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USB -to- Serial Adapters

Overview    |     What Works    |     Adapter Attributes    |     DB9-to-DB25    |     Tools    |     Questions


Almost all amateur radio TNCs and many amateur radio transceivers are equipped with RS-232 serial ports for communicating with a computer.  But most computers sold today no longer have RS-232 serial ports.  In fact, on many laptops, and certainly on devices like tablets, there is no room for such a large connector.  So USB-to-serial adapters have become essential for filling the connectivity gap between devices with serial ports and computers without serial ports.

There are many, many vendors offering USB-to-serial adapters and most have at least one model.  But not all are created equal.  Some work better than others and some really don't work very well at all.  Therefore, the choice and use of USB-to-serial adapters has become a real pain point for many users.

The information below is intended to help you select a USB-to-serial adapter that will work well for you.  This web page identifies some key attributes that can be helpful in selecting a USB-to-serial adapter and make recommendations based on what many experienced operators have used successfully.  A list of adapters that have been tested and proven to work with most packet equipment is included.  Of course, as vendors come and go, this list is likely to change. 


Just Tell Me What Works!

This spreadsheet contains details about various USB-to-serial adapters and their compatibility with Outpost running on different versions of Microsoft Windows.

USB to Serial Adapter Comparison - 16-Oct-2014 (PDF - 92kB)


USB-to-Serial Adapter Attributes

Some attributes that be used to select a specific USB-to-serial adapter are described below.  As we gain more information, this section may grow to provide more information.  Some recommendations are given.  But, as always, select what will work best in your environment.


FTDI:  The FTDI chipset is recommended for WIndows and Linux for several reasons: 

KeySpan:  The KeySpan adapter (now owned by Tripp Lite) has been around for many years and is a recommended solution for Windows. 

Prolific:  The Prolific chipset is found on most inexpensive adapters.  Unfortunately, many of these adapters suffer from several problems:

Others:  There are a couple of other chipsets out there.  Not enough is known about any particular one to offer any recommendation.

Physical Characteristics

Following are some additional characteristics that may be important to you when selecting an adapter:

LEDs:  Some adapters have LEDs which can be very useful when troubleshooting.  The most helpful configuration uses multiple LEDs: one is constantly on when the adapter is connected, one blinks when characters are transmitted (from your PC), and one blinks when characters are received (from the serial device).  At least one model has an LED for each of the 9 pins in the DB-9 connector.

Length: The adapter cable must be long enough to reach whatever you are trying to connect to.  But a cable is also an antenna, and you don't want to bring RF interference into your PC or serial device via the USB cable.  So, select as short a cable as will work in your environment.  A length of about three feet seems to work well for most.  Coil up any extra and secure with some Velcro (so you can undo it if necessary).  This will reduce the effectiveness of the cable as an antenna due to the shorter length, and the coil will act as a choke for any common-mode noise on the shield.

Ferrite Bead:  A ferrite bead on the end (or both ends) of the USB cable is another important tool for combatting RF interference.  The ferrite bead acts to resist common mode currents (such as RF interference on the shield).  If you have RF in the shack problems, and especially if you plan to operate portable/mobile with the antenna fairly close by, a ferrite bead is really important.  Of course, you can always purchase clip-on ferrite beads and add them yourself.

Serial Connector:  Some adapter makers offer a variety of serial connector types.  Some are meant to emulate a regular PC serial port DB-9 connector while others are meant to plug directly into a radio.  The standard PC serial port is a male DB-9 connector (9 pins) with female binding posts (nuts) on either side of the connector. Since the USB-to-serial adapter is typically used to emulate a physical port on the PC, most people will want the adapter to have the same configuration as a PC port.  Otherwise, a gender adapter may be required.

Head Shape: An adapter that features a head with flat spaces is convenient for adding labels.  Most people want to label the device with their name and/or call sign. If the adapter is FTDI and there is more than one, it can be helpful to add a label indicating the FTDI unique ID.  If the configuration is fairly static (same adapter always plugged into the same device) it can be helpful to add a label with the COM port number or Linux device name.


DB9-to-DB25 Adapter

Most USB-to-Serial adapters are equipped with a male DB-9 male connector, as described above.  But the RS-232 port on most external TNCs is a female DB-25 connector.  To connect the USB-to-Serial adapter to the TNC will usually require the following adapter:

Of course, check your own equipment before ordering anything.  These adapters are available from online sources such as Amazon, CDW, New Egg, and Cables-to-Go or from local computer and electronics stores such as Fry's Electronics.


USB Tools

The following tools can be helpful in dealing with a variety of USB device issues:


Questions / Comments

If you have a question about any of the above information, someone else probably does, too.  So post your question to our scc-packet Yahoo group where all of our local users can see the question and the answer.

Note:  The scc-packet group is a closed group to prevent SPAM. Only members are able to see and post messages.  You will need to join before submitting a question.

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This page was last updated on 16-Oct-2014