The exact nature of the signals on these ports -- what values will appear and when, is called a protocol. Here are some of the things that a protocol specifies:

• the number of bits used for data accepted and data ready, and what their positions are in the control byte
• the exact sequence of values in the control byte
• who should set which bytes and when
• what character code will be used for the data (ASCII, EBCDIC, other)
• how the device tells the CPU when its done
• how the device tells the CPU about errors or abnormal conditions
• how the CPU gives commands to the device (start, stop, give me your status, wait for a while, etc.)
• how the CPU discovers when the device is out of data

Then there are the even messier but equally important details of the physical protocol:

• how many pins the connectors have
• what the voltages are on each pin
• the shape of the connector
• other electrical details such as how long the wires can be from connector to device

All these things must be decided upon and published so that both CPU and peripheral manufacturers know what to build so that their devices will talk to each other.

SCSI is an example of a standard. SCSI, pronounced "scuzzie," stands for noble concept: Small Computer Systems Interface. It specifies connector shapes, voltages, and a protocol. SCSI disk drives are disk drives whose controller adheres to the SCSI protocol and whose external connectors are the right shape for SCSI plugs. There are SCSI CD-ROMs, tape drives and printers.

Other disk drives and other peripherals might be specific to a certain manufacturer's standard. For example, many disk drives, printers and mice are specific to one type of personal computer. Such a peripheral built for the Macintosh cannot be used with an Intel-based machine, like a Gateway or a Packard-Bell. Apple Desktop Bus (ADB) is one standard specific to Macintoshes. Though the guts of the disk drive or the printer or the mouse might be identical from one peripheral to the next, their connectors, which is their electrical interface to the world, are built to one standard or another.

In recent years, a new and very fast serial bus, called the Universal Serial Bus or USB, has become standard on most personal computers. It is vendor-independent and standardized so that devices built by many companies can be used on Macs or PCs or UNIX workstations. One great thing about USB is that it permits a tree of connections to be made, not just a daisy-chain, permitting up to 127 devices to be connected to a computer with very fast response time. The data rate on a USB wire is 12 MBps (12 megabits per second) which is faster than the original Ethernet speed of 10 MBps!

Even faster serial buses are available now for extremely data intensive peripherals like videocameras. The Firewire bus is an example of this.