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The RS-232 Serial Port

The RS-232 serial port was used to connect a mouse, modem, and⁄or printer. The RS-232 serial port is now totally obsolete. In modern PCs it has been replaced by other serial communication standards, such as USB and Firewire. And I say "good riddance" because it was a real pain in the butt to set up, and it was very unreliable.

I would be very surprised if you ever run into a PC with an RS-232 serial port except possibly in a third-world country. However, I'll briefly describe the it in this article because you may run into a question about it in the CompTIA A+ Certification Exam. They like to make sure that technicians understand legacy systems, but if they do have any RS-232 related questions on the exam, I really think they should get rid of them.

Serial refers to data bits being sent one bit at a time over a single wire. The RS-232 transmits an asynchronous data stream, which means data bits can be sent at arbitrary time intervals, they are not synchronized to a clock signal. Each byte sent is preceded by a start bit, then the data bits were sent, then a stop bit is sent.

The original RS-232 used a 25-pin D connector, but most of the pins had no connection. With the release of the PC AT, the 25-pin connector was replaced with a 9-pin D connector. In many cases this necessitated the use of a 9-pin to 25 pin converter cable. The RS-232 specification allows a maximum cable length of 50 feet.

The RS-232serial port was implemented using a Universal Asynchronous Receiver⁄Transmitter (UART) chip. The UART converts transmit data into serial format and converts receive data into parallel format. Most legacy PCs will use some version of the National Semiconductor 16450 UART chip which had a top speed of 115Kbps. Other vendors produced UART chips with designations 16550, 16750, 16850, and 16950 that were compatible with the 16450 but with larger buffers, they were able to operate at higher speeds.

An RS-232 serial port must be configured to use specific I⁄O addresses and interrupt request lines (IRQs). In order to avoid conflicts with other devices, conventions were established as to which I⁄O addresses and IRQs should be used.

PortAddress Range IRQ
COM1 3F8-3FF4

In legacy systems the I⁄O addresses and interrupt request lines (IRQs) were configured using the BIOS setup screen. In Windows systems they can be configured in Device Manager. In Device Manager, expand the Ports(COM & LPT) branch, then right-click on a port (such as COM1), and set the port's IRQ and I⁄O Range in the ports Properties dialog box.

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