Dell Studio 1558 (Early 2010) - Manual (Page 315)

Ports
315
RS232 Serial Port
A serial port is a serial communication physical interface through which
information transfers in or out one bit at a time. Serial port is usually
identified with RS-232 standard compliant hardware. Some of the devices
that commonly use serial port are dial-up modems, printers, and serial mice.
Pin Signal
1 CD
2 RD
3 TD
4 DTR
5 SG
6 DSR
7 RTS
8 CTS
9 RI