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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:
Error (6)...Timeout interrupt failed: IIR = c1 LSR = 61
8250/16450/16550 Registers
The 8250/16450/16550 UART occupies eight contiguous I/O port addresses. In the IBM PC, there are two defined locations for these eight ports and they are known collectively as <filename>COM1</filename> and <filename>COM2</filename>. The makers of PC-clones and add-on cards have created two additional areas known as <filename>COM3</filename> and <filename>COM4</filename>, but these extra COM ports conflict with other hardware on some systems. The most common conflict is with video adapters that provide IBM 8514 emulation.
<filename>COM1</filename> is located from 0x3f8 to 0x3ff and normally uses IRQ 4. <filename>COM2</filename> is located from 0x2f8 to 0x2ff and normally uses IRQ 3. <filename>COM3</filename> is located from 0x3e8 to 0x3ef and has no standardized IRQ. <filename>COM4</filename> is located from 0x2e8 to 0x2ef and has no standardized IRQ.
A description of the I/O ports of the 8250/16450/16550 UART is provided below.
I/O Port


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