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MSD is helpful in at least determining whether your serial ports are responding. If MSD cannot determine the existence of a port, it does not provide the report indicating that the port exists. This sort of look-and-see test is the first action I usually take to determine why a port is not responding. Windows 95 also shows whether or not your ports are functioning. To check your ports, right-click Your Computer and choose Properties. Choose the Device Manager tab. On the Device Manager screen, if a device is not working properly there will be an exclamation point in a yellow circle next to the device on the list. You can also double-click Ports (COM & LPT), and then double-click the desired port to see whether Windows 95 says that the port is functioning or not. In many cases, it tells you what is conflicting with that specific port. In Windows 98, the Microsoft System Information Utility will provide another way to look at additional modem and port information. To see this, click Start, Programs, Accessories, System Tools, System Information. The System Information window has a tree on the left to choose items in and displays information on the right about the selected device or category. In this tree on the left, you should be familiar with the I/O and IRQ items in the Hardware Resources branch and the Modem and Ports items in the Components branch to troubleshoot modems. Advanced Diagnostics Using Loopback Testing One of the most useful tests is the loopback test, which can be used to ensure the correct function of the serial port, as well as any attached cables. Loopback tests basically are internal (digital), or external (analog). Internal tests can be run simply by unplugging any cables from the port and executing the test via a diagnostics program. The external loopback test is more effective. This test requires that a special loopback connector or wrap plug be attached to the port in question. When the test is run, the port is used to send data out to the loopback plug, which simply routes the data back into the ports receive pins so that the port is transmitting and receiving at the same time. A loopback or wrap plug is nothing more than a cable doubled back on itself. Most diagnostics programs that run this type of test include the loopback plug, and if not, these types of plugs can be purchased easily or even built. The wiring that is needed to construct your own loopback or wrap plugs is as follows:
Future Serial Port ReplacementsTwo new high-speed serial-bus architectures for desktop and portable are becoming available, called the Universal Serial Bus (USB) and IEEE 1394. These are high-speed communications ports that far outstrip the capabilities of the standard serial ports most systems contain today, and may be used as an alternative to SCSI for high-speed peripheral connections. In addition to performance, these new ports will offer I/O device consolidation, meaning all types of external peripherals will connect to these ports. We will only examine USB here as it will be the bus-type that is more widely used for modems in the future. The recent trend in high-performance peripheral bus design is to use a serial architecture, where one bit is sent at a time down a wire. Parallel architecture uses 8, 16, or more wires to send bits simultaneously. At the same clock speed, the parallel bus is faster; however, it is much easier to increase the clock speed of a serial connection than a parallel one. Parallel connections suffer from several problems, the biggest being signal skew and jitter. Skew and jitter are the reasons that high-speed parallel busses like SCSI are limited to short distances of 3 meters or less. The problem is that although the 8 or 16 bits of data are fired from the transmitter at the same time, by the time they reach the receiver, propagation delays have conspired to allow some bits to arrive before the others. The longer the cable, the longer the time between the arrival of the first and last bits at the other end! This signal skew, as it is called, either prevents you from running a high-speed transfer rate, a longer cable, or both. Jitter is the tendency for the signal to reach its target voltage and float above and below for a short period of time. With a serial bus, the data is sent one bit at a time. Because there is no worry about when each bit will arrive, the clocking rate can be increased dramatically. With a high clock rate, parallel signals tend to interfere with each other. Serial again has an advantage in that with only one or two signal wires, crosstalk and interference between the wires in the cable is negligible. Parallel cables are very expensive. In addition to the many additional wires needed to carry the multiple bits in parallel, the cable also needs to be specially constructed to prevent crosstalk and interference between adjacent data lines. This is one reason external SCSI cables are so expensive. Serial cables, on the other hand, are very inexpensive. For one thing, they have very few wires, plus the shielding requirements are far simpler, even at very high speeds. It is for these reasons, plus the need for new Plug and Play external peripheral interfaces, as well as the elimination of the physical port crowding on portable computers, that these new high-performance serial busses have been developed. Both USB and 1394 are available on desktop and portable PCs today.
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