Hardware manual

In 1888 (over 120 years ago) the "Cable Conference" reported its support of twisted-pair (for telephone
systems) and pointed out its advantages. But over 80 years after this approval by the "Cable Conference",
RS-232 failed to utilize it. Since RS-232 was originally designed for connecting a terminal to a low speed
modem located nearby, the need for high speed and longer distance transmission was apparently not
recognized. The result was that since the serial port couldn't handle high speeds, new types of serial interfaces
were devised that could: Ethernet, USB, Firewire, etc. The final outcome was the demise of the serial port
which due to it's ancient technology became obsolete for high speed uses.
4.12 Inefficient PCI Interface to the Computer (in some
cases)
The serial port communicates with the computer via the PCI bus, the LPC bus, X-bus, or ISA bus. The PCI
bus is now 32 or 64 bits wide, but the serial port only sends a byte at a time (8 bits wide) which is a waste of
PCI bus bandwidth. Not so for the LPC bus which has only a 4-bit wide bus and thus provides an efficient
interface. The ISA bus is usually 16-bits wide and the efficiency is intermediate as compared to efficient LPC
and inefficient PCI.
5. Multiport Serial Boards/Cards/Adapters
5.1 Intro to Multiport Serial
Multiport serial cards install in slots in a PC on the ISA or PCI bus. They are also called "... adapters" or "...
boards". Each such card provides you with many serial ports. Today they are commonly used for the control
of external devices (including automation for both industry and the home). They can connect to computer
servers for the purpose of monitoring/controlling the server from a remote location. They were once mainly
used for connecting up many dumb terminals and/or modems to serial ports. Today, use of dumb terminals
has declined, and several modems (or digital modems) can now be built into an internal card. So multiport
serial cards are not as significant as they once were.
Each multiport card has a number of external connecters (DB-25 or RJ45) so that one may connect up a
number of devices (modems, terminals, etc.). Each such physical device would then be connected to its own
serial port. Since the space on the external-facing part of the card is limited there is often not enough room for
all the serial port connectors. To solve this problem, the connectors may be on the ends of cables which come
out (externally) from the card (octopus cable). Or they may be on an external box (possibly rack mountable)
which is connected by a cable to a multiport card.
5.2 Dumb vs. Smart Cards
Dumb multiport cards are not too much different than ordinary serial ports. They are interrupt driven and the
CPU of the computer does most all the work servicing them. They usually have a system of sharing a single
interrupt for all the ports. This doesn't decrease the load on the CPU since the single interrupt will be sent to
the CPU each time any one port needs servicing. Such devices usually require special drivers that you must
either compile into the kernel or use as a module.
Smart boards may use ordinary UARTs but handle most interrupts from the UARTs internally within the
board. This frees the CPU from the burden of handling all these interrupts. The board may save up bytes in its
large internal FIFOs and transfer perhaps 1k bytes at a time to the serial buffer in main memory. It may use
the full bus width of 32 bits for making data transfers to main memory (instead of transferring only 8-bit bytes
Serial HOWTO
4.11 RS-232 Cable Is Low Speed & Short Distance 19