How can we use a usb port (in a laptop) for general purpose input/output operations, such as robotics control??

I want to control an external hardware such as a robot or say simply an array of LEDs using my laptop's USB port. Is there a way to do so. If not can we use some other ports like Serial port or RS-232 cable?


Basically you need something that will know how to talk USB at the other end of the line. Same thing goes for any other port, but USB itself is pretty complicated, so I'll focus a bit more on it.

Unlike simple serial or parallel ports, doesn't just send data over to the other side. Instead, there's a large (well compared to RS-232) amount of communication between the device and the computer and the device needs to identify itself to the computer and so on and so on.

So if you aren't particularly interested in how exactly USB works and just want to use it to control something, you have two basic approaches: Get some sort of converter, such as a USB to serial port chip (FTDI makes may types of such chips and is very popular) or cable and then use USB just to send serial commands from the computer. The second approach is to get a microcontroller which can talk USB. There are many such microcontrollers today and there are libraries available that will allow you to simply program a microcontroller to work as a USB device. There are also numerous PC side examples which you could use to work with your micro. The bad side of this is that if you want to go a bit beyond what pre-made libraries offer you'll either have to go the serial to USB converter way (and it's not difficult to implement a virtual serial port inside of a microcontroller) or to learn how to work with USB, which is difficult.

With traditional serial port, what you have is basically just a few wires which are toggles between various states by the computer. It's much easier to program it on both the PC side and on microcontroller side. Also, since you can directly control pin states, you don't have to use microcontrollers at all. You can simply build a circuit which will react when a certain pin state goes high or low and that's it.

Do note that traditional PC serial ports use RS-232 signaling levels, so zero is positive voltage and one is negative. Also the voltages are pretty high at around 12 V.

Another type of port that was extremely popular with hobbyists in the past, but is getting rare today, is the parallel port. It's main advantage is that you basically have an 8 bit bus which you can control, so you have much more pins you can directly toggle from computer. Main problem with it today is that you'd basically have to get a parallel port card for new computers since many do not have it anymore on their motherboard and the USB to parallel port converters often only work for printer use. Another problem are drivers, since in the post Windows XP era, drivers which allow you to directly experiment with the port are a bit rare.

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There are lots of approaches and interface projects for accomplishing this, so you may get many different answers - I'll give one that I would use.

It would be quite difficult to do this without using some type of interfacing circuit. The reason is that a USB port is a Serial port. All the information is sent through a single communication wire. It would be necessary to "unpack" (or deserialize) the information being sent.

To do this, we often employ a microcontroller. A good example would be the Arduino (http://arduino.cc/en/Main/ArduinoBoardUno). This microcontroller can be programmed by you using a development tool (http://arduino.cc/en/Main/Software#toc1) to understand the information being sent to it over the USB connection.

In turn, you would then decide how to use that information, such as turning on an LED or advancing a motor. Many examples of how to interface an Arduino to the real world are discussed here.

In general, here's a simple overview of how it would work...


simulate this circuit – Schematic created using CircuitLab

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