USB to DB9 connector full guide - how to work with USB to Serial DB9


  1. USB to DB9 adapters differences
  2. Interface types
  3. Connection examples
  4. Choose the right USB to DB9 adapter

What are the differences between USB to Serial adapters?

USB to serial DB9 adapters come in a variety of shapes, sizes, and configurations. Size differences are not the most important variable in a USB to DB9 connector but they are the most obvious physical distinction between adapters. This is usually the result of the number of ports that an adapter provides its users. It makes sense that an adapter that has four ports will be larger than one that only contains a single interface, and that is indeed the case. Other physical differences that are easily observed are the inclusion of thumbscrews or screw nuts used to attach the DB9 connector to a device. Some adapters may be equipped with a female DB9 port where others have a male port. Many converters used in industrial applications have a screw terminal header rather than a DB9 connector. A great troubleshooting feature found on some USB to DB9 serial adapters are indication lights for the TX and RX communication lines.

USB to DB9 adapters

That about covers the visible physical differences in adapters. An even more important differentiating factor is the circuitry that the converter contains. There are two main processor chips that are part of a USB to DB9 converter. One is the USB to serial processor chip which is used to convert between USB and serial signals. The other chip is the serial driver chip which amplifies the signal transmitted from a device that is connected to the adapter’s serial port.

Our recommendation is to always use a USB to serial adapter that is manufactured using a high-quality USB processor and driver chip. Some well-known brands that fit this description are FTDI and Silicon Labs. If the merchant selling the adapter cannot tell you who made the chips you should choose another adapter. It likely means that they are using a low-cost processor from an unknown manufacturer. These inexpensive chips suffer from poor design and you will often experience driver issues when installing your adapter or communicating with serial devices.

Interface types

The interface that is used on a USB to DB9 adapter can conform to different protocols. The interface that is most commonly seen is a serial RS232 port. RS485 and RS422 ports are also found on many adapters. Most of the RS232 adapters connect to a serial device by way of a DB9 connector. Screw terminal headers are the primary means of connecting a serial device when the adapter is made for the RS422 and RS485 protocols.

The focus of this article is not the differences in the inner workings of the various protocols. You can find a wealth of information regarding the RS232, RS485 or RS422 protocols on Wikipedia if you would like to pursue it further.

How to use a USB to Serial adapter

The first step in using a USB to 9 pin serial port adapter is to install the adapter’s drivers. These are small program files that enable your computer to locate the adapter and assist in data conversion between your machine and the serial device. Drivers are specific to the operating system that you are using. Most quality adapters will have drivers that support the Windows, Mac, and Linux operating systems. In some cases, adapters of lesser quality will only have drivers for Windows which is an excellent reason to insist on a quality USB to 9 pin serial adapter.

Driver installation is usually fairly simple with a quality USB to serial converter. You might just have to run an executable file that automatically installs the drivers for you. High-quality adapters sometimes include an option for manual driver file installation. This lets you control exactly what is installed and can be seen as an advantage in certain situations. Once you install the drivers you will see a newly created virtual serial port in your operating system’s Device Manager. Here is where you can configure the COM port’s parameters like baud rate and data bits.

USB to Serial adapter

Connecting a serial device to a standard USB to 9 pin serial adapter that has a DB9 connector is a relatively easy task. Simply connect the DB9 connector to your serial device. Of course, one of the DB9 connectors needs to be male and the other female in order to enact the connection, and you will find it easier if one of the connectors has nuts and the other has thumb screws.

There is a more sophisticated solution, that requires no additional cords or drivers - Eltima USB to Serial Converter software. You will just install the software and work with all needed apps thanks to USB to DB9 serial adapter.

Connection Examples

1-port RS232 interface

Here is a basic setup that employs a USB to serial adapter that contains an RS232 port. This simple connection is made between a single computer and one serial device using a 1 port USB to COM adapter. To ensure reliable data transfer you should use a USB cable that is at most 5 feet long.

Multi-port RS232 interface:

Let’s look at a typical setup that uses a 4-port USB to serial converter with an RS232 interface.

This configuration will create 4 virtual serial ports that can be seen in your system’s Device Manager. You can change the individual numbers of your COM ports as well as configure several parameters for each interface.

RS485 interface:

As we previously noted, converters that feature interfaces for the RS485 or RS422 protocols often lack a DB9 connector. Instead, they connect to the communication lines via a screw terminal header. Below is an example of a setup using an RS485 adapter. One of the overriding advantages of an RS485 interface is the ability to use much longer cables. RS485 connections can use cables that are as long as 4000 feet which makes them a great solution for extending data signals. Conversely, an RS232 USB to DB9 converter is used primarily to simply convert between the two types of data signals.

Port-powered and externally powered adapters

USB to serial adapters can be powered in one of two ways. The first method is to draw power from the USB port. This is usually the way that a single port converter is powered, and some four-port USB to serial adapters also are powered in this way. This is a simple and convenient way to connect as there are no additional power cables required. Larger adapters that have more than 4 serial interfaces need an external power supply in order to function as the USB port cannot deliver enough power. Isolated industrial grade adapters also need to use a self-contained, external power source.

Optical isolated adapters

You can obtain both isolated and non-isolated USB to serial converters. Non-isolated adapters will often employ a high-speed surge suppressor on each communication line. This configuration is satisfactory for general use and protects against low-voltage spikes and static electricity.

Isolated USB to serial adapters are used when more protection from rogue electricity and reliability is required, for instance, to protect high-value equipment that is connected to your network. They accomplish this extra protection by separating the internal data lines and through the use of optocouplers and TVS (Transient Voltage Suppressors). These help prevent power surges, high voltage spikes, and line noise from causing damage to your computer or interfering with the machine or the connected serial device. Additional protection can be gained by isolating the external power supply from the rest of the adapter. If you suspect high voltage problems on your network be sure to use an isolated USB to serial adapter.

How to choose the right USB to Serial adapter

Some points to consider when on the market for a USB to RS232 serial converter is whether you can get by with a commercial grade adapter or if you need to step up to an industrial strength model. The environment in which the device will be used is the determining factor here. The next question is if you should go with an isolated or non-isolated adapter. Most general applications can get by with a port-powered adapter, but you may need to go with an externally powered one based on your usage scenario. Your last choice is whether or not you want the convenience of having LED light on the TX/RX lines. Remember that when troubleshooting your equipment, these lights can be extremely helpful.