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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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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.


Tip:  
The 16550 UART chip is pin-for-pin compatible with the 16450 UART. If your 16450 UART is socketed, it is a cheap and easy way to improve serial performance to install a 16550 UART chip in the socket.

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 don’t 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.

Table 1.4 UART Chips in PC or AT Systems.

Chip Description

8250 IBM used this original chip in the PC serial port card. The chip has several bugs, none of which is serious. The PC and XT ROM BIOS are written to anticipate at least one of the bugs. This chip was replaced by the 8250B.
8250A Do not use the second version of the 8250 in any system. This upgraded chip fixes several bugs in the 8250, including one in the interrupt enable register, but because the PC and XT ROM BIOS expect the bug, this chip does not work properly with those systems. The 8250A should work in an AT system that does not expect the bug, but does not work adequately at 9600 bps.
8250B The last version of the 8250 fixes bugs from the previous two versions. The interrupt enable bug in the original 8250, expected by the PC and XT ROM BIOS software, has been put back into this chip, making the 8250B the most desirable chip for any non-AT serial port application. The 8250B chip may work in an AT under DOS, but does not run properly at 9600 bps.
16450 IBM selected the higher-speed version of the 8250 for the AT. Because this chip has fixed the interrupt enable bug mentioned earlier, the 16450 does not operate properly in many PC or XT systems, because they expect this bug to be present. OS/2 requires this chip as a minimum, or the serial ports do not function properly. It also adds a scratch-pad register as the highest register. The 16450 is used primarily in AT systems because of its increase in throughput over the 8250B.
16550 This newer UART improves on the 16450. This chip cannot be used in a FIFO buffering mode because of problems with the design, but it does enable a programmer to use multiple DMA channels and thus increase throughput on an AT or higher class computer system. I highly recommend replacing the 16550 UART with the 16550A.
16550A This chip is a faster 16450 with a built-in 16-character Transmit and Receive FIFO buffer that works. It also allows multiple DMA channel access. You should install this chip in your AT system serial port cards if you do any serious communications at 9600 bps or higher. If your communications program makes use of the FIFO, which most do today, it can greatly increase communications speed and eliminate lost characters and data at the higher speeds.

High-Speed Serial Ports

Some 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 doesn’t 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 2–32 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, it’s 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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