Comment 9 for bug 1087519

For one, check that your kernel support registration of more than 4 ports if you want to use /dev/ttyS4. The standard config is set to 4.

In the case that it tells you port is in use, check for a dangling lock file. cu will create a lock in order to prevent double access. 'lsof' may help you to find if an application is accessing the port (too).

dmesg may reveal how many serial ports are allocated during boot.

Kees