CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [303]
Figure 18-1 Serial port
So what type of settings do you need to configure on a serial port? Find a PC with a real serial port (a real 9-pin connector on the back of the PC). Right-click the COM port and choose Properties to see the properties of that port in Device Manager. Open the Port Settings tab and click the Advanced button to see a dialog box that looks like Figure 18-2.
Devices such as modems that have built-in serial ports don’t have COM port icons in Device Manager, because there’s nothing to change. Can you see why? Even though these devices are using a COM port, that port is never going to connect to anything other than the device it’s soldered onto, so all of the settings are fixed and unchangeable—thank goodness!
When you are configuring a serial port, you will have a lot of different settings to configure, many which may or may not make sense. The convenient part about all this is that when you get a new serial device to plug into your serial port, the instructions will tell you what settings to use. Figure 18-3 shows an instruction sheet for a Cisco switch.
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NOTE If you need a serial port to support some older device, but have a motherboard that doesn’t have one, don’t fret. You can always get a PCI expansion card with classic, 9-pin serial ports.
Figure 18-2 Serial port settings
Figure 18-3 Serial port instructions
USB Ports
You should be familiar with the concept of USB, USB connectors, and USB hubs from the discussion of those concepts in Chapter 3, “The Visible PC.” Here’s a more in-depth look at USB and some of the issues involved with using USB devices.
Understanding USB
The cornerstone of a USB connection is the USB host controller, an integrated circuit that is usually built into the chipset and controls every USB device that connects to it. Inside the host controller is a USB root hub: the part of the host controller that makes the physical connection to the USB ports. Every USB root hub is really just a bus—similar in many ways to an expansion bus. Figure 18-4 shows a diagram of the relationship between the host controller, root hub, and USB ports.
Figure 18-4 Host controller, root hub, and USB ports
No rule says how many USB ports a single host adapter may use. Early USB host adapters had two USB ports. The most recent ones support up to ten. Even if a host adapter supports a certain number of ports, there’s no guarantee that the motherboard maker will supply that many ports. To give a common example, a host adapter might support eight ports while the motherboard maker only supplies four adapters.
The most important point to remember about this is that every USB device connected to a single host adapter/root hub shares that USB bus with every other device connected to it. The more devices you place on a single host adapter, the more the total USB bus slows down and the more power they use. These issues are two of the biggest headaches that take place with USB devices in the real world.
USB devices, like any electrical device, need power to run, but not all take care of their own power needs. A powered USB device comes with its own electrical cord that is usually connected in turn to an AC adapter. Bus-powered USB devices take power from the USB bus itself; they don’t bring any AC or DC power with them. When too many bus-powered devices take too much power from the USB bus, bad things happen—devices that work only some of the time and devices that lock up. You’ll also often get a simple message from Windows saying that the hub power has been exceeded and it just won’t work.
Every USB device is designed to run at one of three speeds. The first USB standard, version 1.1, defined two speeds: Low-Speed USB, running at a maximum of 1.5 Mbps (plenty for keyboards and mice), and Full-Speed USB, running up to 12 Mbps. Later, the USB 2.0 standard introduced Hi-Speed