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The 16550 UART was the first serial chip used in the PS/2 line. This chip could function as the earlier 16450 and 8250 chips, but it also included a 16-byte buffer that aided in faster communications. This is sometimes referred to as a FIFO (first in/first out) buffer. Unfortunately, the 16550 also had a few bugs, particularly in the buffer area. These bugs were corrected with the release of the 16550A UART, which h is used in all high-performance serial ports.
Because the 16550A is a faster, more reliable chip than its predecessors, it is best to look for serial ports that use it. If you are in doubt about which chip you have in your system, you can use the Microsoft MSD program (provided with Windows, MS DOS 6.X, and Windows 95) to determine the type of UART you have. Another way to tell if you have a 16650 UART in Windows 95 is to right-click My Computer, and then click Properties. This brings up the System Properties dialog box. Choose the Device Manager tab, Ports, and then the communications port that you want to check. Choose the Port Settings tab and then click the Advanced button. This will bring up the Advanced Port Settings box. If you have a 16650 UART, there will be a check mark in the use FIFO Buffers option. The original designer of these UARTs is National Semiconductor (NS). Many other manufacturers are producing clones of these UARTs, such that you probably dont have an actual NS brand part in your system. Even so, the part you have will be compatible with one of the NS parts, hopefully the 16550. In other words, you should check to see that whatever UART chip you do have does indeed feature the 16-byte FIFO buffer as found in the NS 16550 part. Some manufacturers have also begun making integrated chips which take on the functions of multiple chips. Boca Research for instance sells serial and parallel cards with little more than one Integrated Circuit (IC) on them. Most of these integrated chips function as a 16550 would; however, you should make sure that they have 16550 compatibility before purchasing them. Table 1.4 lists the standard UART chips used in IBM and compatible systems.
High-Speed Serial PortsSome modem manufacturers have gone a step further on improving serial data transfer by introducing Enhanced Serial Ports (ESP) or Super High Speed Serial Ports. These ports enable a 28,800 bps modem to communicate with the computer at data rates up to 921,600 bps. The extra speed on these ports is generated by increasing the buffer size. These ports are usually based on a 16550AF UART or a 16550AF UART emulator with dual 1,024-byte buffers and on-board data flow control, and can provide great benefit in an environment where both your computer and the receiving computer are equipped with these ports. Otherwise, just one of the computers having an ESP doesnt yield any benefit. As the need for additional serial devices continues to increase, users are beginning to need more than the standard two com ports which are standard in PCs. As a result, multi-port serial cards were created. These cards generally have 232 ports on them. Often they also provide higher baud rates than can be achieved on a standard serial port. Most of the multi-port serial cards on the market use standard 16550 UARTs with a processor (typically an 80×86 based processor) and some memory. These cards can improve performance slightly because the processor is dedicated to handling serial information. However, its not always the best method for high-performance applications. Some of the better multi-port serial cards have broken the model of the 16550 UART in favor of a single integrated circuit. These cards have the advantage of higher sustainable throughput without loss. One such card is the Rocketport by Comtrol. It comes in ISA and PCI versions with up to 32 ports. Each port is capable of 232K baud sustained.
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