I/O (Input/Output) Ports

I/O - stands for In/Out. When purchasing the components for your system, you need to keep in mind certain limitations. You will only have 2 Com (serial) ports and 1 Printer (parallel) port under normal circumstances.
Determine how many printers you wish to use, and whether you need two printer ports or just a switch box to manually select which printer you want to use.
If you are running Windows, or a GUI environment, you will need a mouse. There are basically two choices: Serial or PS/2. Running a PS/2 is a new option to "clones" and custom systems and frees up a communications (COM or serial) port... but still takes up an IRQ, which is really limiting.
Are you planning to use a modem? If yes, will it be internal or external? and other such details.
Mice and modems generally use serial ports while printers usually use parallel ports. If you are using an external modem is the BAUD rate above 9600? If so then you must make sure that your serial ports have a 16550 UART or similar. This is a chip that allows these high speed communication devices to operate a full speed without data corruption or loss.


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