Serial Versus Parallel Information Transfer

Information flows through the computer in many ways. The CPU is the central point for most information. When you start a program, the CPU instructs the storage device to load the program into RAM. When you create data and print it, the CPU instructs the printer to output the data.

Because of the different types of devices that send and receive information, two major types of data transfers take place within a computer: parallel and serial. These terms are used frequently, but if you're not familiar with the differences between them, check out Figure 3.6.

Figure 3.6. Parallel data transfers move data 8 bits at a time, whereas serial data transfers move 1 bit at a time.

Parallel Information Transfers

Parallel transfers use multiple " lanes " for data and programs, and in keeping with the 8 bits = 1 byte nature of computer information, most parallel transfers use multiples of 8. Parallel transfers take place between the following devices:

• CPU and RAM

• CPU and interface cards (see Chapter 8)

• LPT (printer) port and parallel printer

• SCSI port and SCSI devices

• ATA / IDE host adapter and ATA/IDE drives

• RAM and interface cards (either via the CPU or directly with DMA)

Why are parallel transfers so popular?

• Multiple bits of information are sent at the same time.

• At identical clock speeds, parallel transfers are faster than serial transfers because more data is being transferred.

However, parallel transfers also have problems:

• Many wires or traces (wire-like connections on the motherboard or expansion cards) are needed, leading to interference concerns and thick, expensive cables.

• Excessively long parallel cables or traces can cause data to arrive at different times. This is referred to as signal skew (see Figure 3.7).

  Figure 3.7. Parallel cables that are too long can cause signal skew, allowing the parallel signals to become "out of step" with each other.

  

• Differences in voltage between wires or traces can cause jitter .

As a result of these problems some compromises have had to be included in computer and system design:

• Short maximum lengths for parallel, ATA/IDE, and SCSI cables

• Dual-speed motherboards (running the CPU internally at much faster speeds than the motherboard or memory)

Fortunately, there is a second way to transmit information: serial transfers.

Serial Transfers

A serial transfer uses a single "lane" in the computer for information transfers. This sounds like a recipe for slowdowns, but it all depends on how fast the speed limit is on the "data highway ."

The following ports and devices in the computer use serial transfers:

• Serial (also called RS-232 or COM) ports and devices

• USB (Universal Serial Bus) 1.1 and 2.0 ports and devices

• Modems (which can be internal devices or can connect to serial or USB ports)

• IEEE-1394 ( FireWire , i.Link) ports and devices

• Serial ATA (SATA) host adapters and drives

Serial transfers have the following characteristics:

• One bit at a time is transferred to the device.

• Transmission speeds can vary greatly, depending on the sender and receiver.

• Very few connections are needed in the cable and ports (one transmit, one receive, and a few control and ground wires).

• Cable lengths can be longer with serial devices. For example, an UltraDMA/66 ATA/IDE cable can be only 18 inches long for reliable data transmission, whereas a Serial ATA cable can be almost twice as long.

Although RS-232 serial ports are slow, newer types of serial devices are as fast or faster than parallel devices. The extra speed is possible because serial transfers don't have to worry about interference or other problems caused by running so many data lines together.

For more information about serial, parallel, USB, and IEEE-1394 ports, see Chapter 8, "Input/Output Devices and Cables." For more information about RAM, see Chapter 7, "RAM." For more information about ATA/IDE, Serial ATA, and SCSI, see Chapter 14, "Storage."