Serial Port: Difference between revisions
Revision as of 08:40, 28 August 2019
Introduction
A Serial Port is the generic term for a port on a computer that can be connected to other devices. This may be a proprietary connector with a few wires in it; an AppleBus/AppleTalk connector to connect a printer, modem or another computer; or even an Ethernet port. But by far the most common usage is for an RS-232 port with a 9-pin D-connector (DE-9) for asynchronous communications (see Communications). All these ports have one thing in common: the data is sent over a small number of wires (usually one in each direction) in serial fashion, rather than all at once in parallel.
But since "Serial Port" is usually used to refer to the COM Port that is found on the PC (to distinguish it from the rarely-seen-nowadays "Parallel Port"), this page will concentrate on that. Refer to the specific pages for other port types. This page will cover the physical port itself, its signals and how they're commonly used, and the most common protocol used on this port.
Physical Port
The original IBM PC computer came with two ports: a parallel port for connection to a printer, and a serial port to connect to a modem for external communications. Of course, over time those ports were used for other purposes, but that was their original intended usage.
Confusingly, both ports used the then already established standard connector called the DB-25.
- The D-series range has (usually) two columns of pins offset from each other to provide both ease of connection, and a definite polarity to the plug to prevent it from being plugged in upside down. Anecdotally, it was so-named because the metal shielding shroud protecting the pins looked like the letter 'D'.
- The letter after the 'D' indicates the layout of the pins; and often the number of them, making the suffix number redundant. For example, the first in the series is the DA-37, followed by the DB-25. The later DE-9 was adopted for COM ports on the IBM PC-AT, since only 9 of the 25 defined serial port pins were used.
To prevent plugging a device into the wrong port, the DB-25s used were different genders: the parallel port was female, needing a male printer cable, while the serial port was male, needing a female modem cable.
DB-25
IBM used the DB-25 connector to match other computing products that it was selling. The existing layout allowed for multiple serial ports within the one connector - the so-called Primary and Secondary ports. It also had clock pins for high-speed Synchronous ports as well. But for the IBM PC, only the Asynchronous Primary port pins were used, which meant only 9 of the 25 pins were needed.
DE-9
In the later IBM PC "Advanced Technology" (AT), they used the smaller DE-9 connector for more flexible port layout on the rear of the PC.