No not the kind that you ride on to get to the shops but the ones that your data rides on to get between the motherboard and the other devices that you have either in or connected to your computer. There are a number of these buses that your computer uses to pass data between the components of your computer and they work in slightly different ways. Here I am going to introduce you to three of these buses and give you some idea what they do.
The first bus I want to cover is the IDE (Integrated Drive Electronics) bus. Your computer probably has two of these which are known as the primary and secondary. Each IDE bus can have two devices connected to it known as the master and the slave. This means that you can connect up to four IDE (or ATAPI) devices to these buses. The most common IDE devices are hard disks and the disk drive(s) in your computer are most likely of this type.
Other devices such as CD-ROM drives, DVD drives, and ZIP drives use an interface called ATAPI to interface with the IDE bus. The speed of the IDE buses have increased over the years from the original 33 to 66, 100, and now 133. On many motherboards the secondary IDE bus may not support as high a speed as the primary.
Usually the IDE sockets on the motherboard are colour coded so that you can tell the supported speed of each. The IDE bus can only communicate with one device on each bus at a time so if you expect to regularly be copying information between two IDE devices you will be better off to have one on the primary bus and one on the secondary. Your computer is probably configured to load the operating system from a hard drive connected as master to the primary IDE bus although modern computers can also boot from a CD-ROM. One final note regarding the IDE bus is that the total cable length that can be used with IDE devices is very short meaning that all IDE devices will need to be fitted inside the computer case unless they are fitted into a conversion box that allows them to be connected using one of the other two buses that I am about to discuss.
The next bus that I am going to mention is the SCSI (Small Computer System Interface) bus. Most PCs need to have a separate SCSI card fitted if they are to provide a SCSI bus. SCSI can have all the same types of devices connected to it as an IDE bus does however the devices have to be designed to attach via the SCSI bus so they are not interchangeable. The SCSI bus also supports other devices such as tape drives, scanners etc which can also be connected to the SCSI bus. The SCSI bus can have up to seven devices attached to one bus and with wide SCSI you can have as many as fifteen devices attached to the one bus. SCSI devices can either be fitted inside of your computer case where they connect to an internal connector on the SCSI card or they can be connected to an external socket on the back of the card. SCSI comes in several versions including SCSI 1 which uses a 25 pin external connector, SCSI 2 which uses a 50 pin external connector, and SCSI 3 which uses a 68 pin external connector.
You can get conversion cables to convert between these versions but if connecting multiple devices this way the cable ends with the higher number of pins needs to be nearer the computer. Unlike IDE the SCSI bus allows multiple devices to communicate with the motherboard (or each other) at the same time. SCSI devices are usually daisy chained together (which means that one device is plugged into the next) and each external device has two connectors to allow for this to be done. SCSI devices are given addresses on the SCSI bus by assigning each device a separate number. These addresses can be set using jumpers or switches on each device and each device on the bus needs to be set to a different address. Most SCSI buses give priority to the lowest numbered address but a few work the other way around.
Finally, I will introduce you to USB (Universal Serial Bus). This provides a much slower interface than either the IDE or SCSI bus but allows a much larger range of devices to be attached including cameras, printers, modems, networks, and even keyboards and mice (although I do not recommend using it with these last two). Some USB devices have even been designed to be powered via the bus however if you have a lot of USB devices you hav want to look at providing additional power sources for some of the devices. A single bus can support up to 127 devices and the original intention was to have devices provide two connections so that they can be daisy chained together just like SCSI.
Most USB devices do not have the second connector fitted so instead of being able to plug the devices into one another you need instead to get USB hubs which convert one connector into four or more connectors (and in some cases can supply an additional power source). Most computers today have two USB connectors on the back of the motherboard and a few even provide connectors on the front of the case. All of these connectors are attached to the same bus. USB shares the communication link between all of the devices attached to the bus so long before you get anywhere near the 127 device maximum all of the attached devices will have become so slow as to be unuseable. A new faster version of USB (called USB 2) has recently been introduced and is supported by some motherboards. It is backwards compatible with existing USB devices.
So that's a brief intro to three of the buses that you can use to attach devices to your computer. Hopefully this will assist you in deciding which is most appropriate to use when next you want to buy a new device to attach to your computer.
There is one other bus that you might come across when looking at devices and that is the firewire bus. This is a faster alternative to USB although there is not such a large range of devices available to use with it. Like SCSI you will need a special interface card if you want to use devices that use this bus. Most digital video devices are designed to use this bus.
Stephen Chapman runs a computer help website at felgall.com/
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