Serial Interface

Before you start

Objectives: learn what is serial interface and how does it work.

Prerequisites: no prerequisites

Key terms: com, device, port, problem, data, modem, mouse, connection, byte, printer

How Does it Work

Let’s say that we have two devices connected using serial connection.


Image 315.1 – Serial Connection

In a serial connection, one device sends signals to other device in a serial fashion, meaning that it sends one bit at a time across a wire. One of the great advantage of serial connection that we can have very long cables. The maximum length for an RS-232 serial cable is 50 feet or 15 meters. The key disadvantage is that it’s rather slow. Other problem that came up was the signaling. Now, signaling was not a problem for slow devices, but it was a problem with modem for example. If we’re just getting a serial stream of data coming in how do we know where one byte ends and where the next byte starts? Because of that we use different types of encoding schemes in order to determine signals which indicate when particular data begins and when it ends. Timing was is an issue too, since devices need to know how fast bits were going to be coming in. The serial interface was used by different types of devices like the mouse, external modems and serial printers. For mice and for modems serial worked well, but for printers it was not so good. With printers the issue of speed really manifested itself. It could take a very long time to get a page of data sent to printer when we’re dealing with a serial interface.

Serial devices use COM ports for system resources. COM ports 1 and 3, and COM ports 2 and 4 are shared. If we install a serial device on both COM 1 and COM 3, the devices might cause a resource sharing problem. Serial ports on the back of a computer are always male connectors. The original serial standard defined a 25-pin connector, but since only 9 pins of the connector were used, manufacturers moved to a 9-pin connector.

9 pin

Image 315.2 – Serial Port (RS-232, 9 pin, male)

RS-232 9PIN

Image 315.3 – Serial Cable (RS-232 cable with female serial connectors)

Many newer computers no longer include a serial port, so if we need to to add serial ports to a computer, we have to install an adapter card.


In serial connection data is transmitted one bit at a time. Serial devices use COM ports for system resources. On the back of a computer serial ports are always male connectors.