Cabling Concerns

Except for internal modems, serial and parallel devices require cables. Figure 16.1 shows several types of serial and parallel cable connectors for reference. With the exception of the USB Series B and Centronics parallel port connectors, all these connectors plug into ports on the back of x86 computers. The USB Series B and Centronics parallel port connectors both connect to devices, such as printers, that interface via the USB and parallel ports, respectively.

USB ports also require interrupts, but the interrupt assigned to USB ports varies substantially 16

from one machine to another. Because most USB ports are PCI devices that are built in to the motherboard, it's usually not necessary to explicitly configure a USB port's interrupt. S A

The term I/O port can be confusing, because it can refer either to an interface into which you can plug a device or to a small area of memory used by the x86 architecture to transfer data between the CPU and its devices. Serial and parallel ports, like other hardware, require access to certain I/O port ranges, as detailed in Table 16.1.

Like shared interrupts, shared I/O ports on serial ports can be a problem. You should be sure to change the I/O ports for such serial devices in much the same way as you change their inter-rupts—either disable a built-in device and let the add-on device take over the built-in device's settings, or assign an unused I/O port to the device. You can find a list of used I/O ports in Linux by typing cat /proc/ioports. This command produces output similar to the following:

Part IV

2-to-25 pin RS-232

serial cable USB series B adapter connector

9-pin RS-232 serial connection

USB series A 25 pin parallel Centronics connector port connector parallel port connector

9-pin RS-232 serial connection

USB series A 25 pin parallel Centronics connector port connector parallel port connector

Figure 16.1

Serial and parallel cable connectors come in a wide variety of shapes and sizes.

Because cables are well standardized, there are few caveats when cable shopping. There are a few, however, including

• IEEE 1284 parallel cables Many printers require cables that conform to the IEEE 1284 standards. These standards specify a minimum number of pins that must be connected for bidirectional communication. Some cheaper cables might not conform to the IEEE 1284 standards. Such cables often work with earlier printers, but not many later models.

• Modem versus null modem cables A null modem cable is one that's designed to connect two computers via their serial ports. This cable must be wired differently than a cable to connect a computer to a modem (often called a modem cable, or a straight-through cable). With Ethernet so common, the need for null modem cables is slim, but these cables are still sold, so you should be careful not to buy one if you need a regular modem cable.

Null modem cables usually contain two 9- or 25-pin female plugs, whereas straight-through cables usually have one 9- or 25-pin female plug and one 25-pin male plug. You should generally go by the description on the cable's packaging, but if you have loose cables lying around, this rule of thumb can help you identify them.

Chapter 16

• Number of pins Be sure the cables you buy have the correct number of pins. If they don't, you'll need an adapter like the one shown in Figure 16.1. These adapters can cost almost as much as a new cable.

Some cables use unusual connectors on the device end, in order to interface with unusual devices. Palmtop computers and digital cameras, for instance, often use extra-small connectors in order to save space. These cables tend to be expensive and difficult to find, so you should be careful not to lose or damage them.

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