I have a USB serial port that works fine under Red Hat 8. It is based on a Prolific chip and uses the pl2303.o driver. It appears as a serial port /dev/ttyUSB0

My problem is that the same USB serial port doesn't work on another Red Hat 8 computer.( Let's name this computer B) . The line is always busy on that computer when I try to use /dev/ttyUSB0. The kernel and /lib/module drivers are the same and lsmod shows the same drivers loaded. cksum confirm that the kernel and drivers are the same. A USB thumbdrive works fine on either computer.

The only difference that I can think of is that the computer B has a USB keyboard. The other has a PS/2 connector. The USB keyboard works fine from boot. I think the BIOS releases the keyboard OK since when I rmmod hid the keyboard stops working.

I have tried booting with no keyboard ; it did not made a difference, still no serial port.

Can anyone help me by explaining how to get the serial line unstuck on computer B ?

In general, does anyone know why and how serial lines get stuck with Line busy ?

thanks