USB Interface

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USB

USB (Universal Serial Bus) has become the universal connection for "personal" printers.  The connection is simple to handle. The connectors are robust with a metal shield arround them. Devices are hot pluggable, there is no risk of electrical damage from plugging things in whilst they are powered up.  New devices are identified and connected to the driver within a few seconds on recent computers running Microsoft XP or Linux.
  • USB is fast - USB 2.0 transfers up to 480 million bits per second and even the USB 1.1 provided on older devices usually manages 12mbps. The low speed is adequate for basic printers.
  • Cables can be up to 5 metres long and can be extended to 30 metres with active extensions.
  • The connection is bidirectional - the printer becomes part of the computer it is connected to so that the control software can see precisely what is going on.
USB is electrically similar to a network standard – it connects to hubs using a bi-directional wire pair and half-duplex differential signalling (full duplex is introduced with USB 3.0). A protocol negotiates how information is passed.

USB normally uses an oblong "A" plug at the computer host end of the cable and a squarish "B" plug at the device end. Printers usually do use the standard "B" plug.  Lots of handheld devices use a Mini-B USB receptacle which helps where space is short, but most printers are big enough that space is not an issue.

Getting the correct cable might be an issue because in a fit of meanness the printer industry has more or less standardised on not supplying one. Users have to buy the cable seperately. This may be an attempt to give stores an incentive to sell printers - they make the profit on the cable! It's a wierd world.
 
There are four signaling rates on USB devices:

USB 1.1 provided two speeds:

Low speed is 1.5 million bits per second. The reason for this low speed seems to have been the possibility of using cheap cable on keyboards, mice and joysticks. It may also reduce their power budget a bit because slower chips tend to use less power.

Full speed is 12 million bits per secon and is the basic rate for USB.

Hi-Speed was introduced with USB.20 in 2001. Singalling is at 480 million bits per second using the same cables and plugs as  for full speed. Hi-speed devices are expected to fall back to full-speed if they cannot negotiate a hi-speed connection.

Super Speed is introduced by USB 3.0 and is at 4.8 gigabits per second. Connectors are partly backward compatible.

Printers

Printers are full-speed or hi-speed. Printer manufacturers have  a knack of describing  the printers as  USB 2.0 compatible  but in some cases that seems to mean they are actually full-speed not hi-speed; its just that the implementation is compatible with other features of USB 2.0. At the low cost end of the printer market where this happens it probably makes no difference as any limitation in speed is the printer, not the connection.

Printers are usually connected directly to the computer with a single cable

A USB computer interface can support up to 127 USB devices cascaded via 5 tiers of hubs. In practice it is normal to find that computers have three or four USB Host Interfaces and that a  printer, scanner and perhaps a couple of disk drives are directly attached. The bandwidth of the USB bus is shared - so if a user does have several disks and a high speed printer on one bus installing an extra USB host controller for the printer is likely to improve performance.

USB is not entirely like network standards such as Ethernet, for instance it doesn't need a lot of user provided set-up information. Typically the user just selects "install printer" from a CD before plugging the new device in for the first time. On Linux systems most commonplace printers are already available.

Although USB looks somewhat like a network it is electrically very different. The USB device becomes part of the host computer it is attached to.  In principle a USB device can work entirely using the host computers memory and processing power. In practice even the most rudimentary printer has some sort of microprocessor of it's own. Workgroup laser printers just treat the USB as an interface and exchange a conventional language across it then do the Raster Image Processing themselves.

Low cost USB printers commonly do expect the host computer to provide most of their processing power. Large, high esolution colour images may slow the host computer down markedly which is likely to be a problem if the printer is shared over a network. The user with the printer is likely to complain. Because USB printers come and go as the PC is powered up and down and to slow down when other devices are plugged in other users are likely to complain as well. USB printers are suitable for one or two users - not a "workgroup".  Printers intended for workgroups are likely to work better using an Ethernet port. Ethernet Ports on Printers

There are some USB print servers. Zyxel's NPS 520 print server works with printers and multifunction machines The Draytek Vigor 2800 series provide a print server.  Larger groups of users will benefit from using Ethernet to USB  print servers rather than plugging the USB into a users computer.
 
 
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© Graham Huskinson 2010

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