Zenith Data Systems Z-100 Series Installation Manual page 36

Page
3.6
General Theory of Operation
• Horizontal Displayed Register (R1)-This 8-bit, write-only register
selects the number of characters in each row that will be displayed
on the screen. In normal operation, this register should contain the
value 50H or 80 decimal.
• Horizontal Sync Position Register (R2)-This 8-bit, write-only regis-
ter determines the position of the sync pulse in each scan line. Any
value greater than the value stored in the horizontal displayed register
(R1) and less than the value stored in the horizontal total register
(RO) may be used. When the value programmed in R2 is increased,
the display shifts to the left. When the value is decreased, the display
shifts to the right. The default value is 52H or 82 decimal.
• Sync Width Register (R3)-This 8-bit, write-only register sets the
pulse width of the sync pulse. The horizontal sync is controlled by
the lower 4 bits, and the pulse width of the vertical sync by the upper
4 bits. Increasing this value lengthens the pulse. The default value
is OFH or 15 decimal.
NOTE: The sum of R1
+
R2
+
R3 must be less than or equal to the
value in RO.
• Vertical Total Register (R4)-This 7-bit, write-only register and R5
are used to determine the refresh rate of the video monitor and the
vertical retrace timing. This value is expressed in character rows (nor-
mally 14 scan lines). These values are based on a horizontal rate
of 18,432 scan lines per second and are determined by the formula:
value
=
horizontal scan lines per second/refresh rate
x
1/scan lines
per character.