The RS-232 Serial Port
By Stephen Bucaro
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 In modern PCs it has been replaced
by other serial communication standards, such as USB and Firewire.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.
| Port | Address Range | IRQ |
| COM1 | 3F8-3FF | 4 |
| COM2 | 2F8-2FF | 3 |
| COM3 | 3E8-3EF | 4 |
| COM4 | 2E8-2EF | 3 |
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.
More Computer Anatomy Articles:
• Laptop Computer Bottom Access Covers
• PC Sound Circuitry
• Understanding Modern PC Specifications
• How Does the CPU Cache Work?
• Plug and Play Resource Allocation
• Basic PC Anatomy Inkjet Printers
• Understanding PC Data Buses
• PC Video
• PC Keyboards
• Building Your Own PC! - The Motherboard is the Core of the PC
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