what you're doing here, but connecting a terminal 
to a serial interface doesn't create a device. If the UART (real or 
emulated) is detected, that's as good as you'll get.

You might be hitting hardware flow control problem. However, I can't 
figure for certain what the "DB9 to USB through cable" is about. I'm 
guessing that on host A you have some kind of serial port breakout which 
you believe is COM0. You have connected this to a DB9 IBM-PC style 
connector using a Startech passive adapter. Into this you have a null 
modem cable connected to a Serial USB adapter on another host.

Does the other hose enumerate the USB->Serial adapter?

Try looping back pins 2+3 on both hosts and see if you get your data 
echoed back on each.

Look in /dev for devices /dev/cua*

Note that I think USB serial devices use a capital U; native UARTs uses 
a lower case u.

Let's say you find cuau0

Force hardware flow control OFF:

stty -f /dev/cuau0 -crtscts

Connect to it using cu:

cu -l /dev/cuau0 -s 9600

Type something and see if it echos back. (Use ~. on a blank line to exit)

If you don't have a cuau device in /dev/ that's your first problem. If 
you don't have echo back then your UART is the problem. If you have echo 
back on both then your cross-over cable is the problem. You may need to 
loop back the hardware handshaking lines - disabling them in software is 
obviously not the answer if they're enabled at power-on.

If you're not getting your USB->Serial converter appearing as 
/dev/cuaU[0-9] run usbconfig and see what it finds.

Regards, Frank.