Zenith Z-100 PC series Service Manual page 259

Page
3.12
Circuit Description
The first group of registers, RO through R3, establishes the horizontal
format and timing parameters. Registers R4 through R9 determine vertical
format and timing parameters. The remaining registers, R10 through R17,
deal with cursor characteristics, screen memory addressing, and the light
pen interface. Typically, registers RO through R11 are loaded when the
system is turned on and should not need to be accessed thereafter. The
other six registers, R12 through R17, need to be accessed on an ongoing
basis during display operations. R12 and R13 establish the 14-bit starting
(top-of-page) address of screen memory. The contents of these registers
may be manipulated to perform scrolling of the screen contents. R14 and
R15 establish the 14-bit cursor address which establishes the cursor sym ­
bol position on the screen. R16 and R17 are used only if a light pen
is interfaced.
• Horizontal Timing and Format Registers - The contents of register
RO determines the total time allotted for one scan line in terms of
character clock (ClK) cycles (total of displayed and undisplayed char­
acters minus 1) per horizontal line, thus determining the horizontal
sync (HSYNC) frequency.
R1 is the character row register. It is loaded with the total number
of characters minus 1 in character clock (ClK) units.
The horizontal sync register, R2, establishes the point at which the
HSYNC signal makes its negative-to-positive transition, specified in
terms of ClKs. The reference point for the beginning of the HSYNC
pulse is the left-most character position displayed on the scan line.
R3, the HSYNC width register, uses only the four least-signi'ficant bits
of data to establish the duration of the HSYNC pulse in the range
of 1 to 16 character clocks. This allows the HSYNC pulse duration
to be adjusted to the requirements of the CRT monitor in use.
Figure 3.3 shows the interrelationship between the four horizontal tim­
ing and format registers (RO through R3) in terms of ClK.
1,1'