The Pololu USB-to-serial adapter is great for connecting microcontroller projects to a personal computer. The adapter uses 3.3 V signal levels so you can connect it directly to any microcontroller running at up to 5 V without the hassle of an RS-232-to-TTL level converter. The tiny unit is less than 1.0" x 0.7" including its mini-B connector, and the simple layout of the ground, transmit, and receive lines allows for easy mounting that takes up as little as four breadboard rows.

The USB adapter's drivers make it look like a standard serial port to the operating system, allowing you to connect to all of your existing software, such as servo controller interface programs, through a USB port. The adapter is compatible with USB 2.0 standards and allows baud rates of up to 921.6 kbps. Support is initially available for Windows 98 through XP; Mac and Linux support will follow shortly.

With the trend toward removing serial ports from new computers, the Pololu USB-to-serial adapter provides one of the most economical, small, and simple solutions to the common problem of interfacing small projects to PCs.

Device specifications

PCB size: 25.4 x 16.5mm including connector
USB standard: USB 2.0 (full speed)
Connector: USB mini-B
Data I/O voltage levels: 3.3 V (5 V tolerant)
Maximum data rate: 921.6 kbps
Supply current: 25 mA

PIN / TYPE / FUNCTION
DCD / In / "Data carrier detect" control input (active low)
DTR / Out / "Data terminal ready" control output (active low)
(often used with DSR)
DSR / In / "Data set ready" control input (active low)
(often used with DTR)
RTS / Out / "Ready to send" control output
(often used with CTS)
CTS / In / "Clear to send" control input
(often used with RTS)
GND / Power / Ground
TX / Out / Asynchronous serial data transmit (idle high)
VBUS / Power / USB bus voltage (+5V)
RX / In / Asynchronous serial data receive
SUSPEND / Out / Driven high when in USB suspend state
SUSPEND(negated) / Out / Driven low when in USB suspend state
RST / I/O / Device reset
RI / In / "Ring indicator" control input (active low)
VDD / Power / 3.3 V voltage regulator output

Typical connection (see Additional Images)
To connect a serial device to your computer's USB port, you will most likely only need to use the GND, TX, and RX lines. The TX and RX lines use 3.3-volt levels, but the USB adapter input lines will tolerate up to 5.5 volts. Therefore, a direct connection to a microcontroller's I/O lines is usually possible.

For some programs, it will be necessary to connect the handshaking lines (RTS, CTS, etc.) if the software does not establish a connection without those lines. In general, the handshaking lines can be left disconnected, used for their intended purpose, or used as additional I/O lines to the computer.

Driver installation instructions
Before you connect your Pololu USB-to-Serial adapter to a computer, you must install its driver by following these steps:

Do steps 1-6 only once, even if you are planning to connect multiple adapters or USB servo controllers. These pre-installation steps need only be carried out once, and the USB servo controller and USB-to-serial adapter drivers are the same.

Download this driver: 3810-001 Driver.zip
IMPORTANT: You must extract the files in usb01a.zip into a temporary directory. DO NOT attempt to double-click or run any of the files until you have extracted them.
Open the temporary directory and run setup.exe. The "Install Driver" dialog will open.
Browse to the directory in which you would like the driver to be installed and click "Install".
In Windows XP, a "Software Installation" dialog will appear twice, warning you that this driver has not been tested by Microsoft; click "Continue Anyway" both times.
When the installation is complete, a "setup" dialog that says, "Installation Successful" will appear; click "OK".

Follow steps 7-10 for each new adapter you connect to your computer:

Connect the Pololu USB-to-Serial adapter to your computer; Windows will detect the new hardware and open the "Found New Hardware Wizard".
Select the automatic software installation option; if multiple drivers are found, you will be prompted to "select the best match for your hardware" from a list. Select the one for the Pololu USB-to-Serial Adapter.
The "Hardware Installation" dialog will appear, warning you that this driver has not been tested by Microsoft; click "Continue Anyway". The installation should then complete; click "Finish".
You will have to repeat steps 8 and 9 again (you have to install one driver for the USB device and another for the serial device). After that, you are ready to use the USB-to-Serial adapter!

Some software will not allow connection to higher COM port numbers. If you need to change the COM port number assigned to your USB adapter, you can do so in the Windows device manager.