Raster Scan Terminology - Wyse WY-60 Maintenance Manual

RASTER SCAN TERMINOLOGY
The terminal has a 14-inch cathode ray tube (CRT) display.
Because the terminal has a video output, most of its circuitry
provides the appropriate inputs to the CRT display (Le., clock
and video information).
An electron beam sweeps the face of the CRT. The electron beam
begins in the upper left-hand corner of the display and sweeps
from left to right. One sweep of the beam is called a "scan
line." When the beam returns from the right side of the screen
to the left side, beginning the next scan line, it must be
disabled (turned off). This prevents a streak from appearing on
the face of the CRT. Disabling the beam is called "horizontal
blanking." At the end of 416 scan lines, the electron beam
arrives at the lower right-hand corner of the display. In order
to return to the upper left-hand corner, the beam must be
disabled again so a vertical streak doesn't appear on the face of
the CRT. Disabling the beam at this time is called "vertical
blanking." The amount of time it takes the beam to move from the
upper left-hand corner of the display and return is a "field."
The terminal uses a 60 Hz field rate in either 60 Hz or 50 Hz AC
input.
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