USB to Serial adapter Wiki by Magneto Tech Research

USB to Serial adapters Wiki

What is a USB to Serial adapter and how is it used? How do I choose the right model and how do I connect it? What to look for when shopping for a USB to Serial adapter.
Here are answers to all your basic (and possible advanced) USB to Serial questions in an easily understandable language.

What is a USB to Serial adapter?

The USB to Serial adapter comes in many models, sizes and shapes, here are a few examples:

USB to Serial adapter miniUSB to Serial adapter StandardUSB to Serial adapter PROUSB to Serial adapter DualUSB to Serial adapter 4-portUSB to Serial adapter Industrial

What are the differences between USB to Serial adapters?

So what is the main differences between these USB to serial adapters? The most obvious difference is the physical size of the adapters; this is mostly due to the number of ports the particular adapter has. For example a one port adapter is much smaller than a four port adapter. However mostly the physical size of the USB to serial adapter is actually the least important feature, more importantly are the electrical features of the adapter. More about that later. Other differences are that some adapters has a female DB9 port instead of a male DB9 serial port. Some have thumb screws for attaching the DB9 connector to a device while others have screw nuts for connecting devices. Most USB to Serial adapters has a DB9 connector however some adapters has a screw terminal header instead, this applies to industrial adapters and converters in particular. One of our favorite features of the USB to serial adapter is the type of adapter which have indication lights for the TX and RX communication lines. This is a very helpful feature when troubleshooting.

Okay, but how about the inside of the adapter? Well, the circuitry inside the adapter is even more important that the outside. The main parts of the inside circuitry consists of two processor chips: a USB to Serial processor chip, this chip converts all the USB data to serial data and all the serial data to USB data, and a Serial driver chip, this chip provides the power for your the device connected to the serial port of the adapter; it kind of amplifies the signals.

We highly recommend a USB to serial adapter designed with a well known brand of USB processor and driver chip. Examples of well known brands are FTDI and Silicon Labs. The seller of the adapter should be able to tell you which processor brand is inside the chip, if they cant tell you, don't buy it. The problem with an unknown or low-cost processor and driver ship is that it will most likely cause you more problems when installing the drivers or when trying to communicate with your serial device. The reason is that low-cost processor chips are often poorly designed and have incompatible drivers.

Interface types.

USB to Serial adapters comes in several different variations in regards to interface types. The most commonly used interface type is serial RS232. Other frequently used interfaces are RS485 and RS422. Regardless of which interface type the adapter has it usually looks the same, with the one difference that RS232 adapters usually have a DB9 connector for connecting to the serial device where as the RS485 and RS422 adapters usually have a screw terminal header for connecting a serial device.
We will not go into the details of how the RS232, RS485 or RS422 protocol works, but if you are interested in this then there are lots of RS232, RS485 and RS422 information on Wikipedia.

How to use a USB to Serial adapter.

Before you can start using a you need to install the drivers for the adapter. The drivers are small files / programs which makes it possible for your computer to find the adapter and help in converting the data sent and received from your serial device.
Usually drivers are available for Windows, Linux and Mac operating systems, however some low-cost adapters only have drivers for Windows, which is another reason for selecting a quality USB to Serial adapter.

Installing the drivers are easy if you have a good USB to serial adapter, usually you can install the drivers simply by starting an exe file and the drivers will automatically installed. However, some of the higher level high quality adapters offers an option to install the driver files manually, this can be an advantage it you want to control exactly what is installed.
After the drivers has been successfully installed you will be able to find the new virtually created COM port in your operating systems Device Manager where different parameters for the COM port, such as baud rate and data bits can be configured.

A standard RS232 USB to serial converter with a DB9 connector is usually very easy to connect to a serial device, simply connect the DB9 connector to the serial device, the DB9 connectors needs to be female / male to be able to connect, and it is an advantage if one of the connectors has thumb screws and the other has nuts.

Connection Examples.

1-port RS232 interface
Lets take a look at one of the most simple setups using a USB to serial adapter with RS232 interface:

USB to Serial setup
This setup simply connects one computer with a serial device by a 1-port USB to serial adapter. It is recommended to keep the USB cable less than 5 feet to maintain reliable data transfers.

Multiport RS232 interface:
Below is a typical setup of a 4-port USB to serial converter with RS232 interface:

USB to serial schematic
With this setup there are four virtual COM ports visible in Device Manager within the computer's operating system. The COM port number can be individually changed and several parameters for each COM port can be configured.

RS485 interface:
As mentioned earlier, the USB to RS485 converter with a RS485 or RS422 interface usually has a screw terminal header instead of a DB9 connector for connecting the communication wires. Below is an example of a setup using RS485. One of the advantages of using a RS485 connection is that the communication wires can be up to 4000 feet long. For this reason RS485 is often used to extend data signals where as RS232 is usually only use as a converter between two types of data signals.

usb to serial rs485 setup

Port-powered and externally powered adapters.

In regards to powering the USB to Serial adapter, there are two ways of doing this. Usually a single port USB to serial adapter is powered by the USB port, even many four port converters are powered by the USB port. This is the most convenient way because it does not require any additional power cables. However, most adapters with more than four serial ports are powered by an external power supply. The reason is that a standard USB port does not have sufficient power to provide power for multi-port adapters. Also, most isolated industrial grade adapter is using an external power supply due to its isolation requirements.

Optical isolated adapters.

Some USB to Serial adapters can be either non- isolated or isolated. An isolated USB to Serial adapter have optocouplers and TVS (Transient Voltage Suppressors) separating the data lines inside the unit to prevent high voltage spikes, surges, ground-loops and noise from damaging and interfering with your computer or the device you have connected to the serial port. Some USB to Serial adapters even have the external power supply isolated from the rest of the adapter, this provides additional protection and reliability. Usually isolated adapters and converters are used only if the user wants to additionally protect the device or equipment connected to the network (in case the equipment is of high value), or if there are reasons to suspect high voltage spikes or similar in the network.
A non-isolated converter usually only has high-speed surge suppressor at each communication line as a simple protection against static electricity and low-voltage spikes which is sufficient for general use.

How to choose the right USB to Serial adapter.

When shopping for a USB to RS232 serial adapter you should first determine if you need an industrial strength or a commercial/consumer grade adapter, this depends on in what kind of environment you plan to use the adapter. Next you should determine if you need an isolated or non-isolated adapter, and then you can decide if you need an externally powered adapter or if you can use a port-powered adapter. Usually a port-powered adapter will suffice for most general applications. Finally you should decide if you want LED lights for the TX/RX lines, this can be very helpful when troubleshooting and is highly recommended.

Detailed specifications for other USB to serial adapters:

The Mini USB to Serial adapter
The Standard USB to Serial adapter
The PRO USB to Serial adapter