Useful website for RS232 / Serial connectivity
One of the advantages of Linux (UNIX) over some other operating systems is that it can be maintained on the command line, and hence is useful for accessing over a modem or serial connection (e.g. via a Baytech or Terminal Server device).
The problem is that there are so many different standards for RS232 cabling, which was traditionally on a DB9 or DB25 connector, although is increasingly on RJ45 connectors.
It’s not just different connectors, but then different types of cable, e.g. straight; crossover (null modem); or rollover (a Cisco thing that has now become more common with other manufacturers).
Here is a useful resource to try and make it a little easier to understand: