Repair manual

2-29
THEORY OF OPERATION
f. Communications Bus
The communications bus consist of 16 lines. It functions as an
address/data bus and a latch /address bus.
As an address/data bus, it provides the signal flow path for all data and
instruction codes between the Digital Palette and the host computer. It
also provides the signal flow path for all data and instruction codes
between the microprocessor and the electrical devices that control the
operations of the Digital Palette.
As a latch/address bus, it provides the signal flow path for address
instructions between the control PALs and the applicable address latches
for a particular operation (i.e., loading the VRAM shift registers).
2. Erasable Programmable Read Only Memory (EPROM)
Two 8-bit EPROM’s (Figure 2-9, sheet 1) are used in conjunction with the
Microprocessor Control network to contol the operating functions of the Digital
Palette. Depending on the Digital Palette’s configuration, the two 8-bit
EPROM’s can total 64 Kbytes or 128 Kbytes of memory. For 64 Kbyte memory
configurations, each 8-bit EPROM contains 32 Kbytes of memory. For 128
Kbyte memory configurations, each EPROM contains 64 Kbytes of memory.
(On newer boards there are only two 64Kbytes of memory.)
The two 8-bit EPROM’s are considered the Digital Palette’s firmware because
they contain the necessary operating program that make the Digital Palette
work. Collectively, they store the Digital Palette operating programs that:
o Initializes the Digital Palette when it is first powered Up (Turned On).
o Runs POWER ON diagnostics
o Checks to see if the Digital Palette has optional DRAM memory available.
o Provides a resident test image for the user to self test the exposure operation
by holding in the RESET pushbutton on the back panel of the Digital Palette.
o Controls the auto luminance operation.
o Controls the positioning of the filter wheel.
o Controls the blinking of the front panel LED to indicate when an exposure
sequence is taking place.
o Controls the sequential exposure of the computer-generated image.
o Controls vertical deflection up to 4K lines, depending on the configuration of
the Digital Palette.
o Controls the ejection or the advancement of the film frame after the
sequential exposure computer-generated image is completed.