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The <replaceable>g</replaceable> is the product grade field. If an <literal>I</literal> precedes the package-type letter, it indicates an <quote>industrial</quote> grade part, which has higher specs than a standard part but not as high as Military Specification (Milspec) component. This is an optional field.
So what we used to call a NS16550AFN (DIP Package) is now called a PC16550DN or PC16550DIN.
Other Vendors and Similar UARTs
Over the years, the 8250, 8250A, 16450 and 16550 have been licensed or copied by other chip vendors. In the case of the 8250, 8250A and 16450, the exact circuit (the <quote>megacell</quote>) was licensed to many vendors, including Western Digital and Intel. Other vendors reverse-engineered the part or produced emulations that had similar behavior.
In internal modems, the modem designer will frequently emulate the 8250A/16450 with the modem microprocessor, and the emulated UART will frequently have a hidden buffer consisting of several hundred bytes. Due to the size of the buffer, these emulations can be as reliable as a 16550A in their ability to handle high speed data. However, most operating systems will still report that the UART is only a 8250A or 16450, and may not make effective use of the extra buffering present in the emulated UART unless special drivers are used.
Some modem makers are driven by market forces to abandon a design that has hundreds of bytes of buffer and instead use a 16550A UART so that the product will compare favorably in market comparisons even though the effective performance may be lowered by this action.
A common misconception is that all parts with <quote>16550A</quote> written on them are identical in performance. There are differences, and in some cases, outright flaws in most of these 16550A clones.
When the NS16550 was developed, the National Semiconductor obtained several patents on the design and they also limited licensing, making it harder for other vendors to provide a chip with similar features. As a result of the patents, reverse-engineered designs and emulations had to avoid infringing the claims covered by the patents. Subsequently, these copies almost never perform exactly the same as the NS16550A or PC16550D, which are the parts most computer and modem makers want to buy but are sometimes unwilling to pay the price required to get the genuine part.
Some of the differences in the clone 16550A parts are unimportant, while others can prevent the device from being used at all with a given operating system or driver. These differences may show up when using other drivers, or when particular combinations of events occur that were not well tested or considered in the <trademark class="registered">Windows</trademark> driver. This is because most modem vendors and 16550-clone makers use the Microsoft drivers from <trademark class="registered">Windows</trademark> for Workgroups 3.11 and the <trademark class="registered">Microsoft</trademark> <trademark class="registered">MS-DOS</trademark> utility as the primary tests for compatibility with the NS16550A. This over-simplistic criteria means that if a different operating system is used, problems could appear due to subtle differences between the clones and genuine components.
National Semiconductor has made available a program named <application>COMTEST</application> that performs compatibility tests independent of any OS drivers. It should be remembered that the purpose of this type of program is to demonstrate the flaws in the products of the competition, so the program will report major as well as extremely subtle differences in behavior in the part being tested.
In a series of tests performed by the author of this document in 1994, components made by National Semiconductor, TI, StarTech, and CMD as well as megacells and emulations embedded in internal modems were tested with COMTEST. A difference count for some of these components is listed below. Since these tests were performed in 1994, they may not reflect the current performance of the given product from a vendor.
It should be noted that COMTEST normally aborts when an excessive number or certain types of problems have been detected. As part of this testing, COMTEST was modified so that it would not abort no matter how many differences were encountered.
Part Number
Errors (aka "differences" reported)
Reference modem with internal 16550 or an emulation (RC144DPi/C3000-25)
Modem with an internal 16550 (SC11951/SC11351)


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Source string comment
(itstool) path: row/entry
Source string location
article.translate.xml:1041 article.translate.xml:1047 article.translate.xml:1053
String age
a year ago
Source string age
a year ago
Translation file
articles/serial-uart.pot, string 244