Hardware manual

23. Appendix A: Very Obsolete Hardware/Software
23.1 Replacing pre 1990 UARTS
Many 486 PCs (old) and all Pentiums (or the like) should have modern 16550As (usually called just 16550's)
with FIFOs. If you have something really old (pre 1990), the chip may unplug so that you may be able to
upgrade by finding a plug-in 16550A chip and replacing your old existing 16450 UART. If the functionality
has been built into another type of chip, you are out of luck. If the UART is socketed, then upgrading would
be easy if you could find a replacement. The new and old are pin-to-pin compatible. It may be more feasible
to just buy a new serial card on the Internet (few retail stores stock them as of 2000) or find a used one.
23.2 Two Ports with the Same IO address
Modern kernels should not allow the opening of ports with the same IO address. But one may probe for ports
even though they are not open. If two ports have the same IO address then old fashioned probing by sending
commands to the address will erroneously indicate only one port. But modem device detection at boot-time
should discover both ports and report the conflict. In olden days, all sorts of errors were reported/observed for
devices illegally attempting to use the same IO address. See Probing.
23.3 Configuring by modifying source code
In the past, to get a certain serial port supported, one might need to modify the C source code, perhaps by
adding a #define to it. Today, the use of parameters for modules or the kernel, or the use of configuration
options should handle all cases (except possible for antique hardware ??).
23.4 Modems on Multiport Cards Obsolete for Sending at
56k
For a modem to transmit at nearly 56k requires that it be a special digital modem and have a digital
connection to a digital phone line (such as a T1 line). Modems used with serial cards (the modems may either
be on the serial card or on another card) normally have no such digital connection so they can't be used at the
56k speed, and thus are obsolete unless one doesn't need to send at 56k. In other words they are obsolete for
ISP servers but might be OK for small business or home use.
A partial exception to the above are modem banks that connect to multiport serial cards where the modem
bank can access multiplexed digital phone lines. Thus one could use a multiport serial card with a few 56k
digital modems for sending at 56k. For both analog and digital modems there is one modem on each serial
port so there needs to be an external cable (modem bank to multiport) for each modem. This can lead to a
large number of cables. So it's less clutter (and cheaper) to use internal modems without a multiport card. This
makes even this "exception" obsolete for high volume work. It's somewhat analogous to the lower cost of an
internal modem for a desktop PC as compared to the higher cost (and more cabling) for an external modem.
See Modem-HOWTO: Modem Pools, Digital Modems.
Serial HOWTO
22.5 Internet 80