COMPONENT MAINTENANCE MANUAL
IN 1502H RADAR INDICATOR
TM109102
1-5
May/01
At the end of each line, an X-retrace interval, which is about 12.8 microseconds. Total
time for each line, including X-retrace, is therefore about 64 microseconds.
The vertical scan time for each field is about 16.38 milliseconds (256 x 64
microseconds). At the end of each field, a Y-retrace pulse is generated by the
interface unit. However, because the two fields of each frame start at different lines,
the vertical sweep controlling the vertical scan must be started at different times to
allow the beam to begin the horizontal sweep for the interlaced lines halfway between
the lines for the previous field.
As shown in Figure 1-2, the horizontal lines are numbered from zero to 511, starting
at the bottom of the screen. The first field (field #0) begins scanning horizontally at
line 510 and proceeds to scan every other line (508, 506, . . . 4, 2, 0). When the
horizontal scan of line 0 is complete, the interface unit stops sending video and
transmits a Y-retrace signal which returns the electron beam to the top of the screen.
At the completion of field #0, the Y-retrace interval is equal to 15 X-line intervals, or
about 960 microseconds (64 microseconds x 15).
The vertical sweep must be started before video is available and for the next scan it is
initiated by the leading edge of the next X-retrace pulse. This places the start of field
#1 at line 511, and the odd numbered lines (511, 509, . . . 3, 1) are now scanned
horizontally and the scan is terminated by the next Y-retrace pulse.
To place the electron beam at the proper location (line 510) for the start of video for
filed #0, the interface unit initiates the vertical sweep early. This is accomplished by
terminating the Y-retrace pulse 32 microseconds, or one-half horizontal line, before
the start of the first horizontal line for the field. This results in a Y-retrace time equal to
only 14-1/2 X-line intervals instead of the 15 used for the previous Y-retrace. Video
from the interface unit begins when the electron beam reaches the beginning of line
510 and the cycle is repeated.
Total time for one frame (two fields plus two Y-retraces) is approximately 34
milliseconds [64 x (256 + 15 + 256 + 14.5) microseconds]. This gives a refresh rate of
nearly 30 frames and 60 fields per second.
(3) Alphanumeric Format
The interface unit generates the alphanumeric character display from dot-matrix data
stored in memory. Each character is formed according to a fixed format, 16 bits wide
and 32 lines high, including spacing between characters.
Since each line has 512 pixels (individually addressed picture elements), the
maximum number of characters per line is 32. Since each character requires 32 lines
and each frame has 512 lines, 16 rows of characters can be provided.
Characters can be shifted horizontally up to 15 pixels. This feature is useful, for
example, in shifting the numeric heading call outs at the top of the screen left or right
as the aircraft heading changes.
The document reference is online, please check the correspondence between the online documentation and the printed version.