Wednesday, October 13, 2010

USB Info: Frequently Asked Questions

USB Cables, Connectors, and Networking with USB

Q1: How long of a cable can I use to connect my device?
A1: In practice, the USB specification limits the length of a cable between full speed devices to 5 meters (a little under 16 feet 5 inches). For a low speed device the limit is 3 meters (9 feet 10 inches).

Q2: Why can't I use a cable longer than 3 or 5m?
A2: USB's electrical design doesn't allow it. When USB was designed, a decision was made to handle the propagation of electromagnetic fields on USB data lines in a way that limited the maximum length of a USB cable to something in the range of 4m. This method has a number of advantages and, since USB is intended for a desktop environment, the range limitations were deemed acceptable. If you're familiar with transmission line theory and want more detail on this topic, take a look at the USB signals section of the developers FAQ.

Q3: How far away from a PC can I put a USB device?
A3: With the maximum of 5 hubs connected with 5m cables and a 5m cable going to your full speed device, this will give you 30m of cable (see section 7.1.19 for details). With a low speed device, you will be able to get a range up to 27m, depending on how long the device's cable is. With a straightforward cable route, you will probably be able to reach out 25m or so from the PC.

Q4: I need to put a USB device X distance from my PC. What do I do?
A4: If X is less than 25m or so (see previous question), buy a bunch of hubs and connect them serially with 5m cables. If you need to go farther than that, put another PC, or maybe a laptop, out where you need the device to be and network it with the first PC using something that's intended to be a long-range connection, such as Ethernet or RS-485. If you need to use nothing but USB, consider using USB based Ethernet adapters to hook the PCs together.

Q5: How can I connect two PCs to each other with USB?
A5: You need a specialized USB peripheral known as a USB bridge (sometimes called a USB to USB adapter) to do this. Without a USB bridge, connecting a USB port of a PC directly to another PC's USB port can damage the computers.

Q6: You mean I can't make a direct cable connection like a null modem?
A6: Correct. In fact, if you try this with an illegal A to A USB cable, you'll short the two PCs' power supplies together, possibly destroying one or both machines or causing a fire hazard. Even there were no danger to the machines from the problem with two power supplies, there still wouldn't be any way to get the two PCs talking to each other, since USB doesn't support that particular kind of communication. A reasonably priced solution to handle this need is the USB bridge.

Q7: So why do people make A to A cables, anyway? What kinds of cables do I need to connect USB devices together?
A7: A number of cable vendors seem to have reached the conclusion that USB is like a PC's serial port, only faster, so you need all sorts of special hardware to create the USB connection you need to make. This is completely incorrect. The only kind of cables you'll ever need to connect normal USB products are A to B cables, A to mini B cables or mini A to mini B cables of various lengths. Some special kinds of devices use nonstandard connectors and so come with their own special cable.

Q8: What if I want to network a whole bunch of PCs together with USB?
A8: If you need to connect just a few machines, USB bridges and a hub or two will work. USB was not designed to be a LAN, however, and there are certain safety hazards associated with trying to use USB with large numbers of PCs. There's also a large performance penalty compared to a real LAN. If you need a LAN, use a technology intended to be used as a LAN, such as Ethernet.

Q9: Is there any way for two PCs to share a USB printer or some other peripheral?
A9: At the moment, the simplest and best solution is just to plug the printer into whatever PC needs to use it. You can make this somewhat easier by plugging any peripherals you want to share into a hub, and running cables from the PCs to the hub, so all you have to do is change which PC the hub is plugged into.

Q10: Is there any way I can put a USB device on a network, like a network printer?
A10: To do this, you'd need something like a USB to Ethernet bridge that was capable of acting as a USB host.



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