USB 1.1 and USB 2.0 specification
Introduction to USBThe USB, or universal serial bus has replaced the RS232 and parallel communications in a lot of situations. USB is now the most used interface to connect devices like printer, mice and scanners to personal computers. Many people ask if there is a simple way to convert RS232 to USB. Afterall, they are both serial interfaces. The answer is a little disappointing for most: No, RS232 and USB can't be connected by just soldering the right connector to the cable, but there are RS232 to USB adapters sold for this purpose.
Let's look at USB in some more detail. The name of USB tells us in fact a lot. Universal should't need to have a lot of explanation. The definition of USB tries to address a lot of uses of the interface. Not only communication with modems as was the case with RS232, but with all kinds of devices. The second character in USB stands for serial. This is what confuses a lot of people. Serial is not a family of interchangable communication interfaces. It just tells us that every bit of information is send in a specific time slot and that no two items of information can be send at one moment. This is not only the case for USB and RS232. Ethernet networks also communicate in a serial way. Actually most communications—even high-speed—are performed in a serial way. Parallel is the minority for the simple reason that you need more lines and thus more expensive material to transfer the data. We see parallel interfaces mostly used at short distances, like connecting a harddisk to the mainboard. The last character in USB stands for bus. This tells us something about the higher architecture allowed. USB is not designed primarly for 1:1 communication like RS232 or the parallel printer interface, but it is a bus architecture where you can attach more than two devices to.
USB versionsJust spelling the word USB has given us already a lot of information about the interface, but not enough yet to know all about it. First of all the versions. You might have heard about USB 1.1. The manual of some devices will tell you that you need at least USB 2.0 to operate the device properly. But what are those versions?
Higher USB version numbers describe the USB interface with more features and higher speed. It's simple as that. The USB interface is already under development for more than ten years. Version 0.7 of the USB interface definition was released in November 1994 and the first "real" definition of USB, USB 1.0 came out in January 1996. It was a combined effort of some large players on the market to define a new general device interface for computers. Major pushers of the project were Compaq, Intel, Microsoft and NEC.
The USB definition is in a lot of ways comparable with the RS232 definition. It doesn't only specify things like communication speeds and low level interfacing, but also protocols, and the mechanical characteristics of the connectors to be used. This made USB different from other standards that had seen the light since RS232, like the RS422 and RS485 that focussed mainly on the low level interfacing and signal definition and less on the practical implementation. The necessity for a well defined way of practical implementation has many times been overlooked by those developing standards. The main reason I started my website back in 1997 was the huge amount of different RS232 cable layouts for different purposes and the total lack of practical information about them. With USB the four parties wanted to get rid of these problems. In fact it was one of the three main motivations as described in the USB 1.1 specification.
Although I wouldn't recommend the specification documents for reading to most users as they are quite technical, this first page is interesting because it describes the main motivation to develop the new USB interface. The first motivation has—in my opinion—become obsolete. This motivation was the ability of USB of Connection of the PC to the telephone. The parties were facing a huge gap between the developments of computer and telephony industry and they thought there would be a large market in between with CTI, computer telephony integration. In fact I was also convinced about it and as you can read on my work page, I started my company mainly in this type of business.