Why USB devices need a HOST. |
Go back and buy one. --- thanks!
> Hi Larry, > just wondering if you can help me. > I want to connect a USB moden to a LAN or RS232 interface without the need for a PC in between. > Is there any device you know of that will do this ? >The answer is no. The reason is simple once you know how USB works (which is not simple). However, there is a new cool thing on the horizon that will address this limitation of USB. It's called OTG. OTG is an extention of the USB 2 specification although as of August 2002 and I've not seen any devices with OTG yet, but then - I don't get out much. More about this later, but first - let's talk a little about why USB is not anything like your fathers COM port.
OK, so why can't we just connect USB things to a COM port, isn't serial just serial? There are many reasons. Not only are the voltage levels very different, USB defines a complex communications protocol that is used between a host and the devices. USB requires a USB HOST. The host has a number of complex tasks it must be able to do to manage the Universal Serial Bus (USB). "Bus" is a key word here. Like the bus on a mother board, many things can attach to it, in parrallel, at the same time and the all need to be coordinated by someone - in this case the host is the master of the bus and tells everyone when they can use the bus and when they can't. When a new device it added to the bus, it must play a little "mother may I" game with the host. The host will give it a name and negociate with it about lots of stuff. On one single 4 wire USBus can be attached up to 128 USB devices at the same time. And the bus supplies power to them! The 2 data lines are "differential" and balanced I think. And what's more, the communication speed on this bus is 12 mega-baud for USB 1. USB 2 is an astounding 480 mega-baud! On top of all of that, as if that was not incompatable with everything that every existed before it, all communication between the HOST and the devices is done with well defined packets of data. You can attach low speed devices together with high speed devices and they can appear to all work and talk at the same time. The devices can be plugged in and unplugged without turning off the power! It's very good stuff. Each device has an identifier. A VID (Vendor ID) and a PID( Product ID) so the host knows what to expect and can locate a driver for it. If the device is unknown to the host, it will ask you for a driver. You can start to see why a host is needed. And each device needs to be very smart to be a USB device, to ID itself, to follow directions and respond to requests from the host. This stuff is not easy, and inside each device is one or more computers with memory and specialized controllers.
On the horizon for USB.
is On-The-Go! OTG is an extention of the new spec for USB 2.0 which says that
a device, with OTG, can be a device, or become a limited host. This new capability will
let a USB device connect to another USB device without a host. New (backwards compatable)
plugs are designed and a way for two OTG devices to decide who will be the host has
been worked out. I've not seen one yet but it's sure to come because it will let just
about every palm thing connect with every other palm thing. Cameras to phones to PDA's
to GPS's to PC's to you-name-it. Very exciting stuff. If you need a reason to stick around
here, this is it, ... but save your money as like other cool new high tech things it'll be
expensive at first. I'm drinking 2 beers a day (to stay healthy) and saving up my money.
mybest
larry