Main Control Unit - OKIDATA OL830 Service Manual

2.2.02 Main Control Unit

The control unit controls reception of the data from the host through the interface, processes
command analysis, bit image development, and raster buffer read. It also controls signal flow with
the Engine Controller Board and the Operator Panel.
Reception Control
The control unit has three interface ports: centronics parallel and optional RS-232C serial, and
Appletalk/RS-422 serial ports. Through the menu, one of the I/F ports can be selected at a time.
The other ports output BUSY in emulation mode, but not in PostScript mode. The Centronics
Parallel port is the default port for the OL830.
Command Analysis Processing
The OL830 contains the PostScript Interpreter and the following emulation modes (which are
selected through the printer MENU.)
LaserJet Series II: Hewlett Packard
Diablo 630 : Xerox
In the PostScript mode, each typeface style is stored as a series of outlines, one for each
character, comprised of lines and Bezier curves. The shape of each character in a typeface style
is actually determined by a mathematical formula. When the document is to be printed, the host
sends information to the printer in the form of the PostScript page description language. The
printer then reads the file it receives, sizes and orients each character outline as required, and
prints the page.
In the HP and Diablo modes, the OL830 prints fonts as bit-mapped outlines. The outline of each
character is determined by placing dots in a grid. A complete set of these grids, one for each
character in the font, is stored in ROM. Since no special calculations are used in the HP mode,
each typeface, style, weight, orientation and size is stored separately.
Down-line-loaded (DLL) fonts and macro registration are stored in the DLL and macro areas of
memory (RAM). Emulation and PostScript interpretation are performed under program control by
the CPU.
Font Processing
In the emulation mode, the bit image in the font ROM is loaded by the CPU into the raster buffer
(RAM). The raster buffer has a band buffer structure; new data is written to the raster buffer once
the buffer has been read and becomes empty.
In PostScript mode, the outline data stored in the programmed ROM is loaded into the RAM area
called the font cache. The required font pattern is then transferred to the raster buffer. The buffer
has a full-page buffer structure. One page of data is written at a time. After a page is written,
another raster buffer read operation starts.
Raster Buffer Read Operation
The bit image written to the raster buffer by the CPU is converted to serial dot data to be read in
response to the VIDEO SYNCH signal.
Service Guide OL830
Chapter 2 Principles of Operation
Page: 18