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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)
To date, the author of this document has not found any non-National parts that report zero differences using the COMTEST program. It should also be noted that National has had five versions of the 16550 over the years and the newest parts behave a bit differently than the classic NS16550AFN that is considered the benchmark for functionality. COMTEST appears to turn a blind eye to the differences within the National product line and reports no errors on the National parts (except for the original 16550) even when there are official erratas that describe bugs in the A, B and C revisions of the parts, so this bias in COMTEST must be taken into account.
It is important to understand that a simple count of differences from COMTEST does not reveal a lot about what differences are important and which are not. For example, about half of the differences reported in the two modems listed above that have internal UARTs were caused by the clone UARTs not supporting five- and six-bit character modes. The real 16550, 16450, and 8250 UARTs all support these modes and COMTEST checks the functionality of these modes so over fifty differences are reported. However, almost no modern modem supports five- or six-bit characters, particularly those with error-correction and compression capabilities. This means that the differences related to five- and six-bit character modes can be discounted.
Many of the differences COMTEST reports have to do with timing. In many of the clone designs, when the host reads from one port, the status bits in some other port may not update in the same amount of time (some faster, some slower) as a <emphasis>real</emphasis> NS16550AFN and COMTEST looks for these differences. This means that the number of differences can be misleading in that one device may only have one or two differences but they are extremely serious, and some other device that updates the status registers faster or slower than the reference part (that would probably never affect the operation of a properly written driver) could have dozens of differences reported.
COMTEST can be used as a screening tool to alert the administrator to the presence of potentially incompatible components that might cause problems or have to be handled as a special case.
If you run COMTEST on a 16550 that is in a modem or a modem is attached to the serial port, you need to first issue a ATE0&amp;W command to the modem so that the modem will not echo any of the test characters. If you forget to do this, COMTEST will report at least this one difference:


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articles/serial-uart.pot, string 250