serial port : Computer Hardware Buyers’ Glossary

serial port
A connector to which you can connect a serial mouse, an external modem or a serial printer (not recommended!). Modern computers aften have no serial ports, now that USB (Universal Serial Bus) has become so popular. On the computer the serial port is a 9 or 25-pin D shaped male connector. There can be up to four COM (Component Object Model) ports. They must be assigned the following way for all software to work. If they are improperly done, likely some software will work. If there is an internal modem taking up com2: then there must not be an external com2: port. Never assign com1: and com3: without a com2:, or any other skip. Unfortunately, there is only about a 1 in 3 chance your retailer will properly configure your COM ports.
port address IRQ (Interrupt Request)
com1: 3F8 4
com2: 2F8 3
com3: 3E8 4
com4: 2E8 3
It is possible to make serial ports work even if you don’t follow the standard pattern of IRQs (Interrupt Requests) and port addresses, but life gets extremely complicated. There are then three different numbering schemes, the hardware one, based on standard port assignments, which allows you to skip com3: and then have com4:, the BIOS (Basic Input Output System) scheme that renumbers com4: to com3: if you skip com3:, and the Windows scheme on top of the BIOS scheme, which allows com5: and above, and logical remapping of COM numbers to arbitrary physical port addresses. Your software may use any one of the three numbering schemes. You must keep all three schemes in sync, never skipping a port, always using the standard port addresses and IRQs or you will create massive confusion.

The serial port is going the way of the dodo bird, replaced by the USB port which has smaller simpler cables and the ability to double your ports using a splitter.

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