While the common monochrome terminal is not a color terminal it may have a fixed "color" display other than white such as green or amber. All terminals have black (electron beam turned off = zero brightness). A real color terminal can change the color of the text and background to many different colors while a monochrome terminal can only change the brightness of a fixed color.
However, changing the brightness, etc. gives a lot of possibilities. For example, a black and white (monochrome) terminal can have white, grey, and black by varying the brightness. Some words can be black on a light grey background while other are highlighted by black on white. In addition there is white on black, underlining, and blinking.
Color works like the color on a computer monitor or TV screen. The CRT has three colors of dots on it with each color controlled by its own electron beam (3 beams). Monochrome has inherently better resolution since it doesn't depend on dots permanently fixed on the screen. For text terminals the only use of color is to differentiate text and this advantage is not always worth the cost of worse resolution. Thus monochrome may be better since it also costs less.
For dual sessions the terminal has two serial ports of equal status. Each port is connected to a serial port on a different computer. Thus one may log in to two different computers with each session displaying in a split-screen window. Alternatively, each session may run full-screen with a "hot" key (or the like) to switch between sessions. One could also connect to two different serial ports on the same computer and log in twice (similar to "virtual terminals" at the console). The program "screen" will make any ordinary terminal (single session) connected to a single computer run two or more "sessions".
Many terminals have a connector on the rear for such a port. It may be labeled as "Aux" or "Printer", etc. Some printer ports are for parallel printers while others are for serial printers. If a printer is connected to the printer or auxiliary port, then pressing certain keys will print the screen. One may also have everything that displays on the screen go also to the printer. If the port is an auxiliary port, one may connect this to another computer and almost have dual sessions as above. However, the video memory inside the terminal may not retain both sessions so you may need to refresh the screen when switching to the other session. There will likely not be a hot key either but possibly a programmable function key may be programmed to do this. There exists various key combinations and escape sequences for controlling such a port. See Printer Esc.
There is a program called vtprint which is designed to send a
print job to your terminal to be printed on a printer attached to the
terminal. It's homepage is
http://people.qualcomm.com/garrett/vtprint . It's also a
included (as of 1998) in the Debian distribution of Linux.
xprt (also in Debian) seems to do something similar, but only
for X-Window terminals ??
Many terminals permit the storage of more than one page in their video memory. Sometimes the page size is the same as the screen, but sometimes it is larger so that scrolling will reveal unseen parts of a page. So when one looks at a screen, there may be hidden text on the same page above or below the display. In addition, if there is more than just one page, there may be hidden text on these other pages. One use for pages is on terminals that support dual sessions. Each session may have its own page and one may switch back and forth between them.
Even if you only have a one-page-terminal with the page sized equal to what is displayed on the screen, you will still see other pages of a file (etc.) as the host sends more data to the terminal. One advantage to having additional pages stored in the terminal memory is so that you can jump to them instantly without waiting a second or so for them to be transmitted from the host.
Multiple pages is supported by ncurses. There is also a commercial program called "Multiscreen" which supports multiple pages but probably not for Linux ?? Multiscreen is reported to be part of SCO and is something like the virtual terminals on a Linux PC console. The Linux program "screen" makes it look like you have multiple pages but they are stored in the computer and but you can have only one page-like window for each running program.
A character-set is normally represented by a list (or table or chart) of characters along with the byte code assigned to each character. The codes for a byte range from 0 to 255 (00 to FF in hexadecimal). In MS-DOS, character-set tables are called "code-pages". You should examine such a table if you're not familiar with them. They are sometimes included in printer and terminal manuals but may be difficult to find.
ASCII is one of the most common character-sets used on text terminals. It is a 7-bit code but can be made into 8-bit if the first bit (high order bit) is always set to 0. Other character-sets are usually available (except on very old terminals where the only choice is ASCII). The first half of most character-sets are the conventional ASCII characters and the second half (the characters with the high-order bit set to 1) belong to a wide variety of character-sets. Character sets are often ISO standards. To get specialized character sets on a terminal, you may need to download a soft-font for that character-set into the memory of the terminal.
Besides ASCII, there are some other common character-sets, all 8-bit. CP stands for Code Page character sets invented by IBM: CP-437 (DOS ECS), CP-850 (Multilingual Latin 1 --not the same as ISO Latin-1), ISO-8859-1 (Latin-1), ANSI (derived from Latin-1). MS Windows uses ANSI while the Internet often uses Latin-1. There are several ISO-8859 character sets in addition to Latin-1. These include Greek (-7), Arabic (-6), Eastern European (-2), and Russian (-5). There are many others. For example, KOI8-R is more commonly used for Russian than IS0-8859-5. Unicode is a very large character-set where each character is represented by 2 bytes instead on just one byte.
More info re character-sets are:
Most terminals made after the mid 1980's can accept downloaded
soft-font. This means that they can display almost any character set
provided that you can find the soft-font for it. If you can't find
the needed soft-font, you can always create your own. A free font
editor for this is called BitFontEdit (written by the author of this
document) and (in 1998) was at
Europe:
http://www.funet.fi/pub/culture/russian/comp/cyril-term/
N. America:
ftp://cs.utk.edu/pub/shuford/terminal/BitFontEdit.tar.gz
Terminal keyboards often have a number of keys that one doesn't find on a PC keyboard. Few (if any) actual terminals will have all of these keys and most will have additional keys not listed here. Some have a large number of special purpose keys such as terminals made for use with cash registers. There are often many more key meanings than shown here since these keys often have extended meanings when used in conjunction with other keys (such as shift and control).