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For the purposes of this book, the word terminal doesn't necessarily mean the old, dumb, ASCII-based terminals
that many feel are remnants of days gone by. Although those machines are definitely terminals (and fully usable
with Linux), modern terminals can range from inexpensive graphics-based systems to complex X workstations
that have more in-built computing power than most PC machines (at a suitably hefty price). A terminal can also be
any other computer (PC, Macintosh, UNIX system, Commodore 64, Amiga, and so on) that runs a terminal
emulation package, which makes the machine act like a terminal.
You may want to add terminals to your system to allow other users to work with Linux at the same time you do (it
is a multiuser system) or to provide access to your database of videotape movies (not to mention games) by
running a terminal into your living room. If you are a parent, you can run terminals into your children's rooms,
letting them use the system for education and entertainment. You may want to let friends access your system when
they visit, or call in over the modem.
For whatever reason you need new terminals, you can add them to your Linux system by connecting them through
an existing serial port on the back of your PC or through a multiport card with many serial ports on it.
Using Multiport Cards
Because many PC machines have a maximum of four serial ports (and the majority of systems only have two),
expanding your Linux system using serial ports can be limiting. If you use an external modem, a serial printer, or
other serial port devices, you may not have any serial ports left for terminals. In this case, you must use a multiport
card.
Multiport cards are an easy and effective method of adding serial ports to a Linux system. Multiport cards have a
plug-in board that is placed in a slot on your PC system and an oversized connector on the outer board edge to
which a cable is attached. The cable either leads directly to a number of serial ports (in which case the cable is
called an octopus) or to a hardware device that has serial ports laid out on it.
Multiport cards come in two basic types. The first is essentially a fast, somewhat intelligent serial port server.
These cards are inexpensive because they are simple to manufacture. However, they lack any on-board processing
or memory, so every device connected to the multiport card takes its toll on the Linux system's CPU and RAM.
The other type of multiport card is the intelligent controller. These cards cost much more, but have an on-board
CPU to offload the Linux system's CPU. The on-board CPU can handle all the communications requests and, in
some cases, is smart enough to provide terminal commands. These cards usually have RAM mounted on the board
too, which provides a cache system for speeding up access. In general, these intelligent boards are much better for
supporting four or more terminals and other devices than their dumber brethren, but you may be spending money
on features you don't need if you have less than that number of attached devices.
Multiport cards can provide from 2 to 32 additional serial ports per card, and you can add multiple boards to the
increase capacity even further, although this situation is very rare for Linux installations. A couple of
manufacturers even offer systems that can support 256 terminals spread out in a cluster arrangement. Some
multiport boards include parallel ports for printers, and a few high-end boards are designed to use SCSI devices.
Each port on the multiport card is usually wired for use by any serial device, including terminals, modems,
printers, scanners, and so on. However, a few cards designed for simple terminal use support only a few of the
wires in a serial port. These cards cannot support modems, printers, and similar complex devices properly.
Different card manufacturers have different supported systems, so if you decide to go with a multiport card for
your system, check the specifications carefully.
The types of connectors on multiport cards differ, too. Most use either standard DB25 25-pin connectors or DB9
9-pin connectors, identical to the ones found as PC serial ports. Some cards use the RJ45 connector, which looks
like a wide modular telephone-style jack. Adapters are used to connect the RJ11 connector to a standard serial
cable. Again, if you decide to use a multiport card, check the types of connectors and make sure the wiring of the
connector is consistent with your serial devices or that converters and adapters are readily available.
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