Serial ports on printers are also known as communication ports (COM ports) or alternatively, RS232. On a printer, they are basically there to allow for connection of hardware like; mouse, sound card, modem, fax machines or principally any hardware that requires an internet request (IRQ) setting.
Even though parallel ports have slowly replaced serial ports, there are printers that still have them. They serve to transmit data and they also receive because they are bi-directional. They are advantageous because they require fewer cables to operate as opposed to parallel ports. This means that the cost you incur on cable is lower.
However, you may find that a printer has a serial port and the computer you want to connect it to does not. The way to solve this is to turn the USB port on the computer into a serial port. This is done by using a USB to serial adapter. With this, you can operate your printer and computer as if they both had serial ports.
Finding cables for the serial ports on a printer is not hard really. Even though they are not as common as they used to be, they are still available. Someone with the knowledge can also build their own cables with relative ease.
A printer with serial ports will generally come with a manual on how to set it up so it shouldn't really be a problem. You should always be sure on what kind of cables you should use. It is also important to make sure that the bits per second have been set.
Select the highest bits per second for a printer that has serial ports. If you have a printer that has serial ports, it is important to know how they work; know what goes where to guarantee maximum performance.