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In addition to these <quote>dumb</quote> UARTs, many vendors produce intelligent serial communication boards. This type of design usually provides a microprocessor that interfaces with several UARTs, processes and buffers the data, and then alerts the main PC processor when necessary. BecauseAs the UARTs are not directly accessed by the PC processor in this type of communication system, it is not necessary for the vendor to use UARTs that are compatible with the 8250, 16450, or the 16550 UART. This leaves the designer free to components that may have better performance characteristics.
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. BecausSince these tests were performed in 1994, they may not reflect the current performance of the given product from a vendor.
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. BecauseAs 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.
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. Because ofDue 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.
An improved version of the INS8250 using XMOS technology with various functional flaws corrected. The INS8250A was used initially in PC clone computers by vendors who used <quote>clean</quote> BIOS designs. Because ofDue to the corrections in the chip, this part could not be used with a BIOS compatible with the INS8250 or INS8250B.
The following step, is not necessary if you are using <citerefentry vendor="current"><refentrytitle>devfs</refentrytitle><manvolnum>5</manvolnum></citerefentry> in FreeBSD 5.<replaceable>X</replaceable>.
A etapa seguinte, não é necessária se você estiver usando o <citerefentry vendor="current"><refentrytitle>devfs</refentrytitle><manvolnum>5</manvolnum></citerefentry> no FreeBSD 5.<replaceable>X</replaceable>.
You can omit this part if you are running FreeBSD 5.X with <citerefentry vendor="current"><refentrytitle>devfs</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
Você pode omitir esta parte se você estiver executando o FreeBSD 5.X com <citerefentry vendor="current"><refentrytitle>devfs</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
Next, appropriate entries in <filename>/dev</filename> for the devices must be made using the <filename>/dev/MAKEDEV</filename> script. This step can be omitted if you are running FreeBSD 5.X with a kernel that has <citerefentry vendor="current"><refentrytitle>devfs</refentrytitle><manvolnum>5</manvolnum></citerefentry> support compiled in.
Em seguida, as entradas apropriadas em <filename>/dev</filename> para os dispositivos devem ser criadas usando o script <filename>/dev/MAKEDEV</filename>. Esta etapa pode ser omitida se você estiver executando o FreeBSD 5.X com um kernel que tenha sido compilado com o suporte ao <citerefentry vendor="current"><refentrytitle>devfs</refentrytitle><manvolnum>5</manvolnum></citerefentry>.