Terminal-host is the first generation of computers worked exclusively in the terminal-host mode. A terminal, essentially a keyboard, video screen, and a little bit of circuitry, had no intelligence at all – no CPU, no DRAM, no hard drive or other from of storage. The central computer (first a mainframe but later also a minicomputer) did all the work. The user entered keystrokes at the terminal and these were then sent over the wire to interrupt the CPU which responded as appropriate. In some cases, the response would be to modify the screen, in others it would perform some action such as printing or retrieving information. In any case, the screen image was kept in the main computer. When a change to the terminal’s display was needed, the host would update the image in its memory, then send commands to the terminal’s monitor instructing it to change the displayed image.

In later iterations, the terminal acquired more intelligence. Some terminals had enough memory to store extra screens. For example, a monitor might display half of a word processing page and the second half would be prepared by the host and sent to the terminal for storage. When the user pressed the down arrow key, the next twentyfive lines or so could be retrieved locally instead of from the host (which could then automatically send the next half to the monitor’s memory in the background). Also, as graphics became popular, more advanced systems featured intelligent terminals. Much like the video bard in a microcomputer, the terminal had a specialized processor that would draw images in its local memory, and then on the screen. Putting some intelligence in the terminal helped to reduce the demands that new users and applications were making on host machines. Still, it wasn’t the requirements of new connections and new software that drove the fundamental change in computing that led to the microcomputer


Graphical terminal

A graphical terminal is the most known (and the most used) kind of terminal-host architecture. Its purpose is display images as well as text. Graphical terminals are divided into vector-mode terminals, and raster mode.

A vector-mode display directly draws lines on the face of a cathode-ray tube under control of the host computer system. The lines are continuously formed, but since the speed of electronics is limited, the number of concurrent lines that can be displayed at one time is limited. Vector-mode displays were historically important but are no longer used. Practically all modern graphic displays are raster-mode, descended from the picture scanning techniques used for television, in which the visual elements are a rectangular array of pixels. Since the raster image is only perceptible to the human eye as a whole for a very short time, the raster must be refreshed many times per second to give the appearance of a persistent display. The electronic demands of refreshing display memory meant that graphic terminals were developed much later than text terminals, and initially cost much more.

Most terminals today are graphical - that is, they can show images on the screen. The modern term for graphical terminal is "thin client". A thin client typically uses a protocol like RDP for Microsoft Windows, or X11 for Unix-terminals. The bandwidth needed depends on the protocol used, the resolution, and the color depth.