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Various manufacturers make versions of the 16550A; National Semiconductor was the first. Its full part number for the 40-pin DIP is NS16550AN or NS16550AFN. Make sure that the part you get is the 16550A, and not the older 16550. You can contact Frys Electronics or Jameco Electronics to obtain the NS16550AN, for example. Serial-Port ConfigurationEach time a character is received by a serial port, it has to get the attention of the computer by raising an Interrupt Request Line (IRQ). Eight-bit ISA bus systems have eight of these lines, and systems with a 16-bit ISA bus have 16 lines. The 8259 interrupt controller chip usually handles these requests for attention. In a standard configuration, COM1 uses IRQ4, and COM2 uses IRQ3. When a serial port is installed in a system, it must be configured to use specific I/O addresses (called ports), and interrupts (called IRQs for Interrupt ReQuest). The best plan is to follow the existing standards for how these devices should be set up. For configuring serial ports, you should use the addresses and interrupts indicated in Table 1.5. (For more discussion of IQR and I/O addresses, see the section System Resources later in this chapter.)
You should ensure that if you are adding more than the standard COM1 and COM2 serial ports, they use unique and non-conflicting interrupts. If you purchase a serial port adapter card and intend to use it to supply ports beyond the standard COM1 and COM2, be sure that it can use interrupts other than IRQ3 and IRQ4. Another problem is that IBM never built BIOS support for COM3 and COM4 into its original ISA bus systems. Therefore, the DOS MODE command cannot work with serial ports above COM2 because DOS gets its I/O information from the BIOS, which finds out what is installed in your system and where during the POST. The POST in these older systems checks only for the first two installed ports. PS/2 systems have an improved BIOS that checks for as many as eight serial ports, although DOS is limited to handling only four of them. To get around this problem, most communications software and some serial peripherals (such as mice) support higher COM ports by addressing them directly, rather than making DOS function calls. The communications program Procomm, for example, supports the additional ports even if your BIOS or DOS does not. Of course, if your system or software does not support these extra ports or you need to redirect data using the MODE command, trouble arises. Windows 95 has added the support for up to 128 serial ports. This allows for the use of multi-port boards in the system. Multi-port boards give your system the ability to collect or share data with multiple devices, while using only one slot and one interrupt. A couple of utilities enable you to append your COM port information to the BIOS, making the ports DOS-accessible. A program called Port Finder is one of the best, and is available in the general hardware data library of the PCHW forum on CompuServe. Port Finder activates the extra ports by giving the BIOS the addresses and providing utilities for swapping the addresses among the different ports. Address swapping enables programs that dont support COM3 and COM4 to access them. Software that already directly addresses these additional ports usually is unaffected.
Modem StandardsBell Labs and the CCITT have set standards for modem protocols. CCITT is an acronym for a French term that translates into English as the Consultative Committee on International Telephone and Telegraph. The organization was renamed the International Telecommunications Union (ITU) in the early 1990s, but the protocols developed under the old name are often referred to as such. Newly developed protocols are referred to as ITU-T standards. A protocol is a method by which two different entities agree to communicate. Bell Labs no longer sets new standards for modems, although several of its older standards are still used. Most modems built in the last few years conform to the CCITT standards. The ITU is an international body of technical experts responsible for developing data communications standards for the world. The group falls under the organizational umbrella of the United Nations, and its members include representatives from major modem manufacturers, common carriers (such as AT&T), and governmental bodies. The ITU establishes communications standards and protocols in many areas, so one modem often adheres to several different standards, depending on its various features and capabilities. Modem standards can be grouped into the following three areas:
Other standards have been developed by different companies (not Bell Labs or the ITU). These are sometimes called proprietary standards, even though most of these companies publish the full specifications of their protocols so that other manufacturers can develop modems to work with them. The following list shows some of the proprietary standards that have become fairly popular:
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