Display; 98770A And 98780A Display Interface Module (Dim) - HP 9020 Service Manual

2-34
Theory of Operation
A sample and hold circuit checks the voltage level of the battery assembly at power-up. When
power is off, a capacitor is charged to the battery level. When power comes back on, the voltage
level of the capacitor is automatically read. If the level is below 3 volts, the RTC and NVM data may
be invalid and should be restored by the lOP. This fault condition is indicated to the lOP by the
battery failure status bit in the status buffer. This bit must be read within four seconds of power-up.
Display
One of the following displays is installed on the computer:
• 98770A High-Performance Color Graphics Display
• 98780A Monochromatic Display
• 98760A Standard Color Graphics Display
The 98770A and 98780A displays have their own service manuals that include theory of operation.
The 98760A display theory of operation follows in this chapter.
Note
Ignore references to ROMs in the following manuals. The ROMs do not
apply to the HP 9000 computer.
For theory of operation of the 98770A or 98780A display, refer to the applicable service manual:
• 98770A Color Graphics Display Service Manual
• 98780A Monochromatic Display Service Manual
The 98770A and 98780A displays have an associated Display Interface Module (DIM) mounted
below the display. The DIM is described below. The 98760A display theory of operation follows the
DIM description.
98770A and 98780A Display Interface Module (DIM)
A Display Interface Module (DIM) is mounted to the bottom of the 98770A or 98780A display to
interface the lOP bus from the processor with the display. The DIM is described in the following
paragraphs.
Figure 2-20 is a block diagram of the DIM. DIM translates the general lOP interface from the
computer mainframe into the specialized alphanumeric bus required by the display. DIM contains a
full screen buffer of alphanumeric characters along with refresh sequencing hardware. DIM also
buffers the 1/0 bus before sending it to the graphics sections of the display. DIM's primary function
is to refresh the alphanumeric display. DIM contains sufficient memory to store all the alphanumeric
data for one full frame. Only characters which are changed need to be written to DIM.