AIX

shockneck, for your benefit.

The handshaking used in single user mode, a diagnostic boot, and the early part of a normal boot differs from the handshaking when AIX takes over and configures the tty in multi user mode before it issues the login prompt.

Before AIX takes over the port and configures the TTY the system uses hardware flow control, but the default for AIX is software flow control and this is where the extra wires, often not found in a normal null modem cable, come into play.

So if your cable is not wired correctly for an AIX system, i.e. it is a "standard" rs232 null modem cable or a "laplink" cable then you are likely to have this problem of seeing everything OK in single user, diagnostic, and the first part of the multi user boot but you will not see the login prompt prompt in a multi user boot.

Hm, I don't think that applies with clocal being configured as stty login and runmode option. Unfortunately even with original IBM cables some servers cannot be accessed if a tty is configured at baud rate proper with 8N1 xon and login enabled only. Try to use IBM type 10N7712/3 on Power3 System and it will probably fail without the clocal option. It works on some late Power3 systems and all Power4/5/6 systems though because the serial ports changed. However when using clocal all Power3/4/5/6 servers can be accessed by those IBM cables and also via a blue Laplink cable. Well, at least with "my" blue Laplink cable So configuring clocal makes it much easier finding a suitable cable. It does not have to be an 'original' one.
Btw the number of wires in those cables is highly overestimated IMHO. An AIX tty can be controlled with just two wires (if I let aside shielding) for send and receive when flow control is set to none.

Good for you.

I'm always willing to learn so post the pin out of your magic "blue Laplink" cable then (will other coloured wire work?), so we can all share your joyful experience of AIX terminal access (bet it has more than two wires, if you are honest)